the suitability of active personal dosimeters as the legal dosimeter for pet radioisotope workers

The suitability of active personal dosimeters as the

legal dosimeter for PET radioisotope workers

Steven J Crossley

Supervisors: Dr. Roger Price, Dr. Mike House

Masters Thesis submitted as part of the M.Sc. by Thesis and Coursework

in the School of Physics, University of Western Australia

Date of submission: 28th of September 2016

Abstract

Staff working with PET radiopharmaceuticals wear active personal dosimeters and

a passive dosimeter which provides the legal dose record for regulatory purposes.

Given the capabilities of current active dosimeters with a dose logging capability it

may be asked whether the active dosimeters could be used as the legal dosimeter,

removing the need for a passive dosimeter.

A series of controlled experiments were performed exposing active dosimeters

and two types of approved passive dosimeters to a range of doses from vials con-

taining 18FDG. Reported doses from passive and active monitoring of staff were

compared over 24 months. A questionnaire was used to gauge worker preferences

and acceptance of different personal dosimeters.

It was found that the active dosimeters agree well with the TLD results over

the range of doses tested in the controlled experiments. Agreement with the OSL

dosimeters was not as good. Active dosimeters gave more repeatable results than

either of the passive dosimeters.

There was poor agreement between the passive and active dosimeters in the

worker results for both radiopharmaceutical production workers and nurses and

technologists working with PET patients. Large numbers of the passive dosimeters

reported “below the detection limit” when the active dosimeters reported doses above

the supplier stated detection limits.

Workers were positive in their response to using active dosimeters, and felt that

they were useful in aiding their radiation protection.

Controlled experiments have demonstrated that active dosimeters are capable of

accurately and reliably reporting doses from 18FDG. Comparisons of worker doses

were far less conclusive and demonstrated the difficulty of obtaining accurate dose

data from personal dosimeters of any kind. The main hurdle to the use of active

dosimeters to provide the legal record of worker exposure seems to be regulatory

rather than technical.

Acknowledgements

I would like to acknowledge the assistance I have received form my co-workers in the

Medical Technology & Physics Department (MTP) at Sir Charles Gairdner Hospital.

The RAPID group within MTP, in particular Peter Gibbons and Chris Jones, for their

dispensing of doses of radiopharmaceutical and helping with the ordering, setting up

and running of the active dosimetry system. The Medical Physics group in MTP for

help with ordering and reporting advice for passive dosimeters and assistance with

literature searches and for there feedback during thesis writing. I’d like to thank

Phil Parr and Barry Turk for their mechanical skills in making my experimental

rig. Janette Atkinson and Dr Roger Price have my gratitude for allowing me the

time and resources to carry out my research in the department. The nurses and

technologists in Nuclear Medicine also deserve thanks for their willingness to assist

me in trialing the active dosimeters in their department.

My project coordinator Dr Mike House has been a great help in pulling this

thesis together and in helping me complete the rest of the Masters course. I would

like to thank him for his patience.

Last, but by no means least, my wife Kelly has helped greatly with patience, ad-

vice, assistance and motivation throughout my Masters and indeed our life together.

Contents

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Types of personal dosimeter . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Passive Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1.1 Film Badges . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1.2 Thermo-luminescent and Optically Stimulated Dosime-

ters (TLD & OSL) . . . . . . . . . . . . . . . . . . . 4

1.2.2 Active Personal Dosimeters . . . . . . . . . . . . . . . . . . . 5

1.3 The Accuracy of Personal Radiation Dosimeters . . . . . . . . . . . . 10

1.4 Active Dosimeters for Legal Assessment of Occupational Dose . . . . 11

1.5 PET Radiopharmaceutical Production . . . . . . . . . . . . . . . . . 12

1.6 PET Radiopharmaceutical Dispensing and Use . . . . . . . . . . . . . 14

1.7 PET Centre workers . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.8 Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Experimental Methods & Materials 17

2.1 Radiation Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2 Passive Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.1 TLD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.2 OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3 Active Dosimetry System . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3.1 DMC 2000 and DMC 3000 . . . . . . . . . . . . . . . . . . . . 20

2.3.2 Logging Station & Database . . . . . . . . . . . . . . . . . . . 21

2.3.3 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.4 Controlled performance comparison of passive and active dosimeters . 24

2.4.1 Radiation Safety . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4.2 Physical layout of experiment . . . . . . . . . . . . . . . . . . 25

2.4.3 Conducting an Exposure . . . . . . . . . . . . . . . . . . . . . 28

i

CONTENTS ii

2.4.4 Exposures Performed . . . . . . . . . . . . . . . . . . . . . . . 29

2.4.5 Obtaining Results . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.4.6 Normalising results from separate exposures . . . . . . . . . . 31

2.4.7 Displaying Results . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4.8 Statistical Assessment of Difference of Means . . . . . . . . . . 33

2.5 Comparison of staff doses recorded by passive and active dosimeters . 34

2.5.1 Gathering RAPID Staff doses . . . . . . . . . . . . . . . . . . 34

2.5.2 Gathering PET Centre Staff doses . . . . . . . . . . . . . . . . 34

2.5.3 Comparison of doses . . . . . . . . . . . . . . . . . . . . . . . 35

2.6 User Experience Survey . . . . . . . . . . . . . . . . . . . . . . . . . 36

3 Results 37

3.1 Results below the detection limit . . . . . . . . . . . . . . . . . . . . 37

3.2 Controlled performance comparison of passive and active dosimeters . 39

3.2.1 Comparison of doses around the experimental rig . . . . . . . 39

3.2.2 Comparison of results from the same dosimeter type . . . . . . 39

3.2.3 Comparison of dosimeter results with theoretical dose . . . . . 41

3.2.4 Effects of angling the dosimeters . . . . . . . . . . . . . . . . . 43

3.2.5 Comparison of passive dosimeters . . . . . . . . . . . . . . . . 47

3.2.6 Comparison of active dosimeters with OSL dosimeters . . . . . 50

3.2.7 Comparison of active dosimeters with Thermoluminescent dosime-

ters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.2.8 Summary of inter and intra-dosimeter type comparisons in

controlled experiments . . . . . . . . . . . . . . . . . . . . . . 52

3.2.9 Statistical significance of agreement of means . . . . . . . . . 54

3.3 Comparison of staff doses recorded by passive and active dosimeters . 55

3.3.1 RAPID Staff doses . . . . . . . . . . . . . . . . . . . . . . . . 55

3.3.1.1 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.3.1.2 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.3.1.3 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.3.1.4 The effect of reported wear position on correlation . 60

3.3.2 PET Centre Staff doses . . . . . . . . . . . . . . . . . . . . . . 61

3.3.2.1 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.3.2.2 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.4 User Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.4.1 Profession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.4.2 Time using Passive and Active Dosimeters . . . . . . . . . . . 66

iii

3.4.3 Ease of use of Dosimeters . . . . . . . . . . . . . . . . . . . . 66

3.4.4 Comfort wearing Dosimeters . . . . . . . . . . . . . . . . . . . 67

3.4.5 Wear Position of Dosimeters . . . . . . . . . . . . . . . . . . . 67

3.4.6 Frequency of checking results . . . . . . . . . . . . . . . . . . 67

3.4.7 Level of trust in dosimeter results . . . . . . . . . . . . . . . . 68

3.4.8 Rate of not wearing a dosimeter . . . . . . . . . . . . . . . . . 68

3.4.9 Usefulness of results and feedback . . . . . . . . . . . . . . . . 69

3.4.10 Prefer to wear Active, Passive or Both . . . . . . . . . . . . . 69

3.4.11 Additional Comments . . . . . . . . . . . . . . . . . . . . . . 69

4 Discussion 71

4.1 Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.2 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.3 Repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.4 Limits of Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.5 User compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.6 User Acceptance of Active Dosimeters . . . . . . . . . . . . . . . . . . 75

4.7 Approval of Personal Radiation Dosimetry Services . . . . . . . . . . 76

4.8 Standards for Personal Radiation Monitors . . . . . . . . . . . . . . . 77

4.9 Calibration of APDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.10 Record Keeping and Data Analysis . . . . . . . . . . . . . . . . . . . 78

4.11 Incident investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.12 Economic Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.12.1 Costs of Passive Dosimetry . . . . . . . . . . . . . . . . . . . . 80

4.12.2 Costs of an Active Dosimetry System . . . . . . . . . . . . . . 81

4.12.3 Lifetime of MGP Active dosimeters . . . . . . . . . . . . . . . 81

4.12.4 Comparison of costs per year . . . . . . . . . . . . . . . . . . 82

4.13 Legislative issues in Western Australia . . . . . . . . . . . . . . . . . 85

5 Conclusion and Future Work 87

5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Bibliography 89

A User Experience Survey 94

B Example Dose Reports 97

List of Figures

1.1 Electron trapping in TLD/OSL . . . . . . . . . . . . . . . . . . . . . 4

1.2 Electron relaxation during TLD/OSL readout . . . . . . . . . . . . . 5

1.3 Doped silicon semiconductor structures illustrating free electrons (n-

type) and electron holes (p-type). . . . . . . . . . . . . . . . . . . . . 6

1.4 Diode with no applied voltage . . . . . . . . . . . . . . . . . . . . . . 7

1.5 A reverse bias diode acting as a radiation detector . . . . . . . . . . . 8

1.6 Example diagram of an FDG synthesis system (IBA Synthera) . . . . 13

2.1 Radiation Detection Company TLD . . . . . . . . . . . . . . . . . . . 19

2.2 Landauer OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3 Active dosimeters DMC2000S, DMC2000X, DMC2000XB and DMC3000 20

2.4 Logging Station with dosimeter in cradle . . . . . . . . . . . . . . . . 22

2.5 Logging Station showing dose results at log out . . . . . . . . . . . . 23

2.6 Passive dosimeters arranged on 1m radius rail . . . . . . . . . . . . . 25

2.7 Personal dosimeters on holders. . . . . . . . . . . . . . . . . . . . . . 26

2.8 Dosimeter holders on the rail at 0, 30 and 60 degrees . . . . . . . . . 27

2.9 Example plan layout of experimental setup . . . . . . . . . . . . . . . 28

3.1 Comparison plots for results from the same dosimeter type . . . . . . 40

3.2 Comparison of Active dosimeter results with theoretical dose . . . . . 42

3.3 Comparison of OSL dosimeter results with theoretical dose . . . . . . 42

3.4 Comparison of TLD results with theoretical dose . . . . . . . . . . . 43

3.5 Plots showing the effect of angulation on MGP dosimeters . . . . . . 44

3.6 Plots showing the effect of angulation on OSL dosimeters . . . . . . . 45

3.7 Plots showing the effect of angulation on TLDs . . . . . . . . . . . . 45

3.8 Plot of Normalised Mean Results against angle . . . . . . . . . . . . . 46

3.9 Initial comparison of passive dosimeter results . . . . . . . . . . . . . 47

3.10 Comparison of OSL and TLD results after repeat exposures . . . . . 48

3.11 Comparison of OSL and MGP results . . . . . . . . . . . . . . . . . . 50

iv

LIST OF FIGURES v

3.12 Comparison of TLD and MGP results . . . . . . . . . . . . . . . . . . 51

3.13 Mean dose per Dosimeter Type vs Mean dose per exposure . . . . . . 53

3.14 Comparison of RAPID staff dose results for 2012 . . . . . . . . . . . 56



3.17 RAPID staff dose results for 2014 for staff wearing the passive and

active dosimeters in the same position on the body. . . . . . . . . . . 61

3.18 Comparison of PET Centre staff dose results for 2013 . . . . . . . . . 62

3.19 Comparison of PET Centre staff dose results for 2014 . . . . . . . . . 64

4.1 Lifetime of MGP dosimeters . . . . . . . . . . . . . . . . . . . . . . . 82

List of Tables

2.1 Active Dosimeter Models Used . . . . . . . . . . . . . . . . . . . . . . 21

2.2 Alarm Settings on Active Dosimeters . . . . . . . . . . . . . . . . . . 22

2.3 Exposures performed in initial controlled experiments . . . . . . . . . 30

2.4 Exposures performed in repeated controlled experiments . . . . . . . 30

2.5 RAPID Staff Numbers 2012-2014 . . . . . . . . . . . . . . . . . . . . 34

2.6 Numbers of staff wearing active dosimeters when working in PET only

(Nov 2012 to December 2013) . . . . . . . . . . . . . . . . . . . . . . 35

2.7 Numbers of staff wearing active dosimeters while working with PET

and Nuclear Medicine patients (January to December 2014) . . . . . 35

3.1 Reported Minimum Detection Limits for Passive dosimeters as stated

by suppliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.2 Number of excluded dosimeter results in the controlled experiments . 38

3.3 Position dependence of results . . . . . . . . . . . . . . . . . . . . . 39

3.4 Summary of changes in dose readings when angling dosimeters . . . . 46

3.5 Summary of linear fits to comparisons of dosimeter results . . . . . . 52

3.6 Mean difference and 1.96σv values for dosimeter comparisons . . . . . 52

3.7 T-test Results (for p=0.05) for agreement of different dosimeter types 54

3.8 Bland-Altman Results for 2012 RAPID Doses . . . . . . . . . . . . . 57



3.11 Bland-Altman Results for 2013 PET Centre Doses . . . . . . . . . . . 63

3.12 Bland-Altman Results for 2014 PET Centre Doses . . . . . . . . . . . 64

3.13 Professions of those surveyed . . . . . . . . . . . . . . . . . . . . . . . 66

3.14 Experience using Active and Passive Dosimeters . . . . . . . . . . . . 66

3.15 Ease of Use of Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . 66

3.16 How comfortable are dosimeters . . . . . . . . . . . . . . . . . . . . . 67

3.17 Wear Position of Dosimeters . . . . . . . . . . . . . . . . . . . . . . . 67

vi

LIST OF TABLES vii

3.18 Frequency of checking dosimeter results . . . . . . . . . . . . . . . . . 67

3.19 Level of trust in dosimeter results . . . . . . . . . . . . . . . . . . . . 68

3.20 The rate at which workers forget to wear dosimeters . . . . . . . . . . 68

3.21 Usefulness of results and feedback . . . . . . . . . . . . . . . . . . . . 69

3.22 Prefer to wear active, passive or both . . . . . . . . . . . . . . . . . . 69

4.1 UK HSE Pass/Fail criteria for dosimetry services for monitoring whole

body gamma exposure (Health and Safety Executive, 2010) . . . . . . 77

4.2 Economic Comparison of Active and Passive Dosimetry . . . . . . . . 83

Chapter 1

Introduction

1.1 Background

Exposure to ionising radiation is potentially harmful both in terms of large acute

doses, causing tissue effects, and small but chronic exposure increasing the risk of

stochastic effects, in particular cancer (International Commission on Radiological

Protection, 2007). While the statistics of cancer induction make it impossible to

prove that doses of a few milli-sieverts increase the risk of cancer in humans, data

at higher doses indicate a linear relationship between radiation exposure and the

probability of cancer induction. Radiation safety standards and legislation assume

that this relationship is linear for low exposures all the way down to zero; this is

known as the linear no threshold hypothesis (LNT) (International Commission on

Radiological Protection, 2007). In order to assess the risk from an exposure, or

a series of exposures, it is essential to know the dose to which the individual was

exposed (International Commission on Radiological Protection, 2007), even when

the exposure level is low.

International recommendations have been made to limit the dose to which ra-

diation workers are exposed (International Commission on Radiological Protection,

2007). Based on these recommendations governments, in Australia and around the

world, have put in place laws (Western Australia, 1984; The Health and Safety Ex-

ecutive, 1999; South Australia, 2000) to limit the risks to occupationally exposed

workers from radiation. Dose limits are set to keep the risk from ionising radiation,

calculated using LNT, similar to workplace risks of other kinds, accepted by workers

in other occupations. In order to monitor compliance with dose limits, many juris-

dictions have also mandated the use of personal radiation monitors (The Health and

Safety Executive, 1999; Western Australia, 1984; Bolognese-Milsztajn et al., 2004).

1

1 Introduction 2

Personal radiation monitors are devices worn by individual workers which are

used to provide a permanent record of their radiation exposure. The principal is

that the monitor is exposed to the same radiation fields as the individual, and is

capable of recording the exposure. The dose that the monitor has been exposed to

is then read from the dosimeter to provide a record of the exposure of the individual

(National Council on Radiation Protection and Measurements, 1995).

The materials used to record radiation exposure in personal dosimeters have

evolved over time, but the general working practice has remained the same, with

personal dosimetry being provided as a service by approved suppliers. It has been

argued that developments in active dosimeter technology could change this model.

Employers could provide their own dosimetry service while improving radiation safety

through feedback on dose rates from active dosimeters (Luszik-Bhadra et al., 2007).

1.2 Types of personal dosimeter

1.2.1 Passive Dosimeters

Different types of passive dosimeter record dose in different ways, but all are worn for

a fixed period of time, usual one or three months, by a specific individual and then

returned to the supplier for reading (American Association of Physicists in Medicine

[AAPM], 1995). The dosimeters are recording their radiation exposure from the

time of their manufacture to the time of their reading. The supplier then provides

a report of the cumulative dose for each individual over the period the dosimeter

was worn. The effect of background radiation is mitigated by the use of “control”

dosimeters which come from the same batch as the dosimeters which will be worn.

The control dosimeters travel to and from the workplace with the dosimeters which

will be worn by staff, but are kept away from occupational exposure. The reading

of the control dosimeter is subtracted from that of the worn dosimeters to give

the occupational dose reading. Passive dosimeters provide a retrospective record of

received dose which is reported some time after the exposure occurs. The dosimeter

is sent to be read at the end of the wear period, and there is a delay between the

end of the wear period and the reporting of the dose. This delay can extend to three

or four months (Lummis, 2013). Passive dosimeters have a decades long history of

use in radiation protection, and their performance across a wide range of radiation

energies and types is well understood and documented (Luszik-Bhadra et al., 2007).

There comes a point where a measurement is too small to reliably distinguish it

from background radiation. Passive dosimeters start recording background radiation

1 Introduction 3

from the moment they are manufactured or are reset through heating or exposure to

a strong light source. When a worker is only exposed to small amounts of radiation in

their occupation, this small amount of radiation can be swamped by the background

signal acquired over the months between manufacture and reading of the dosimeter.

Even with background subtraction there is a limit to how small an exposure can

be reliably detected. Because of these issues, passive dosimeters have a minimum

detectable dose below which no reliable dose information can be obtained, and thus

readings below this level are not reported. The value of the minimum detectable

dose varies from provider to provider, but is largely governed by the material used

to record the dose.

1.2.1.1 Film Badges

Film badges are the oldest type of passive personal dosimeter still in use today, but

are being phased out in some jurisdictions, including France and Germany (Luszik-

Bhadra et al., 2007). A film badge contains a small sheet of radiation sensitive film

protected from light by an opaque packet. The film is housed in a plastic holder

that can be attached to clothing. Radiation incident on the film causes chemical

changes which make the film darker when developed, increasing its optical density.

The film is “read” with a densitometer as the optical density of the developed film

is proportional to the radiation dose it has been exposed to. Due to its composition

and density, film does not absorb radiation in the same way that human tissue does.

Exposed to the same radiation, film will absorb a different fraction of the energy

from the radiation than tissue would. Film is not “tissue equivalent”. As the film is

not tissue equivalent the optical density of the film is not directly related to tissue

dose. The use of a range of filter materials placed in the holder between the radiation

source and the film give a range of optical densities on the film. The set of optical

densities can be used in the calculation of tissue dose. A filter is a known thickness

of a pure material with known radiation absorption qualities. If two areas of a film

are exposed to the same radiation source with different, known filters, the difference

in the energy deposited in the two regions gives information on the energy of the

radiation. This spectral information can be combined with the dose to the film to

deduce the dose to tissue. Typically film badges have a minimum detectable dose of

100μSv (Bushberg, 2012). They are lightweight and inexpensive, but easily damaged

by exposure to light, heat or moisture. As the film is replaced in the holder each

month or quarter it is possible to load the film into the holder the wrong way around.

Rotating the film changes the positions of the filters relative to the film, leading to

1 Introduction 4

inaccurate results (Bushberg, 2012).

1.2.1.2 Thermo-luminescent and Optically Stimulated Dosimeters (TLD

& OSL)

TLDs and OSLs are both radiation exposure monitoring devices which make use

of a scintillant material to record exposure over a period of time. Scintillants are

materials which give off visible light when irradiated by ionising radiation. For most

scintillant materials the emission of light is immediate (prompt fluorescence). In

TLDs and OSLs small amounts of specific impurities (dopants) are used to create

electron traps. When electrons in the material are excited to higher energy levels by

ionising radiation they transition to the electron trap rather than returning to the

valence energy level as shown in figure 1.1. Light is only emitted when a stimulus

enables the trapped electron to return to the valence band, emitting a photon of a

particular frequency (see figure 1.2). In the case of TLDs the stimulus is heat, with

an OSL the stimulus is laser light of a particular frequency scanning the surface

of the dosimeter (Bushberg, 2012). The amount of light given off during reading

is proportional to the amount of radiation absorbed. As the materials used in

the dosimeters have a similar effective atomic number to tissue the light output is

broadly proportional to the dose to tissue (Bushberg, 2012). The use of filters of

different materials allows for more accurate determination of equivalent dose, based

on the dose to dosimeter material behind each filter. The use of filters is particularly

important for measuring dose from low energy photons. In modern TLDs and in

OSLs the filters are fixed inside the dosimeters (Obryk et al., 2011).

Figure 1.1: Electron trapping in TLD/OSL

1 Introduction 5

Figure 1.2: Electron relaxation during TLD/OSL readout

From the users’ perspective the badges are handled in the same way as film

badges, they are worn for a given period, and then returned to the supplier. For a

TLD the badge is heated to a particular temperature in controlled conditions, and

the light emitted is detected by a photo-multiplier (PM) tube. The electrical signal

from the PM tube is proportional to the light emitted which in turn is proportional

to the radiation dose delivered to the scintillant (Bushberg, 2012). When reading an

OSL dosimeter the surface of the scintillant is scanned by a laser of one frequency

which causes the de-excitation of electrons, and the emission of light of a different

frequency from the illuminated region (Bushberg, 2012). The scanning of OSL

dosimeters allows for readout of the distribution of dose across the dosimeter which

can give information relating to the nature of the exposure, for example whether it

was a single acute exposure or a number of smaller exposures (Akselrod et al., 2000).

OSL badges can also be scanned more than once if there is a query (McKeever and

Moscovitch, 2003).

Film badges themselves form a permanent record, and can be reviewed if required,

but once TLD and OSL badges are read the badges are stimulated to return all

electrons to the ground state, and the scintillant material is re-used (Bushberg, 2012).

The ability to reuse scintillant materials keeps costs down.

1.2.2 Active Personal Dosimeters

Active personal dosimeters (APDs) contain at least one semiconductor based ra-

diation detector, with electronics to calculate and display equivalent dose. When

semiconducting material absorbs ionising radiation, electrons are promoted to the

conduction band from the valence band, creating electron-hole pairs in a manner

similar to that in TLDs.

1 Introduction 6

Without cooling, applying a voltage across a pure semiconductor to collect the

charge carriers induces a greater number of electron-hole pairs than low doses of

radiation. This renders pure semiconductors inefficient radiation detectors at room

temperature. To overcome this problem a semiconductor diode is used with a reverse

bias. A diode consists of an n-type semiconductor which contains mobile electrons

joined to a p-type semiconductor containing electron holes. The effect of dopants on

the crystal structure of a semiconductor is shown in figure 1.3.

The free electrons and holes are present due to the presence of dopants with

fewer or more valence electrons than the semiconductor material itself. If an element

containing one more valence electron than the semiconductor is present, the spare

valence electron can act as a free electron in the structure of the semiconductor. The

presence of an element containing one fewer valance electrons than the semiconductor

will create a hole into which electrons can move, the movement of an electron to fill

the hole creates another hole. The hole thus acts as a mobile charge carrier in the

semiconductor.

Figure 1.3: Doped silicon semiconductor structures illustrating free electrons (n-type)and electron holes (p-type).

The potassium atom (P) contains an unpaired valence electron, and the boron atom (B)requires an extra electron to form bonds to all the surrounding silicon (Si) atoms.

1 Introduction 7

Figure 1.4: Diode with no applied voltage

With no voltage applied to the diode the mobile charge carriers are distributed

through the diode (figure 1.4). When a reverse bias is applied the mobile charge

carriers move to the edge of the diode, leaving a region at the junction of the two

semiconductors free of charge carriers; the depleted region (figure 1.5a).

When exposed to ionising radiation, electron-hole pairs form in the depleted

region (figure 1.5b). Moved by the applied voltage, the charge carriers generate a

small current which can be amplified and measured (figure 1.5c).

1 Introduction 8

(a) Diode with reverse bias creating the depleted region

(b) An incident gamma photon creating a charge pair

(c) Movement of the charge pair generating a small current.

Figure 1.5: A reverse bias diode acting as a radiation detector

A diode detector acts as an ion chamber. The current generated is proportional

to the energy deposited by the radiation, which is proportional to the dose to tissue

(Bushberg, 2012). As the signal is electrical it can be used by computing circuits as

the input to calculations, the results of these calculations of radiation dose can be

1 Introduction 9

recorded, and displayed to the user in real time.

As active personal dosimeters can give instant feedback on the dose and/or dose

rate, they have primarily been used for operational radiation protection monitoring

(Ginjaume et al., 2007). In particular they are used where there is the potential

for high dose rates, necessitating immediate feedback to minimise exposure. Their

use is mandated for some occupations in Western Australia (Radiological Council of

WA, 2010), including radiochemists working with large activities of PET isotopes.

There have been a number of papers published comparing the performance of

many of the available APDs. The IAEA established a methodology for comparing

the performance of APDs over a range of x-ray, gamma and beta energies, and

compared 13 different dosimeters (International Atomic Energy Association, 2007).

Dosimeters from Artomex, Canberra, Graetz Strahlungsmesstechnik, Polimaster,

SAIC, Synodys Group (MGP and Rodos), Thermo Electron and Unfors were tested.

All but one of the tested monitors reported Hp10 (the personal dose equivalent to

tissue at a depth of 10mm, usually referred to as deep dose), but 3 (Atomex AT3509B,

MGP DMC2000XB and the Thermo Electron EPD Mk2.3) also report Hp0.07 (the

personal dose equivalent to tissue at a depth of 0.7mm; the surface or skin dose).

The Unfors NED is an extremity monitor used for monitoring the dose to fingers or

eyes, and only reports Hp0.07. The monitors were tested using a range of different

radiation sources facing the radiation source, and at 30 and 60 degrees from the

source direction to assess that response is acceptably independent of direction.

The IAEA report concluded that the performance of the active dosimeters was

generally comparable to that of passive dosimeters when measuring gamma radiation,

but only a few accurately reported beta and low energy x-ray radiation doses. Many

of the dosimeters were incapable of measuring pulsed x-ray doses accurately; this

failure has also been reported in other publications (Ambrosi et al., 2010; Ankerhold

et al., 2009; Bordy et al., 2008; Clairand et al., 2008). Pulsed x-rays are used exten-

sively in fluoroscopic procedures in hospitals, and so the selection of an appropriate

model of dosimeter is critical in this setting.

It is clear from the literature that the choice of active dosimeter should be carefully

considered to ensure that it is suitable for dosimetry in all the radiation fields to

which it may be exposed. There are however some dosimeters that perform well

across the board, and could be used in a wide range of occupations, the MPG2000XB

being one such dosimeter (International Atomic Energy Association, 2007). All of

the active dosimeters used in this thesis are MGP2000 or MGP3000 (the successor

to the MGP2000) models.

1 Introduction 10

There have been some papers reporting direct comparison of active and passive

dosimeters in specific workplaces. In nuclear power production Singh et al. (2013)

found good agreement between Saphydose APDs and TLDs when comparing 29000

results during both normal reactor operation, and during refuelling outages. In

the same paper Singh also outlined controlled experiments which showed good

agreement in results from APDs and TLDs when exposed to known doses from a

Cs-137 source. Other experiments comparing multiple types of APD to TLDs in

controlled conditions (Boziari et al., 2011) have produced less convincing results. The

main conclusion of the Boziari paper was to underline the importance of choosing

the correct APD for your work practices, and understanding any limitations it may

have. No comparisons were found in the literature of passive and active dosimeters

for workers exposed to PET isotopes or other positron emitters. Hence, there is

a need to assess the suitability of using active dosimeters in this type of radiation

exposure environment if we are to consider removing passive dosimeters.

1.3 The Accuracy of Personal Radiation Dosime-

ters

Assumptions are made when using a personal dosimeter to assess the exposure of

a worker. The dosimeter only occupies a small volume in space compared to the

worker, and radiation fields are often inhomogeneous across the worker due to the

effects of geometry, and the presence of shielding materials. It is also assumed

that the dosimeter is worn whenever the worker is occupationally exposed, and not

exposed when the worker is not. Passive dosimetry services rely on the dosimeter

being returned on time, with the appropriate control badge, such that background

radiation levels are subtracted appropriately. If this is not the case an estimate of

the background dose will be subtracted introducing greater error into the results.

The reader will appreciate that where large numbers of people are required to keep

track of small objects over long periods of time some of those objects will become

misplaced either temporarily or permanently. The loss of, and damage to dosimeters

leaves gaps in the data which can generate significant error in the estimation of

personal exposure, significant exposure events could be completely missed from the

record.

The range of results provided by different service providers for dosimeters exposed

in controlled conditions has been evaluated (Böhm et al., 1994), and large differences

were found between providers, and from the expected values. Due to the large errors

1 Introduction 11

inherent in measuring small radiation doses using small dosimeters it should be

noted that results for personal dosimetry are more indicative of personal radiation

exposure than they are an accurate measure of it.

1.4 Active Dosimeters for Legal Assessment of

Occupational Dose

Significant improvements in radiation protection have been obtained through the use

of active personal dosimeters (Bolognese-Milsztajn et al., 2004). Their ability to give

instant feedback allows for radiation workers to adjust their technique while working,

and also give dose information for post work assessments and incident investigations

immediately.

There have been a number of suggestions that active dosimeters will replace

passive dosimeters as the legally accepted means of measuring and recording occu-

pational exposure (Ortega et al., 2001; Luszik-Bhadra et al., 2007). The argument

in favour of APDs is that occupational doses will be reduced through the effect of

instant feedback. With such feedback, workers are more aware of the dose rates they

are exposed to and can adjust their work practices to avoid their highest levels of

exposure. Lower levels of exposure can be reported when using electronic monitors,

as they have a much lower minimum detectable level. An active monitor will display

a dose of a single micro-sievert where passive dosimeters can only report doses above

10, 50 or 100μSv depending on the type. In the event of malfunction or damage to

the active dosimeter, the loss of dose information is reduced, as the dosimeter can

be readily replaced soon after the event. In comparison, a problem with a passive

monitor may not be detected until it is sent for reading at the end of the wear

period. Despite the advantages given, and significant improvements in performance

in the recent past, very few jurisdictions use active dosimeters for legal assessment

of occupational dose (Ginjaume, 2011). The main arguments given against replacing

passive dosimeters are that passive dosimeter’s have a long pedigree of reliable use;

have proven reliability in a wide range of radiation fields, are compact in size and

light weight, and are low cost (Ortega et al., 2001; Luszik-Bhadra et al., 2007).

Before the current working practice can be changed, regulatory bodies must be

convinced that active dosimeters are capable of providing a reliable record of worker

exposure, and any legal hurdles involving the nature of personal dosimetry services

must be overcome.

1 Introduction 12

1.5 PET Radiopharmaceutical Production

At Sir Charles Gairdner Hospital, PET radiopharmaceutical production occurs in

the Radiopharmaceutical Production and Development (RAPID) Laboratories.

PET radiopharmaceuticals have two parts, the positron emitting radioisotope

which can be detected by the PET scanner, and the molecule to which it is attached.

The molecule is chosen as it has a particular behaviour in the body of the patient,

which enables a biological function to be detected or evaluated (Ametamey et al.,

2008). Production of radiopharmaceuticals has two main parts, the production of the

required radioisotope, and the incorporation of the isotope into the pharmaceutical

molecule by a series of chemical reactions.

PET isotopes are produced by proton bombardment of a suitable target material

in a cyclotron. A cyclotron accelerates hydrogen ions using a powerful oscillating

magnetic field to produce a high energy (10-20 MeV) beam of protons. The beam

is incident upon a target containing atoms which absorb the protons and undergo

radioactive decay to form the desired positron emitting isotope. The most widely

used PET isotope is Fluorine-18 (18F) which is produced by proton bombardment

of water enriched with Oxygen-18 (18O) as shown in equation 1.1.

188 O +1

1 p →189 F +1

0 n + ν (1.1)

A number of other reactions can be used to produce 18F using different target

materials and particle beams. The 18O p,n reaction has proven the most cost effective

despite the expense of the target material, due to the relatively low beam energy

required, and the large yields that can be obtained (>100GBq) (Bailey et al., 2015).

In addition to the desired radionuclide, other isotopes can be produced. Protons

in the beam can be absorbed by atoms other than the target atoms and be transmuted

to radioactive species. Also components within the cyclotron can absorb the neutrons

produced in the p,n reaction shown in equation 1.1 and become radioactive. All

of the produced radioisotopes can potentially pose a radiation risk to staff working

with the cyclotron.

Once enough of the desired radioisotope has been produced, the target material

is transported to hot-cells, where the desired isotope is separated from other target

materials. In the case of 18F production, the water target is pumped through shielded

tubing from the cyclotron bunker into the hot-cell. For the production of routine

PET radiopharmaceuticals the chemical separation from the target material and

incorporation into the final molecule is a semi-automated process.

1 Introduction 13

In RAPID, prior to the arrival of the target material in the hot-cell, a kit con-

taining the chemical reagents and any disposable piping, filters, reaction chambers

and vessels are attached to a production system and checked by a radiochemist.

The production system transfers the target material, and the intermediate and final

products, through the various reaction chambers for appropriate amounts of time

and may provide heating to speed up chemical reactions where required (see figure

1.6). The final product is a small volume (~10ml) of very high specific activity18Fluorodioxyglucose (18FDG) in aqueous solution (IBA Molecular, 2010). The final

product is transferred to an automated dispensing unit in a separate hot-cell, which

splits and dilutes the product into multiple doses for delivery to customers, and for

quality assurance testing. The automated process is monitored by the radiochemist

to ensure that all the steps of production are progressing correctly. The radiochemist

is responsible for ensuring the final activities dispensed are suitable for the customers’

needs, and for performing the required quality assurance processes. The radiochemist

then removes the shielded product from the hot-cell and packages it for distribution

to the PET centres. A separate sample may be dispensed for individual doses which

must be drawn up by hand. Drawing up doses by hand from a large activity can

expose the radiochemist to a significant radiation dose, particularly to the hands.

Figure 1.6: Example diagram of an FDG synthesis system (IBA Synthera)

1 Introduction 14

1.6 PET Radiopharmaceutical Dispensing and Use

Once dispatched to the PET centre, the large bolus of 18FDG is transferred to

and loaded into an automated dose dispenser by a nurse or technologist. The dose

dispenser measures the activity of the bolus, which is compared to the expected

activity supplied by the RAPID radiochemist. The dose dispenser can then deliver

individual patient doses via intra-venous lines inserted by nursing staff. Through use

of the automated dose dispenser, staff can be some distance from the line delivering

the patient doses and protected by shielding material from the patient while they

are at their most radioactive. Other radiopharmaceuticals may be delivered as single

doses, in shielded syringes, which are hand injected into the patients. Due to the

need for handling of the dose, exposure of the staff is higher for hand doses.18FDG follows the same metabolic path as glucose, accumulating in cells with

higher metabolic function, such as cancer cells. Concentrations of 18FDG are detected

during the PET scan and can be used to diagnose and track cancer and other diseases.

In order to allow time for bio-distribution and to prevent accumulation in muscle

cells, patients rest between the injection and scanning, typically between 45 and 60

minutes. Immediately after injection the dose rate from the patient is of the order

of 0.092μSv/h/MBq at 1m (Madsen et al., 2006), this means a typical dose rate of

around 23μSv/h at 1m. To minimise radiation exposure to staff and other patients,

the PET patients rest in shielded bays monitored by CCTV. At the end of the rest

period they are escorted to the scanner, positioned and scanned. Scan times vary

depending on the volume of the patient being scanned. The scan rooms are shielded

to reduce dose to the technologists operating the scanner and people in surrounding

rooms, including those above and below the scan room. Due to the penetrating

nature of positron annihilation photons there is no monitoring window between the

control and scan rooms and patients are monitored on the scanner by use of CCTV.

Technologists attempt to minimise contact time with the patients, but it is often

unavoidable, particularly with patients with reduced mobility.

1.7 PET Centre workers

Hospital workers are one of the largest groups of occupationally exposed workers to

ionising radiation (Covens et al., 2007), and medical applications account for the

largest collective dose to radiation workers of any industry (Holmberg et al., 2010).

Positron Emission Tomography (PET) relies on the production of pairs of photons

from positron annihilation. Positrons are emitted in the radioactive decay of an

1 Introduction 15

isotope and rapidly annihilate on contact with an electron in the environment. Each

of the photons produced has at least the energy of half the rest mass of the positron-

electron pair, i.e. 511keV. Positron emitting isotopes may also have alternate decay

modes producing high energy gammas, for example 18F decays 3% of the time by

electron capture producing a 1.66 MeV gamma (Delacroix et al., 2002). Due to

the high energy, and therefore high penetrating power of annihilation and other

photons, the dose minimisation precautions required to protect staff working with

PET isotopes present a special challenge (Madsen et al., 2006), greatly increasing the

requirements for shielding materials compared with other medical imaging modalities.

The RAPID group, consisting of radiochemists and cyclotron engineers are re-

quired to use active personal dosimeters in Western Australia (Radiological Council

of WA, 2010). The cyclotron engineers are responsible for the maintenance of the cy-

clotron, the associated radioactive material transport systems and the hot-cells and

synthesis equipment used in the production of radiopharmaceuticals. They are often

exposed to the radioactive products produced by the cyclotron and synthesis process

and are also exposed to neutron activated components of the cyclotron during regular

maintenance, and when undertaking repairs and upgrades. The radiochemists are

responsible for producing radiopharmaceuticals containing the isotopes produced by

the cyclotron. Even with the automation of much of the chemistry, radiochemists

are still exposed when handling and transporting the shielded doses. Radiochemists

are also exposed when performing quality assurance tests which require handing of

samples. Radiochemists are regularly working with tens of GBq of activity and so

maintenance of good radiation hygiene is vital.

Nursing staff and imaging technicians in Nuclear Medicine are not legally required

to wear active dosimeters, but those who work with PET patients receive higher

occupational doses than others in the same department (Covens et al., 2007). The

dose rates involved are lower than in radiopharmaceutical production, but the staff

can be exposed for significant periods of time when they are in close proximity to

the patients. Even when not close to patients radiation shielding is not capable of

reducing the dose rate to zero, and so staff are exposed to above background levels

of radiation for most of the working day, adding to their cumulative exposure.

RAPID workers are accustomed to wearing APDs, whereas Nuclear Medicine

workers are not. This historical difference in usage offers an opportunity to gather

information on attitudes to the use of active dosimeters from the two groups to

investigate if familiarity affects their opinions. As both groups are routinely exposed

to doses measurable by passive personal dosimeters, they are an ideal population for

1 Introduction 16

a comparison study of the two dose measurement methods (passive and active) in

real workplace environments.

1.8 Aims

It is the intent of this thesis to explore whether the type of logging active personal

dosimetry system, currently used by the staff working in PET radiopharmaceutical

production at Sir Charles Gairdner Hospital, would be suitable as a legal dosimeter,

in place of the currently used passive dosimeters, for workers exposed to PET

radiopharmaceuticals and patients.

There are several criteria that would need to be met for the active dosimeters

to be suitable. They must be shown to have adequate detection capabilities for

the radiation emitted by PET radiopharmaceuticals, either as good as, or better

than the passive dosimeters currently used. They must be shown to be as reliable,

or more reliable than passive dosimeters. These two factors will be examined in

controlled exposure experiments. Dosimeters of different types will be exposed to

known quantities of 18FDG, and their results compared. The doses reported by

passive and active dosimeters worn by staff in RAPID and the PET centre will also

be compared.

Active dosimeters must also be accepted by the staff working with PET radio-

pharmaceuticals, and preferably offer benefit to and be appreciated by the staff. This

will be assessed by obtaining feedback from the staff in the form of a questionnaire

after a period of using both types of dosimeter.

Economic factors must be considered. If active dosimeters are to replace passive

dosimeters in areas where the use of active dosimeters is not mandated they must

be cost competitive with the use of passive dosimeters.

The final consideration is legal. Dosimeters must be approved for use by the

regulatory body of the state in which they are used. At present the active dosimetry

system under consideration in this thesis is not approved. The legal hurdles facing

approval of this types of dosimetry system will be explored.

Chapter 2

Experimental Methods &

Materials

This chapter outlines the materials and methods used to compare the performance of

the active and passive dosimeters. A series of controlled experiments were performed

to directly compare the results from passive and active dosimeters when exposed

to a known quantity of 18FDG. The methods used to compare the results from the

passive and active dosimeters worn by staff working with PET radioisotopes are also

discussed. The section ends with a discussion of the method used to obtain feedback

from staff regarding the use of passive and active dosimeters.

2.1 Radiation Source

All of the radiation sources used in the controlled experiments consisted of a 5ml

glass vial containing less than 1ml of 18FDG in aqueous solution. The activity of

the source at a specific point in time, close to the start of each experiment, was

obtained using a calibrated well counter. The activity of the sources used are given

in tables 2.3 and 2.4. The half-life of 18F is 1.83 hours and the gamma constant for

a glass vial at 1m is 0.158 μSvMBq-1(Delacroix et al., 2002). The gamma constant

is a value derived from computer models of a given isotope in a particular geometry.

The constant gives the dose rate per MBq at 1m. Other references (Madsen et al.,

2006)quote different values for the gamma constant. The value from Delacroix et al.

(2002) was chosen as it is based on the same geometry (a small glass vial) used in

the controlled experiments.

17


2.2 Passive Dosimeters

In the controlled experiments two types of passive dosimeter are used, TLD badges

(described in section 2.2.1) and OSL badges (described in section 2.2.2). The dosime-

ters are held vertically in a dosimeter holder clipped to a thin piece of ABS (a

common thermoplastic) slotted into holders on a track. The track and the layout

of the controlled experiment are described in section 2.4.2. Dosimeters used in the

experiments were returned directly after exposure along with an unexposed “control”

badge used to remove the contribution from background radiation.

All PET and RAPID workers wore TLDs (described in section 2.2.1) during the

study. Workers are required to wear the dosimeters on their torso whenever working

in an area where they may be exposed to ionising radiation. They are usually worn

clipped at waist or chest height. TLDs are supplied in batches for use over a one

month wear period (for RAPID and PET Centre staff) and then returned to the

supplier for reading.

Both of the passive dosimeter types used are approved as legal personal dosimeters

in Western Australia, and are in use at Sir Charles Gairdner Hospital. They have

been compared to assess the degree of agreement that could be expected between

approved dosimeter types. Because of time and financial constraints it was not

possible to compare all of the approved passive dosimeters with each other. These

two dosimeter types were chosen as they are readily available, in use in the hospital

where the experiments were undertaken, and utilise two different scintillant materials

and reading methodologies.

2.2.1 TLD

The Pansonic UD-802 TLD dosimeters used in the experiments are supplied by

Global Medical Solutions (GMS) in Sydney and analysed by the Radiation Detec-

tion Company in the U.S.. The TLD itself is sealed in a plastic case with a small thin

window which allows the passage of less penetrating radiation, enabling differentia-

tion between shallow (Hp0.07) and deep (Hp10) dose (see figure 2.1a). There are four

lithium borate chips in the dosimeter (figure 2.1b) with different amounts of filtra-

tion to improve photon energy discrimiation and tissue dose estimation (Radiation

Detection Company, 2015).


(a) External image of TLD showing thin window inupper right

(b) Central component of dosimetershowing 4 TLD chips

Figure 2.1: Radiation Detection Company TLD

A copy of a typical GMS dose report can be seen in Appendix B. The suppliers

claim their TLD badges have a reported minimum detection limit of 50μSv. If the

dose recorded by the dosimeter less the control badge dose is less than this limit the

result on the dose report shows as ’ND’.

2.2.2 OSL

The OSL dosimeters used in the experiments are manufactured and analysed by

Landauer. The crystal itself is sealed in a plastic case with a small thin window which

allows the passage of less penetrating radiation, enabling differentiation between

shallow (Hp0.07) and deep (Hp10) dose (figure 2.2a). The casing holds the integrated

filtration and a thin strip of Al2O3:C (figure 2.2b). A copy of a typical Landauer

(a) External image of OSL show-ing thin window left of centre

(b) Internals of OSL showing filters, hole and grid in the casing,and the Al2O3:C dosimeter

Figure 2.2: Landauer OSL

dose report can be seen in Appendix B. The suppliers claim their OSL badges have

a reported minimum detection limit of 10μSv. If the dose recorded by the dosimeter

minus the control badge dose is lower than this limit, the result on the dose report

shows as ’M’.


Figure 2.3: Active dosimeters DMC2000S, DMC2000X, DMC2000XB and DMC3000

2.3 Active Dosimetry System

2.3.1 DMC 2000 and DMC 3000

The active dosimeters used by staff and in the controlled experiments were DMC

2000 and DMC 3000 personal dosimeters, manufactured and supplied by MGP

Instruments. Four models of dosimeter were used in the controlled experiments

(see table 2.1 for descriptions) as they are used interchangeably by staff in RAPID.

Staff in Nuclear Medicine prefer the 2000 models as they are physically smaller. In

the controlled experiments the dosimeters were selected at random depending on

availability, reflecting the way they are used by staff. All four models were used. This

decision was validated by the small variability in results between active dosimeters,

shown in figure 3.1 and table 3.6. When in use a small screen displays the dose

accumulated since log in and the dosimeter ’chirps’ when radiation is detected. The

rate of the ’chirps’ provides feedback to the user on the dose rate they are currently

exposed to.


Model Measures dose from

DMC 2000 S Gamma onlyDMC 2000 X Gamma & x-rays

DMC 2000 XB Gamma, x-rays & betasDMC 3000 Gamma only (new model)

Table 2.1: Active Dosimeter Models Used

The X and XB models of the MGP contain a thin window to allow calculation

of shallow (Hp0.07) as well as deep (Hp10) dose. In PET the radiation risk comes

from penetrating gammas rather than low energy x-rays or betas, so only the deep

(Hp10) dose is of concern. The dose measurements compared in this study are the

deep (Hp10) doses from the active and passive dosimeters, all of the models of APD

measure deep dose, so they can all be used. Different dosimeters were used in each

controlled experiment as this reflects the way the dosimeters are used by staff. The

IDs of dosimeters used in the controlled experiments were recorded so that any

differences in results between the active dosimeters could be investigated if any were

found.

2.3.2 Logging Station & Database

A dosimeter logging station allows radiation workers to assign a dosimeter to them-

selves before beginning their shift, and log the dosimeter out at the end. At log

out the logging station receives the dose information, displays it to the worker, and

transmits the data to a database for storage. The stations consist of networked

touch screen PCs running the LDM MGR software with a USB dongle or cradle

which enables wireless communication with the MGP dosimeters.

At the start of their shift workers place the dosimeter in the cradle, and enter

their unique identifier code on the touchscreen. The logging station assigns the

dosimeter to that worker, setting the appropriate dose and dose rate alarm levels.

The alarm levels are chosen such that they will not alarm during routine procedures,

only in the event of an unexpectedly high dose or dose rate. As different dose rates

are experienced by different occupations the levels are assigned by occupation.


Figure 2.4: Logging Station with dosimeter in cradle

Dose (μSv) Dose Rate (mSv/h)

Visitor 20 0.1Nurse/Technologist 100 10

Radiochemist 80 10Engineer 200 12

Table 2.2: Alarm Settings on Active Dosimeters

At the end of their shift the worker returns the dosimeter to the cradle, and the

dose data is read from the dosimeter and saved to the database. The total dose

and maximum dose rate per day for the last few days logged in are displayed on

the screen, allowing the worker to quickly review their dose, and check for unusual

readings.

The logging stations read and write data from a relational database running on

a networked PC. This PC also runs the Dosicare software which is used as a front

end to the database. The Dosicare software can be used to add new users, create

or modify user group profiles (with associated dose and dose rate alarm levels), and

generate dose reports.


Figure 2.5: Logging Station showing dose results at log out

2.3.3 Software

The MGP active dosimetery system requires a number of software components to

function. LGM MGR is run on the logging stations, it handles logging dosimeters in

and out of the system. Dosicare is used to modify user and dosimeter information

in the database and to configure and produce reports. It can also be configured to

send email alerts if recorded doses exceed threshold values. All of the MGP software

communicates with a Microsoft SQL Server Express installation which holds the

database of user, dosimeter, and dose information.


2.4 Controlled performance comparison of passive

and active dosimeters

In order to judge the suitability of active dosimeters as a legal dosimeter in PET

applications, one of the first considerations is their ability to accurately record

radiation dose when exposed to radiation from a PET isotope. There are a number

of different providers of passive dosimeters that are approved for use in Western

Australia (Radiological Council of WA, 2010). The methods below explain how a

comparison was made between the results from active dosimeters and two types of

approved passive dosimeters. It is reasoned that if the difference in results between

the active and passive dosimeters is less than, or similar to, the difference between

the results from the two types of passive dosimeters, then the active dosimeters

can be said to be equivalent in terms of accuracy of measurement. The dosimeters

used were chosen because the passive dosimeters are both approved for use as legal

personal dosimeters in Western Australia; the active dosimetry system has been in

use for a number of years in the hospital and is capable of automatically producing

a record of each individual’s exposure.

A controlled experiment was undertaken in order to remove the variations which

occur from person to person and day to day and concentrate simply on the ability of

the dosimeters to measure radiation doses from a PET radiopharmaceutical across the

range of doses typically received by radiation workers in PET radiopharmaceutical

production and use. The controlled experiments consisted of a number of planned

simultaneous exposures of dosimeters to the radiation from a vial containing18FDG

the most commonly used PET radiopharmaceutical.

2.4.1 Radiation Safety

The experiments require the use of an unshielded source of penetrating ionising

radiation (511keV gammas). It is essential to ensure that the experiment is conducted

in a controlled area and that dose rates in any surrounding uncontrolled areas do

not pose a risk to anyone. The controlled area has restricted access with signage

and a physical barrier at the single access point during the period of the exposure.

Dose rate measurements were taken in accessible areas around the controlled area

to ensure that dose rates were below 25μSv/h. All areas surrounding the controlled

area were low occupancy areas (corridors and stairwells). The limited number of

exposures carried out also reduced the risk of cumulative doses to staff from the

experiments.


Lead shielding was placed such that the source could be approached from the

entrance to the controlled area with minimal exposure to the experimenter (Figure

2.6).

2.4.2 Physical layout of experiment

At least three of all three types of dosimeter were arranged simultaneously at a fixed

distance of 1m from the vial of FDG in an arc by means of positioning them on

stands on a circular rail (Figure 2.6) for each exposure. The circular nature of the

apparatus and the central source ensures that the dose to each dosimeter was equal

including the possible effect of scatter from the floor.

Figure 2.6: Passive dosimeters arranged on 1m radius rail

The different types of dosimeter (figure 2.7) were placed alternately around the

arc of the rail. The spread of each dosimeter type around the rig removes any

possibility of systematic bias between dosimeter types caused by geometry.


Figure 2.7: Personal dosimeters on holders.

From left to right: an OSL passive dosimeter, an active MGP DMC2000X dosimeterand a TLD passive dosimeter.

The dosimeter holders contain slots which allowed them to be positioned on

the rail, and to hold the dosimeters parallel with the rail or at and angle of 30° or

60° from the rail (figure 2.8). These slots allowed for accurate positioning of the

dosimeters when comparing the effect of angling the dosimeters relative to the source

of radiation. In a real world application, a source will not always be perpendicular

to a detector, so a dosimeter should still give a reasonable measurement when angled

away from the source. 30° and 60° were chosen to match the methodology used

by the IAEA when comparing multiple types of active dosimeter, including one of

the models of MGP dosimeter used here (International Atomic Energy Association,

2007). In each experiment at least three dosimeters of each type were irradiated

simultaneously. All of the dosimeters in each experiment were at either 0, 30 or

60 degrees from perpendicular to the radiation source. All dosimeters irradiated

simultaneously were at the same angle to the source. Simultaneous irradiation at

the same distance from the same source allows for direct comparison of the results

from the dosimeters in each experiment. The methodology for comparing between

exposures is explained in section 2.4.6.


Figure 2.8: Dosimeter holders on the rail at 0, 30 and 60 degrees

As the active dosimeters contain metal components it was thought that there may

be a small amount of scatter from the active dosimeters. To remove the possibility of

scatter affecting adjacent dosimeters the dosimeters were placed alternately by type

on the holders with a minimum of 10cm space between each one. A minimum of

three of each type of dosimeter were placed on holders around the ring to negate any

unknown geometric effect when comparing between dosimeter types. An example

layout showing dosimeter placement is shown in figure 2.9.


Figure 2.9: Example plan layout of experimental setup

Figure not to scale

To ensure that there was no geometric bias between the positions around the rig

the mean of the reported dose at positions A, B and C in each exposure for each

dosimeter type was calculated. The percentage difference from these means for each

dosimeter type was calculated for each usable result. The average of these percentage

differences for positions A, B and C were then calculated across all dosimeter types.

The results are shown in table 3.3.

2.4.3 Conducting an Exposure

With the dosimeters in place, a shielded vial containing a known activity of 18FDG

was placed in the centre of the apparatus and the shielding removed. The activity


and exposure time can be chosen to deliver a given exposure to the dosimeters using

DT =T1/2ΓA0

ln2

(

1 − e−T ln2

T1/2

)

(2.1)

Where DT is the predicted effective dose to the dosimeter, T1/2 is the half-life of

the isotope, Γ is the gamma constant for the isotope, A0 is the activity of the source

at the start of the exposure and T is the length of the exposure. This equation

accounts for the decay of the isotope during the exposure which is significant for

PET isotopes as they typically have short half-lives. The values for T1/2 and Γ for

the sources used were given in section 2.1.

2.4.4 Exposures Performed

Personal dosimeters must be shown to work accurately over the range of exposure

levels and dose rates experienced by staff, thus a series of exposures were carried out

with different activities of 18FDG. In order to investigate the angular dependence

of the dosimeters exposures were repeated with the dosimeters rotated through

30 and 60 degrees following the methodology of the IAEA (International Atomic

Energy Association 2007). Initial results showed no change in dose rate related

to position around the experimental setup. This meant that there was no need to

repeat exposures with varied positions around the rail.

Tables 2.3 and 2.4 show the exposures performed on dosimeters giving an esti-

mation of the dose to the dosimeters based on equation 2.1. The low doses were

intended to investigate the claimed minimum detectable level for the passive dosime-

ters, in addition to comparing their results with the active dosimeters. The largest

doses were delivered using three separate exposures of the same passive dosimeters

in order to keep the dose rate outside the controlled area to a reasonable level. A

larger number of active dosimeters were used when they were available to increase

the number of points of comparison produced by each exposure.

Some of the OSL dosimeters returned lower than expected results requiring a

repeat of some of the exposures.


Initial Source Exposure Degrees Calculated Number ofExperiment Activity Duration from Dose Dosimeters

(MBq) (hours) perpendicular (μSv) OSLs TLDs MGPs

1 101 2.60 0 25 3 3 32 254 1.00 30 31 3 3 33 253 1.25 60 37 3 3 34 698 1.50 0 117 3 3 35 693 1.48 30 100 3 3 36 708 1.22 60 102 3 3 57 1101 2.55 0 246 3 3 48 1005 2.88 30 261 3 3 49 1010 3.10 60 268 3 3 4

10a 1011 2.800 762 3 3

410b 1009 2.95 410c 1043 2.50 4

Table 2.3: Exposures performed in initial controlled experiments

Initial Source Exposure Degrees Calculated Number ofExperiment Activity Duration from Dose Dosimeters

(MBq) (hours) perpendicular (μSv) OSLs TLDs MGPs

11a 986 2.870 753 3 3

411b 1025 2.67 411c 1012 2.75 4

Table 2.4: Exposures performed in repeated controlled experiments

2.4.5 Obtaining Results

In the controlled experiments dosimeters were logged in using guest log-in codes and

the doses recorded manually at log out for each dosimeter after each experiment .

The passive badges were returned to the companies that supplied them within a few

days of exposure to be read and reported on. Dose reports were returned within a

few months and the results compared to those from the active monitors. The result

from each dosimeter was compared to the results from each of an alternate dosimeter

type from the same exposure. Each comparison provides a point on the Bland-

Altman plots shown in the Results section, starting on page 37. It was expected

that the results from the same dosimeter type in each experiment would be similar.

Comparisons of results from the same dosimeter type in each experiment were made

using the same methodology as the inter-type comparisons. The result from each

dosimeter was compared to the results from the others in the same experiment. Each

result compared had the dosimeter exposed to the same source, for the same amount


of time, at the same angle to the source; the only difference being their location

around the experimental rail. Any significant differences between results from the

same dosimeter type would reveal problems either with the dosimeters themselves,

or with the experimental set up. The results from the intra-type comparisons can be

seen in section 3.2.2. There were some failures of passive dosimeters, these failures

are discussed in chapter 3. Some results were excluded from comparison as they

were obviously in error and would skew the results. Dosimeters which reported the

dose as “below the detectable limit” when the other results for the same exposure

were greater than 20% over the reported detection limit were excluded. The number

and type of excluded dosimeter results are recorded in table 3.2 on page 38.

2.4.6 Normalising results from separate exposures

It was not possible to expose multiple dosimeters of each type at all three angular

positions simultaneously. As shown in table 2.3, the experiments with the dosimeters

0°, 30° and 60° from perpendicular to the source were separate. Each experiment

involved different activities of 18FDG and different lengths of time and thus the

measurements from the separate experiments cannot be directly compared. The

theoretical dose to the dosimeters (see equation 2.1) takes into account the activity

and exposure time for each experiment. The theoretical dose can therefore be used

to normalise the results for comparison of results from separate experiments.

The scaled result from the 30° experiment (Ds30) is calculated from the actual

result from the 30° experiment (D30) scaled by the ratio of the theoretical dose in

the 0° experiment to the theoretical dose in the angled experiment DT 0

DT 30

.

Ds30 = D30DT 0

DT 30

(2.2)

The results of equation 2.2 can now be compared to the results from the 0°

experiment in a Bland-Altman plot. Plots have been generated comparing the

results at 0° with 30° and 0° with 60° for each dosimeter type. The results can be

seen in section 3.2.4.

2.4.7 Displaying Results

The majority of results in Chapter 3 are displayed as plots. One type of plot shows a

direct comparison of results from two dosimeter types, with the dose result from each

dosimeter type forming the axes of the plot. The direct comparison plots include a

least-squares fit to the data. Bland-Altman plots are also used extensively.


The direct comparison plots have the reported dose from one dosimeter type on

the x axis, and the results from the other on the y axis. If the two sets of results

agreed perfectly the least squares linear fit to the points on the graph would be the

line of x = y with a correlation coefficient (r2) of one. The further the linear fit is

from x = y, and the lower the correlation coefficient, the worse the agreement is

between the dosimeter types.

Bland-Altman plots are used to analyse the agreement between two different

measurement methods of the same variable (Bland and Altman, 1999). Each point

in a plot is a comparison between two measurement methods of the same variable,

in the case of these experiments, radiation dose. The point’s position along the

x axis is the average of the results of the two measurements, and the position on

the y axis is the difference between the results. If the two measurement methods

were perfectly correlated, all the points would lie along the line of y=0. A constant

systematic bias can be seen as a shifting of the points up or down away from y=0. If

the points in a Bland-Altman plot slope away from the x-axis, this indicates a need

for a multiplicative correction factor to correlate the results.

In the controlled experiments multiple dosimeters of each type were used. As they

were all exposed to the same radiation they can all be said to be making the same

measurement. This provides multiple points of comparison for each result and there-

fore multiple points in each Bland-Altman plot. As an example, a single exposure of

3 dosimeters of type A, 3 dosimeters of type B, and 3 dosimeters of type C (as shown

in figure 2.9), produce 27 comparison points (A1-B1,A1-B2,A1-B3,A2-B1,A2-B2,A2-

B3,A3-B1,A3-B2,A3-B3, A1-C1,A1-C2,A1-C3,A2-C1,A2-C2,A2-C3,A3-C1,A3-C2,A3-C3,

B1-C1,B1-C2,B1-C3,B2-C1,B2-C2,B2-C3,B3-C1,B3-C2,B3-C3) which would be shown

as 9 points in each of three plots, one comparing A to B, one comparing A to C,

and one comparing B to C. The equivalence of positions 1 to 3 were evaluated by

calculating the difference between the mean result for all three positions with the

result from each position for each dosimeter type.

The Bland-Altman plots in Chapter 3 include lines showing the mean difference

between the results, which demonstrates any systematic bias between the measure-

ment methods. An indication of the spread of results is shown by the ±1.96σv (95%

confidence interval) lines.

In addition to the detailed comparison plots, tables summarising the results and

plots displaying the aggregated data can be found in section 3.2.8.


2.4.8 Statistical Assessment of Difference of Means

For each pair of dosimeter types (active-TLD, active-OSL and TLD-OSL) a test of

the difference of the means was performed. The results of experiments at similar

exposure levels were grouped by normalising the results from the 30° and 60° to the

perpendicular results as per section 2.4.6. The mean and standard deviation of each

set of results was calculated, and from these the standard error of the difference of

the means and the degrees of freedom. As the standard deviations for each set of

results were not similar, equations 2.3 and 2.4 were used.

Standard Error:

SE[X̄A − X̄B] =

√

s2A

nA

+s2

B

nB

(2.3)

Degrees of freedom:

df =[

s2

A

nA+

s2

B

nB]2

[s4

A

n2

A(nA−1)+

s4

B

n2

B(nB−1)]

(2.4)

The test statistic T was calculated using equation 2.5 and compared to the t

distribution to judge agreement between the means.

T =(X̄A − X̄B)

SE[X̄A − X̄B](2.5)

The results of the tests are shown in section 3.2.9.


2.5 Comparison of staff doses recorded by passive


2.5.1 Gathering RAPID Staff doses

Radiochemists and Engineers working in radiopharmaceutical production and devel-

opment (RAPID) at Sir Charles Gairdner Hospital have been wearing both passive

and active dosimeters since the inception of the service in 2003. In 2012 a logging

system was introduced which allowed automatic recording of the results from the

active dosimeters to a central SQL database. It is a requirement of the Radiological

Council that all staff and visitors entering the RAPID area must wear an active

dosimeter. All staff are also legally required to wear an approved passive dosimeter

whenever they are working with radioactive material, or are in an area where they

may be occupationally exposed to ionising radiation. As the staff are wearing both

dosimeters at all times, a comparison of the total monthly whole body exposure

measurements from the two dosimeter types was performed. Staff doses from the

active dosimetry system were extracted from the SQL database with a query which

summed doses over a month for each worker. The monthly doses were copied into a

spreadsheet for comparison with the doses reported by the passive monitors.

The number of staff has varied from year to year, and not all staff have worked

in RAPID every month in a year (table 2.5).

Type of Worker Number of staff (and months of data) in RAPID2012 2013 2014

Radiochemist 5 (53) 5 (59) 5 (59)Engineer 3 (36) 3 (35) 3(36)

Research Chemist 3 (29) 5 (48) 3 (34)Total 10 (118) 13 (142) 11 (129)

Table 2.5: RAPID Staff Numbers 2012-2014

2.5.2 Gathering PET Centre Staff doses

The PET Centre staff did not routinely use active dosimeters prior to commencement

of this project. The PET Centre is within the Nuclear Medicine department, and

the majority of nursing and medical imaging staff who have days working with PET

patients also work some days in Nuclear Medicine. These staff will receive some

exposure from non-PET patients. Due to a limited budget, there were not enough


dosimeters to track all the staff who work with PET patients while they work in

PET and other areas of the Nuclear Medicine department.

It was originally thought that the doses from PET patients would constitute

the majority of the exposure such that the results from the passive badges, and the

active badges should show correlation even if the active dosimeters were only worn

by the staff on the days they were working in PET. It was thought that including a

larger number of staff (table 2.6) would make for more useful data.

Profession Number of staff

Nurse 11Technologist 5

Table 2.6: Numbers of staff wearing active dosimeters when working in PET only(Nov 2012 to December 2013)

The correlation of active and passive dosimetry results from 2013 proved to be

very poor (see figure 3.18) with the passive dosimeters consistently reporting higher

doses. In 2014 specific staff members were asked to wear the active dosimeters

in PET and Nuclear Medicine such that they were always wearing passive and

active radiation monitors when working. The staff chosen were the ones who had

received the highest doses recorded by their passive dosimeters in 2013. Seven active

dosimeters were reserved for use by these workers (table 2.7). Other staff could use

any unused active dosimeters when they were available.

Profession Number of staff

Nurse 5Technologist 2

Table 2.7: Numbers of staff wearing active dosimeters while working with PET andNuclear Medicine patients (January to December 2014)

The doses recorded by the active dosimeters were extracted from the database

and compared with the results from the passive monitors, in the same manner as

the results from the RAPID staff.

2.5.3 Comparison of doses

As with the controlled experiments, the results from the passive and active dosimeters

were compared using least-squares fits to a linear relationship, and Bland-Altman

plots. Each point on both types of plot show the comparison of the results from the

active and passive dosimeters for a particular staff member for a single month.


As with the controlled experiments, if the dosimeters are equivalent the least-

squares fits should be close to y = x, and well correlated with the data. Good

correlation would be demonstrated by a correlation coefficient (r2) close to one.

In the Bland-Alman plots the x axis position of each point shows the average

of the results from the two measurement methods. The y axis position shows the

difference between the active and passive measurement results. For two sets of ideal

measurements all the data points would lie along the line y=0. The Bland-Altman

plots include horizontal lines showing the mean of the difference between the two

measurement methods, and the mean plus and minus 1.96 standard deviations (the

95% confidence interval).

In addition to the standard Bland-Altman plots the staff data is also compared

in Bland-Altman ratio plots. In these plots the y axis value is the result from the

active dosimeter divided by the passive dosimeter result. These ratio plots are useful

when the data in the standard Bland-Altman plot does not follow a horizontal trend.

If there is a systematic bias between two measurement methods, the mean ratio

of one result to the other will show this bias, and the spread of results show how

consistent this bias is. For two equivalent measurement methods the plot should

have points close to y = 1 across the range of values measured.

2.6 User Experience Survey

Staff in RAPID and Nuclear Medicine who have been using both types of personal

radiation monitor were asked to complete a survey (Appendix A). The intent of the

survey was to indicate the level of acceptance of the use of active personal dosimeters,

any preferences between the two types of dosimeter, and the extent to which the

staff felt that either provided a benefit to their radiation safety. The results of the

survey are shown in Section 3.4.

Chapter 3

Results

3.1 Results below the detection limit

As discussed in section 1.2.1 all passive dosimeters have a minimum detection limit.

Suppliers of passive dosimeters do not report doses below this limit, giving the result

as “below the detectable limit”. Some of the controlled experiments exposed the

dosimeters to doses close to the stated detection limit in order to investigate how

the different dosimeter types performed near the limit, and how accurate the stated

limits where.

Minimum detection limit (μSv)

TLD 50OSL 10

Table 3.1: Reported Minimum Detection Limits for Passive dosimeters as stated bysuppliers

In the controlled experiments all results which came back as “below the detectable

limit” are excluded from comparison with the other dosimeter results. In some cases

the results from passive dosimeters came back as “below the detectable limit” when

the results from the other dosimeters, and calculation of the expected dose indicated

the dosimeter was exposed to a dose in excess of the stated minimum detection limit

for the dosimeter type (see table 3.1). The number of results which could not be

used despite an apparent exposure above the stated minimum detection limit are

given in table 3.2.

37

3 Results 38

Excluded Unreliable Results Unreliable resultsresults* (as % of dosimeters of this type used) above 50μSv

TLD 1 1 (3%) 1 (3%)OSL 8 11 (33%) 6 (18%)

Table 3.2: Number of excluded dosimeter results in the controlled experiments

*Badges reporting results below the detection limit when active dosimeterreported doses greater than the stated detection limit + 20%

In the first high dose experiment (3 exposures totalling around 1mSv) the results

of the three OSLs were significantly lower than the results of the other dosimeters,

and the value expected by calculation. These values were not excluded and so are

not included in the first column of table 3.2, but are included in the “Unreliable

Results” columns of the table, taking the total to 11 (6 above 50μSv). The results

of this first high dose exposure can be seen in figure 3.9.

The high dose experiment (10 in table 2.3) was repeated, and the results of the

OSLs from the repeat experiment (11 in table 2.4) were used for all comparisons

shown after figure 3.10. The active dosimeter and TLD results from experiment 10

are still included in comparisons.

Table 3.2 does include three OSLs, used in experiment 9, which reported “below

the detectable limit” when the active dosimeters and TLDs gave results of the order

of 300μSv. The one TLD in table 3.2 reported “below the detectable limit” when the

other two TLDs from the same batch reported 100μSv and 130μSv from experiment

6. There were nine other TLDs that reported “below the detectable limit”. The

expected dose for these nine dosimeters was below or around 50μSv according to

the other dosimeters. These results are not included in the table as the dosimeters

were not expected to report doses at that level. There is no way to compare the

TLD results with those of the other dosimeters in the plots and tables of section 3.2

where the dose is close to or below the reported detection limit.

In the comparison of passive and active dosimeter results for staff there is no

way to know whether the active or passive result is more accurate as there is no

third measurement. In the comparisons of results from staff dosimeters the results

of “below the detectable limit” (BDL) were included as an estimate in the plots

but not in the calculation of the mean difference and the standard deviation. As

the stated minimum detection limit of the used TLDs is 50μSv a value of half the

detection limit (25μSv) was chosen as the estimate for all results returned as “below

the detectable limit” in the plots. To explore the effect of the BDL results, the mean

difference and standard deviations were calculated setting the BDL dose values to

3 Results 39

0μSv, 25μSv and 50μSv and these results are tabulated beneath the comparison

plots. The only months where data were excluded from the plots were those where

the active dosimeters had not been logged in, and the TLD results were “below the

detectable limit”.

3.2 Controlled performance comparison of passive


Comparisons of the performance of the dosimeters have been performed by producing

Bland-Altman plots for pairs of dosimeter types as explained in section 2.4.7. The

sections of this chapter demonstrate the variation in results between dosimeters of

the same type to show the repeatability of each dosimeter type. This is followed by a

comparison of results from each dosimeter type with the theoretical dose. The effect

of angulation of each dosimeter type is then explored. The inter-type comparisons

start with the dose results from the two types of passive dosimeter, then the reported

doses from each of the passive dosimeters are compared with the results from the

active dosimeters.

3.2.1 Comparison of doses around the experimental rig

The means of the differences between the results across all positions and the individ-

ual positions were less than 1.5% (table 3.3). This variation is much smaller than

variations in results found within and between dosimeter types in the subsequent

sections of this chapter.

Position A Position B Position C

Mean difference from-1.4% -0.3% 1.4%mean result across positions

for all dosimeter types

Table 3.3: Position dependence of results

3.2.2 Comparison of results from the same dosimeter type

Any repeated experiments, even with the same equipment will show a spread of

results, the smaller the spread the more repeatable a measurement method is said

to be. The following plots compare the results from the same dosimeter type in

each experiment with each other, comparing the spread of those results with the

3 Results 40

spread between dosimeter types. The direct comparison plots are of no use for these

comparisons as the slope of the plot would always be close to unity. As stated

by Bland and Altman (1999) we would not expect a bias between measurements

of the same type but the size of the standard deviation gives an indication of the

repeatability of each dosimeter type.

(a) Bland-Altman plot of OSL results (b) Bland-Altman plot of TLD results

(c) Bland-Altman plot of MGP results

Figure 3.1: Comparison plots for results from the same dosimeter type

For the OSL dosimeters the mean difference is 3μSv, and the 1.96σv value is 92μSv.

For the TLDs the mean difference is 9μSv, and the 1.96σv value is 109μSv, and for

the active dosimeters the mean difference is 4μSv, and the 1.96σv value is 41μSv.

As expected, the mean difference is very low for all three types of dosimeter. It

3 Results 41

is not statistically different from 0 for any of the dosimeters, according to a standard

one sample t-test with a 95% confidence interval. The active dosimeters show a

much smaller spread of results than the passive dosimeters, demonstrating better

repeatability. The OSL results are in close agreement below 500μSv, but the large

spread of results at around 1mSv has greatly increased the 1.96σv value, as shown in

figure 3.1a. The results from the TLDs gradually get further apart with increasing

dose, resulting in the spreading of points away from y=0 in figure 3.1b. The spread

of results for the MGPs is noticeably smaller, resulting in the lower 1.96σv value.

The agreements between the active dosimeters and each of the passive dosimeters

are similar to the level of agreement within the passive dosimeters. These results

again suggests that the active dosimeters are at least as capable of providing accurate

measurements of radiation dose from 18FDG as the passive dosimeters.

3.2.3 Comparison of dosimeter results with theoretical dose

As shown in section 2.4.3; with a known activity at a fixed distance for a known

period of time it is simple to calculate a theoretical estimate of the dose expected to

be received by the dosimeters using the gamma constant for 18F. This calculation is

not expected to be accurate as it assumes a geometry which is only an estimate, and

that no radiation scattered from other materials reach the detector. As the source

and the dosimeters are close to a concrete floor there is expected to be a scatter

contribution. This contribution should be uniform across the dosimeters due to the

design of the experimental rig.

As with the comparisons between different dosimeter types, direct comparison

plots and Bland-Altman plots have been produced. The plots compare the results

from the three dosimeter types to the calculated theoretical dose.

The active dosimeter results show an excellent fit to a linear model (figure

3.2a) but the dosimeter results exceed the theory by 32%. The sets of points form

vertical lines as all the dosimeter results are compared to the same theoretical result

for each experiment. There were two experiments performed with the dosimeters

perpendicular to the source at the high dose level hence the two lines of data.

The Bland-Altman plot (figure 3.2b) shows the mean difference is -60μSv, and the

1.96σv value is 135μSv. The increasing difference in this plot echoes the substantial

difference between the slope of the previous plot and the ideal agreement of x = y.

3 Results 42

(a) Direct comparison (b) Bland-Altman plot

Figure 3.2: Comparison of Active dosimeter results with theoretical dose


Figure 3.3: Comparison of OSL dosimeter results with theoretical dose

For the OSL dosimeters there is also a good agreement with a linear model (figure

3.3a) but an even higher difference between the reported dose and the theoretical

dose (~50%). In this case the fit does not have an intercept very close to zero. The

higher slope and the negative intercept suggests that the reported high dose results

may be in excess of the actual dose as suggested by earlier comparisons between

dosimeter types. In the Bland-Altman plot (figure 3.3b) the mean difference is

3 Results 43

-50μSv, and the 1.96σv value is 159μSv.


Figure 3.4: Comparison of TLD results with theoretical dose

For the TLD results the direct comparison plot (figure 3.4a) shows good agreement

with a linear model but an underestimation of the dose by theory (or an over-

reporting by the TLDs) of 29%. The intercept is closer to zero than for the OSL

dosimeters but not as close as for the active dosimeters. In the Bland-Altman plot

(figure 3.4b) the mean difference is -85μSv, and the 1.96σv value is 172μSv.

For all of the theory comparisons the measured dose was significantly higher than

the theoretical dose. In all three cases the mean difference was significantly different

to 0, according to a standard one sample t-test with a 95% confidence interval. In

the direct comparison the slope of the results with increasing dose around 1.3 for

the MGPs and TLDs, and 1.5 for the OSLs, suggests either a significant under

measurement of dose at 511keV for all the dosimeters or a scatter component of the

dose rate of around 30%.

3.2.4 Effects of angling the dosimeters

In real life exposure situations the dosimeter is not always perfectly perpendicular

to the source of radiation. It is important that personal dosimeters can accurately

record dose from a range of angles. When testing dosimeters the IAEA exposes

dosimeters at angles of 30 and 60 degrees from perpendicular to judge their ability

to cope with angulation (International Atomic Energy Association, 2007). The

3 Results 44

results of a similar comparison are presented in this section for the three dosimeter

types assessed.

(a) Bland-Altman plot comparing 0° and 30° re-sults

(b) Bland-Altman plot comparing 0° and 60° re-sults

Figure 3.5: Plots showing the effect of angulation on MGP dosimeters

For the active dosimeters at 30° the mean difference is 3μSv, and the 1.96σv value

is 36μSv (figure 3.5a). At 60° the mean difference increases to 11μSv, and the 1.96σv

value is 44μSv (figure 3.5b). The measured values decrease slightly with angulation.

The difference between 0 and 30 degree results is not statistically significant but the

difference in the mean results between 0 and 60 degrees is statistically significant

according to a standard one sample t-test with a 95% tolerance interval.

For the OSL dosimeters at 30° the mean difference is -31μSv, and the 1.96σv

value is 56μSv (figure 3.6a). This is a larger drop and wider spread than for the

active dosimeters at 30° or even 60°. A batch of three OSL dosimeters failed to

report a dose when measuring the doses at 60°; this greatly reduced the data for

the plot (figure 3.5b), but the mean, and in fact all the results at 60°, are lower

than those at 0°, as expected. The difference between 0 and 30 degree and 0 and 60

degree is statistically significant according to a standard one sample t-test with a

95% tolerance interval.

3 Results 45



Figure 3.6: Plots showing the effect of angulation on OSL dosimeters



Figure 3.7: Plots showing the effect of angulation on TLDs

For the TLDs at 30° the mean difference is 17μSv, and the 1.96σv value is 73μSv

(figure 3.7a). A slightly larger drop, and a wider spread than for the active dosimeters

at 60°. At 60° the mean difference is 55μSv, and the 1.96σv value is 72μSv (figure 3.7b).

The difference between 0 and 30 degree results is not statistically significant but the

difference in the mean results between 0 and 60 degrees is statistically significant

3 Results 46

according to a standard one sample t-test with a 95% tolerance interval.

Mean Difference from 0° (μSv)30° 60°

MGP -3 -11

OSL -31 -53

TLD -17 -55

Table 3.4: Summary of changes in dose readings when angling dosimeters

Figure 3.8: Plot of Normalised Mean Results against angle

Figure 3.8 shows the normalised, mean reported dose for each dosimeter type at

zero, thirty and sixty degrees. Lines of best fit for each type at the two exposure

levels were calculated and included in the plot. Overall there is a slight reduction in

measured dose when angling the dosimeters away from perpendicular to the source

of radiation in all the dosimeters. This is shown by the negative value of the mean

difference in all the Bland-Altman plots and the values in table 3.4. As one might

expect, the effect is stronger overall in all three cases with increasing angle. This

can been seen in the downward slope of the lines of best fit in figure 3.8. The active

dosimeters appear less effected by the angle of incidence than the passive dosimeters.

The results suggest that the active dosimeters would be at least as suitable for

personal dose measurement as the approved passive dosimeters in this regard.

3 Results 47

3.2.5 Comparison of passive dosimeters

The plots in sections 3.2.5, 3.2.6 and 3.2.7 include results at 0, 30 and 60 degrees

from perpendicular to the source. All comparisons are like for like, only dosimeters

from the same exposure, and therefore at the same angle, are compared with each

other. The points are plotted in separate series for the 0, 30 and 60 degree exposures

in all of the plots.

(a) Direct comparison of dose results (b) Bland-Altman Plot

Figure 3.9: Initial comparison of passive dosimeter results

It is obvious in figure 3.9 that there is a problem with the results from the OSL

dosimeters in the high dose experiment. The source of this error is unknown. No

fit was made to the data in figure 3.9a as it is clearly not a linear relationship. All

of the data in subsequent plots for the controlled experiment show good agreement

with a linear relationship fitted to the data as would be expected.

The points in the plot form groups of rectangles because the points all come

from comparing each result from one dosimeter type to all the results from the other

dosimeter type in the same experiment. The width of each rectangle shows the

spread of results for the dosimeter type on the x axis; the height shows the spread

of results for the dosimeter on the y axis in each experiment. A similar effect can be

seen in the Bland-Altman plots that follow, the points from each set of comparisons

form parallelograms. The y value of point of the parallelogram furthest from y=0

shows the maximum difference between the two dosimeter types being compared

in that experiment, the y value of the point closest to the x axis (y=0) shows the

smallest difference between the dosimeter types.

3 Results 48

There are more points in the plot than can actually be seen, as the results from

the passive dosimeters are often the same. They coincide as results from passive

dosimetry services are given to the nearest 10μSv. Coincidence of data is much

less likely for the active dosimeters, as the results are given to the nearest 1μSv.

In the plots with linear fits to the data, all points are used in the least squares fit

calculation, independent of whether they can be seen visually in the plot.

The reported doses for the OSLs exposed to the highest dose were considerably

lower than expected from theory, or recorded by the TLDs or active dosimeters. Due

to this discrepancy the exposures were repeated with new TLDs and OSLs in order

to see if this was a flaw in the reporting of a single batch or a consistent problem with

the OSLs. The repeated experiment gave the results in figure 3.10. In the repeated

experiment, the results of the OSLs where much more in line with the results from

the other dosimeter types. It was concluded that there was an error with the batch

of OSLs used in the first set of exposures, or the reading of them. From this point

all comparisons with OSL results in controlled experiments include the results from

this second experiment rather than the first set of exposures.

(a) Direct comparison with least squares fit (b) Bland-Altman plot

Figure 3.10: Comparison of OSL and TLD results after repeat exposures

The results from the repeated experiment where fitted to a linear f(x) = ax + b

relationship using the least squares method. The results show a good agreement

with a linear relationship with an r2 value of 0.96. However the relationship is far

from the expected y = x with a slope a = 0.75 and an intercept b = 79. The results

from the OSLs at the higher exposure level (~1mSv) are higher than those of the

3 Results 49

TLD but agree well in the lower dose experiments. The high values for the OSL

doses cause the decrease in the slope of the fit in figure 3.10a and the high points

to the right of plot 3.10b. The results of other comparisons later in this chapter

support the conclusion that the reported OSL dose in this experiment were higher

than the actual value of a little under 1mSv for this exposure.

The spread of results between dosimeters increases with increasing dose. This

effect can be seen on all the subsequent plots. This could be explained by a systematic

bias on each individual dosimeter which becomes more evident with increasing dose.

The spread of results is greater for the OSL dosimeters at the highest dose exposure

but less than that of the TLDs at the lower dose levels. The spread of results

also appears to increase when turning the dosimeters from 0 to 30° and 60° from

perpendicular to the source.

The Bland-Altman plot including the results of the repeated high dose experiment

(figure 3.10b) shows the mean difference of -21μSv, and just two data points (of 51)

outside the 1.96σv of 106μSv . At the low and medium dose levels the OSL results

are below those of the TLD resulting in a negative mean difference. For the high

dose results however 5 of the 6 points are above the mean and 4 of the points are

positive.

The higher OSL results at the high dose level are obvious in the Bland-Altman

plot, as the mean difference is below zero but five of the six points from the high

exposure experiment are above the mean.

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3.2.6 Comparison of active dosimeters with OSL dosimeters

(a) Direct comparison of OSL and MGP results (b) Band-Altman plot

Figure 3.11: Comparison of OSL and MGP results

The agreement between the active dosimeters and the OSL dosimeters appears to

be better than that between the two types of passive dosimeters, with a = 1.18,

b = −49 and an r2 of 0.99. The OSL dosimeters report a higher dose than the

active dosimeters around 1mSv, excluding those data points would give a fit closer

to y = x.

The mean difference between active and passive dosimeters is just 3μSv with a

1.96σv of 125μSv. Six data points outside 1.96σv in a sample size of 84 is unexpected

and all occur at the highest dose level. The active and passive dosimeters show good

agreement in the low and medium dose exposures but an obvious difference at 1mSv

(over 200μSv difference in one case). The results from the OSLs are mostly lower

than the results from the active dosimeters at lower doses. At these lower doses

(<0.5mSv) there is some overlap in the reported doses from the two dosimeter types.

This can be seen in the points clustered around y=0 in figure 3.11b. The results

from the OSLs are all higher than the results from the active dosimeters in the high

dose experiment, this is obvious from the points on the right of figure 3.11b, which

are all below the line of y=0.

3 Results 51

3.2.7 Comparison of active dosimeters with Thermolumines-

cent dosimeters


Figure 3.12: Comparison of TLD and MGP results

The agreement between the active dosimeter and TLD results seems excellent, with

the slope of the linear fit to the data very close to unity, and an intercept near zero.

There are two sets of high dose comparisons because usable data comparing TLDs

and the active dosimeters was obtained in both the initial and repeated high dose

experiments.

The Bland-Altman plot also shows excellent agreement between TLDs and MGPs

in the controlled experiments. The mean difference is 0μSv, and the 1.96σv value is

105μSv. There is a spreading of the results as dose increases but only 3 points outside

the ±1.96σv values (one at ~350μSv and two at ~900μSv) which is to be expected for

a sample of 92 points. The agreement between the TDLs and the active dosimeters

is the best agreement between the three dosimeter types.

3 Results 52

3.2.8 Summary of inter and intra-dosimeter type compar-

isons in controlled experiments

Slope of Fit Intercept Coefficient of Determination (r2)

OSL-TLD 0.75 79 0.96MGP-OSL 1.18 -49 0.99MGP-TLD 0.99 1 0.97

Table 3.5: Summary of linear fits to comparisons of dosimeter results

On average the TLD and active dosimeters show the best agreement in a direct

comparison, with a slope of close to one, and an intercept close to zero. All of the

data sets show good agreement with a linear fit.

Mean difference 1.96 Standard Deviations Points of comparison(μSv) from the mean (μSv)

OSL -3 92 22TLD 9 109 21MGP -4 42 64

OSL-TLD -21 106 51MGP-OSL -3 125 84MGP-TLD 0 105 92

Table 3.6: Mean difference and 1.96σv values for dosimeter comparisons

Standard one sample t-tests show that of all these comparisons, the only sta-

tistically significant mean difference is the one between the two types of passive

dosimeter (OSL-TLD in table 3.6). The differences in mean dose results between

active and passive dosimeter results are not statistically significant for either passive

dosimeter type with a 95% confidence interval.

3 Results 53

(a) All Results (b) Results below 500μSv

Figure 3.13: Mean dose per Dosimeter Type vs Mean dose per exposure

The plots in figure 3.13 show the variation in each group of dosimeters and the

overall agreement of the dosimeters with each other. The figures plot the mean dose

reported by each dosimeter type, with bars showing the maximum and minimum

dose result reported, against the mean reported dose for all dosimeter types used

in an experiment. For the lowest doses reported the means agree exactly as they

are the same value, only the active dosimeters report any result at this level. In all

other cases there are clusters of vertically overlapping results as the maximum dose

for one dosimeter type overlaps the minimum dose for another. The closely grouped

vertical lines of results arise from the 0, 30 and 60 degree exposures at approximately

the same dose. The greatest differences between the groups of results arise at the

highest doses. The active dosimeter and TLD results for one high exposure overlap,

but the OSL results for that same exposure are very low, as reported earlier. The

low OSL results lower the overall mean, taking the TLD and active dosimeter results

above the line of x = y. The results for the repeated high dose experiment (at the

right of figure 3.13a) show the opposite result, with the OSL results higher than the

TLD and active dosimeter results, in this case though the TLD and active dosimeter

results do not agree as well, as the lowest active dosimeter result is higher than the

highest TLD result. In order to better display the lower dose results, figure 3.13b

shows the results below 500μSv.

3 Results 54

3.2.9 Statistical significance of agreement of means

T-tests were performed, as outlined in section 2.4.8, to test the significance of agree-

ment between the mean results of the different dosimeter types. Test were performed

for the low (~150μSv) and medium (~300μSv) dose experiments (experiments 4-6

and 7-9 in table 2.3). Dose results from the experiments with the dosimeters at 30°

and 60° were normalised using the method explained in section 2.4.6 and included

in the comparison in order to produce a useful number of results for each test.

Low Dose Low Dose Low Dose Med Dose Med Dose Med DoseMGP-OSL TLD-OSL MGP-TLD MGP-OSL TLD-OSL MGP-TLD

Difference20.6 12.1 8.53 4.57 26.3 21.8

of MeansStandard

9.53 12.0 8.96 9.33 18.7 18.5Error

Degrees of10.7 15.0 9.81 12.9 10.3 10.1

FreedomTest

2.17 1.01 0.95 0.49 1.41 1.18Statistic

Confidence(-0.33,41) (-13,38) (-11,29) (-16,25) (-15,68) (-63,20)

IntervalAgreement Yes Yes Yes Yes Yes Yes

Table 3.7: T-test Results (for p=0.05) for agreement of different dosimeter types

Table 3.7 shows no statistically significant difference in the mean results of the

two dosimeter types in each comparison, within the 95% confidence interval. The

confidence interval spans zero in all cases, but only just in the case of the low dose

comparison of active and OSL dosimeters.

3 Results 55

3.3 Comparison of staff doses recorded by passive


3.3.1 RAPID Staff doses

Staff in RAPID have been using the MGP active dosimeters together with the

logging stations since 2012. The following Bland-Altman plots show the level of

agreement between the active and passive dosimeters for 2012, 2013 and 2014. For

privacy reasons the staff are represented by numbers rather than by name. The use

of different symbols gives an indication of agreement between active and passive

dosimeters for different staff members. Each point is a comparison of the monthly

TLD result with the sum of the active dosimeter results over that month for the

same staff member.

In 2013 all staff using the MGP dosimeters were asked to wear the passive and

active dosimeters in the same location on the torso to minimise the differences

between the results. The survey results in table 3.17 show around 70% reported

compliance with this request at the time of the survey (August 2014).

3.3.1.1 2012

The first full year using the logging stations and database was 2012. One TLD result

has been excluded from these plots as it was found that the dosimeter was exposed

while it was not being worn, invalidating the result.

The r2 value of 0.49 indicates that the data dose not fit particularly well to

the least-squares linear fit. There are a large number of TLD results of “below

the detectable limit” (BDL) which is given by the manufacturer as 50μSv. These

results form the horizontal line in the bottom left of figure 3.14a extending out to

177μSv. The lowest reported TLD result is 100μSv. Excluding the comparisons

where the TLDs returned a value of “below the detectable limit” dose not improve

the correlation, r2 drops to 0.38 for the best fit shown with a dashed line. The slope

of the fit excluding the BDL results is closer to one but the intercept is much further

from zero.

In the Bland-Altman plot shown (figure 3.14b) the mean and standard deviation

values were calculated excluding the BDL results. As this excludes a large amount

of data (35 out of 117 points), calculations of the mean difference and the standard

deviation of the differences were calculated assuming three different values for the

unreported TLD results. A one sample t-test shows that the difference between the

3 Results 56

(a) Direct dose comparison (b) Bland-Altman Plot

(c) Bland-Altman Ratio Plot

Figure 3.14: Comparison of RAPID staff dose results for 2012

passive and active dosimeter results is statistically significant using a 95% confidence

interval.

As the stated minimum detection level is 50μSv, values of 0μSv, 25μSv and 50μSv

were used to cover the assumed range of potential values. The results of these

calculations are displayed in table 3.8.

Excluding the BDL results, the mean difference between results from the TLDs

and the summed active dosimeter results for each month was -190μSv with a 1.96σv

of 500μSv. The mean difference and spread of results is much larger than in the

controlled experiments. Picking a value between 0 and 50μSv for the unreported

3 Results 57

Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv

Mean Difference -190 -116 -124 -1311.96 x Std Deviation 500 477 467 458

Table 3.8: Bland-Altman Results for 2012 RAPID Doses

TLD results reduces both the mean difference and the spread of differences. The

best agreement comes from setting the BDL results to 0μSv.

The Bland-Altman ratio plot (figure 3.14c) shows that on average the active

dosimeter result is 0.60 of the equivalent TLD result. The spread is wide with a

1.96σv value of 0.49. In only two of the comparisons is the active dosimeter result

greater than the passive dosimeter result. The points form a better horizontal fit

than the standard Bland-Altman plot, with what seems to be a consistent spread

with increasing average reported dose. This plot does not include any BDL values.

A one sample t-test shows that the ratio between the passive and active dosimeter

results is significantly different to 1 using a 95% confidence interval.

3.3.1.2 2013

The least-squares fit is closer to x = y than for the 2012 comparison but the

correlation coefficient suggests that the fit is no better. Again the active dosimeter

results report doses far in excess of 50μSv for a number of TLD reports of “below

the detectable limit” and the lowest reported TLD result is 100μSv. In the Bland-

Altman plot the mean difference is -136μSv and the 1.96σv is 321, an improvement

on the 2012 results. This may be due to the request to wear dosimeters in the same

location reducing the variation in exposure to the two dosimeters. A one sample

t-test shows that the difference between the passive and active dosimeter results is

statistically significant using a 95% confidence interval.




As with the 2012 results, picking a value between 0 and 50μSv for the unreported

TLD results reduces both the mean difference and the spread of differences. The

effect on the spread of differences is not as great as in 2012 and again, the best

agreement comes from setting the BDL results to 0μSv (table 3.9).

3 Results 58





dosimeter result is 0.58 of the equivalent TLD result, this is similar to the 2012 result.

The spread is even wider with a 1.96σv value of 0.62. There are more instances of

the active dosimeter results being higher than the passive results, particularly some

results for staff members 7 and 4. Both staff members also have many comparisons

for the year where the TLD results are higher than the active dosimeter results,

illustrated by points below 1 on the y-axis. The spread of results seems to reduce

with increasing average reported dose, but the smaller number of points at higher

3 Results 59

doses make it difficult to draw conclusions. This plot does not include any BDL

values. A one sample t-test shows that the ratio between the passive and active

dosimeter results is significantly different to 1 using a 95% confidence interval.

3.3.1.3 2014




For the 2014 data the least-squares fit has a slope close to one and the correlation

coefficient is improved over 2012 and 2013. It can be seen in the plot that there are

a smaller number of points with very large differences between the TLD and active

3 Results 60

dosimetry results in the 2014 data, compared to previous years. The lowest reported

TLD dose is 120μSv.


Mean Difference (μSv) -90 -50 -57 -631.96 x Std Deviation 169 185 175 167


The spread of results has decreased year on year. In all three cases the results

from the active dosimeters are lower than those from the TLDs but are getting closer

each year. One sample t-tests show that the difference between the passive and

active dosimeter results is statistically significant using a 95% confidence interval for

all three years of results.

As with the previous results, picking a value between 0 and 50μSv for the unre-

ported TLD results reduces the mean difference, however for the 2014 results the

spread of differences is increased by setting the BDL values to zero or 25μSv. The

best agreement for the mean of the results again comes from setting the BDL results

to 0μSv (table 3.10).


dosimeter result is 0.56 of the equivalent TLD result, this is similar to the results

from the previous years. The spread is similar to 2012 with a 1.96σv value of 0.51.

The results do not seem distributed along the mean, showing that the ratio between

the active and passive results is not consistent. There is one very obvious outlier from

staff member 19 which is not easy to see in figures 3.16a and b as it is at a relatively

low dose level. This plot does not include any BDL values. A one sample t-test

shows that the ratio of the passive and active dosimeter results is again significantly

different to 1 using a 95% confidence interval.

3.3.1.4 The effect of reported wear position on correlation

It was thought that staff wearing the dosimeters in different locations on the torso

may be a contributing factor to the differences between the passive and active

dosimetry results. To assess this, comparisons (figure 3.17) were made of the results

only for the staff who, when surveyed, reported wearing both dosimeters in the same

location on their torso.

Excluding the staff who wear dosimeters in different locations does nothing to

improve the correlation. The linear fit has a similar slope and intercept to the

3 Results 61


Figure 3.17: RAPID staff dose results for 2014 for staff wearing the passive andactive dosimeters in the same position on the body.

inclusive plot (figure 3.16) and the correlation to that line is worse than for the

inclusive plot. In the Bland-Altman plot the mean difference is increased from

-90μSv to -102μSv and the 1.96σv value is decreased from 169μSv to 159μSv by

including only the results from dosimeters worn in the same body position. A one

sample t-test on this data shows that the difference between the passive and active

dosimeter results is still statistically significant using a 95% confidence interval.

3.3.2 PET Centre Staff doses

3.3.2.1 2013

Staff in the PET Centre started using the dosimeters in 2013. As discussed in 2.5.2

the 2013 data was gathered with a large number of staff only wearing the active

dosimeters when working in PET. The staff were wearing their TLDs in PET and

also in Nuclear Medicine.

The TLD results are much higher than those from the active dosimeters. The

least-squares fit is meaningless with a correlation coefficient as low as 0.13 (figure

3.18a).

3 Results 62



Figure 3.18: Comparison of PET Centre staff dose results for 2013

The Bland-Altman plot (figure 3.18b) demonstrates the poor correlation between

the TLD and active dosimeter results. There is only one active dosimeter result

less than the equivalent TLD result and the difference between the TLD and active

dosimeter results is marked and increases with increasing dose. The mean difference

between TLD and active dosimeter results is similar to that in the RAPID group

despite much lower average dose results. Setting the BDL values to 0, 25 or 50μSv

reduces the mean difference but the standard deviation is even larger (table 3.11).


dosimeter result is 0.36 of the equivalent TLD result, this is much lower than the

3 Results 63



Table 3.11: Bland-Altman Results for 2013 PET Centre Doses

RAPID results. The spread is similar to RAPID with a 1.96σv value of 0.42. There

is no discernible trend in the data. This plot does not include any BDL values.

3.3.2.2 2014

As explained in section 2.5.2, in 2014 a smaller number of staff wore the active dosime-

ters in both PET and Nuclear Medicine in an attempt to gather more meaningful

data.

The direct comparison plot (figure 3.19a) does show an improved correlation

compared to 2013 but it is still far from convincing. The slope of the fit suggests

that the active dosimeter results are still much lower than those from the TLDs. In

the Bland-Altman plot (figure 3.19b) the mean difference is -86μSv and the 1.96σv is

93μSv. Given the lower doses recorded by the dosimeters compared to the RAPID

staff, one would expect that the difference and spread would be lower for PET centre

workers. The results excluding the “below detectable limit” results, are very similar

for the PET centre and RAPID in 2014, the mean difference and standard deviations

are both within a few micro-sieverts of the results from the other group.

3 Results 64



Figure 3.19: Comparison of PET Centre staff dose results for 2014


Mean Difference (μSv) -86 -52 -58 -651.96 x Std Deviation 93 140 123 108

Table 3.12: Bland-Altman Results for 2014 PET Centre Doses

For the 2014 results the mean difference is improved by assuming a value for the

BDL results and including the comparisons, but including the assumed BDL values

increases the standard deviation in all cases (table 3.12).

3 Results 65


dosimeter result is 0.56 of the equivalent TLD result, this is much more like the

RAPID results than the previous year’s results. The spread is similar to 2013 with

a 1.96σv value of 0.43. The two points where the active dosimeter reading was higher

than the TLD results have increased the mean ratio. Without those two points the

mean is reduced to 0.52. This plot does not include any BDL values.

One sample t-tests show that the difference between the passive and active

dosimeter results are statistically significant using a 95% confidence interval for both

the difference and ratio data for 2013 and 2014.

3 Results 66

3.4 User Survey Results

A copy of the user survey can be found in Appendix A. 22 questionnaires were

completed.

3.4.1 Profession

Profession Number

Technologist 9Nurse 5

Radiochemist 7Other (Engineer) 1

Table 3.13: Professions of those surveyed

The technologists and nurses work in the PET center. The radiochemists and engi-

neers work in RAPID. The radiochemists include those that do regular productions

of 18FDG and those who produce research radiopharmaceuticals.

3.4.2 Time using Passive and Active Dosimeters

Timespan Passive Active

Less than 3 months 33-6 months6-12 months 1 1

Longer than 12 months 21 18

Table 3.14: Experience using Active and Passive Dosimeters

Almost all of the workers have been wearing both types of dosimeter for more than

12 months and so are accustomed to wearing them.

3.4.3 Ease of use of Dosimeters

Ease of Use Passive Active

Very Easy 18 174 30 20 0

Complex/Difficult 0 0

Table 3.15: Ease of Use of Dosimeters

3 Results 67

Nobody reported finding the dosimeters difficult to use.

3.4.4 Comfort wearing Dosimeters

Passive Active

Very Comfortable 14 135 82 11 0

Difficult/Uncomfortable 0 0

Table 3.16: How comfortable are dosimeters

Almost all the staff reported finding both dosimeters comfortable to wear.

3.4.5 Wear Position of Dosimeters

Passive Active

Waist 13 9Chest 9 13

Same for both 16

Table 3.17: Wear Position of Dosimeters

There is a preference for wearing the active dosimeters at chest height and the

passive dosimeters at the waist when staff wear them in different positions. 16 of 22

staff members wear the dosimeters in the same location (as requested), evenly split

between waist and chest.

3.4.6 Frequency of checking results

How often Passive

Every Month 11Most Months 5Sometimes 4

Hardly Ever 2Never 0

How often Active

Every Day 18Most Days 2Sometimes 1

Hardly Ever 0Never 1

Table 3.18: Frequency of checking dosimeter results

Results from the passive dosimeters are only available once per month and half

the staff report checking the results every month. Most staff report checking their

3 Results 68

results from the active dosimeter every day, probably because it is displayed on

screen during log out.

3.4.7 Level of trust in dosimeter results

Level of trust Passive Active

Completely 2 611 126 32 0

Not at all 0 0Did not respond 1 1

Table 3.19: Level of trust in dosimeter results

The overall level of trust in the results is high with slightly more trust in the results

of the active dosimeter.

3.4.8 Rate of not wearing a dosimeter

Passive Active

Never 7 6Hardly Ever 9 11Sometimes 2 3

Once per month 3 0Once per week 1 1

N/A 0 1

Table 3.20: The rate at which workers forget to wear dosimeters

Workers report a high level of compliance with wearing dosimeters. The N/A result

is from a nurse in the PET Centre who said that she had stopped wearing an active

dosimeter.

3 Results 69

3.4.9 Usefulness of results and feedback

Passive Active

Very Useful 4 176 26 24 1

No use at all 2 0

Table 3.21: Usefulness of results and feedback

The feedback from the active dosimeters is clearly seen to be more useful than that

from the passive dosimeters.

3.4.10 Prefer to wear Active, Passive or Both

Number

Just Passive 1Just Active 14

Both 7

Table 3.22: Prefer to wear active, passive or both

Given the choice most of the staff questioned would prefer to use just an active

dosimeter, about one third would like to use both and only one member of staff

would prefer to use just a passive dosimeter.

3.4.11 Additional Comments

The following comments were added by the staff:

Finger badges cannot be replaced by electronic dosimeter

TLD result turn-around too slow.

Log-in problems made use of MGPs more difficult

MGP recommended for high radiation areas

National dosimetry register to collect data from all providers using com-

mon unique identifier for workers.

Audible alarm helpful reminder, good for students, possible to equalize

dose among staff by assigning high dose procedures to staff with lowest

dose.

3 Results 70

Don’t wear MGP as end of day alert is annoying

TLD is retroactive so too late. MGP better as checking each day makes

you more aware.

Like the beep as it is a constant reminder. If switch to just electronic

would still want monthly summaries.

Not had any really helpful info back from the MGPs. Like the beeps as

it is a constant reminder.

The comments mainly center on the audible feedback from the active dosimeters,

the delay in reporting from passive dosimeters and some technical issues with the

active dosimeters. The reference to finger badges acknowledges that current active

monitoring systems cannot replace passive dosimeters entirely, as there are no sys-

tems to record extremity doses. RAPID workers are required to wear ring badges

containing TLDs to monitor their extremity doses.

Chapter 4

Discussion

4.1 Reliability

In the course of the controlled experiments there were occasions where passive

monitors produced unexpectedly low results. As shown in table 3.2 a number of

results were excluded from comparison because the reported dose was given as “below

detectable limits” when the results from the other two detection methods were above

the stated detection limit of the passive monitor. In one case a single TLD badge

from a batch reported “below detectable limits” while the other badges of the same

type reported doses around 120μSv, well above the detection limit.

One of the experiments had to be repeated as a set of results from the OSL

monitors came back as approximately one third the reported doses of the active and

TLD monitors. In the repeated high dose experiment the results from the OSLs

were higher than the TLDs and active dosimeters, but the difference was not nearly

as great as in the original experiment.

There were also incidents of staff doses measured by TLDs being unexpectedly

reported as “below detectable limits” when their work patterns were unchanged and

active dosimeters reported typical doses. This may reduce worker confidence in the

results they are receiving. Despite these problems the result of the staff survey in

table 3.19 show a high level of trust in the results though slightly lower for the

passive dosimeters than the active ones.

Given the relatively small number of passive dosimeters used, the number which

seem to have failed to report doses accurately is a cause for concern. The details are

summarised in table 3.2. This lack of reliability does not seem to have been reported

71

4 Discussion 72

in the literature and is worthy of further investigation. Regular blind testing of

dosimetry services using controlled exposures is recommended (Böhm et al., 1994)

but there is no evidence in the literature that this has been carried out in Australia

in recent history.

All of the active dosimeters performed reliably during the controlled experiments.

There were a few older active monitors that failed (while in use in the PET centre)

during the course of this investigation, but their failure was immediately obvious

to users, and so did not significantly reduce the effectiveness of dose monitoring.

If there are spare batteries and a few spare monitors for when there are technical

problems, the reliability of the active dosimeters seems higher than that of the

passive dosimeters in terms of service provision. The database and logging stations

can pose a single point of failure, but with backups a problem of this nature should

pose no greater disruption to monitoring than a shipment of passive dosimeters being

delayed in the mail.

4.2 Equivalence

The results of the controlled experiments show excellent agreement between the

active and passive dosimeters over a range of exposures typically seen by radiation

workers. The data shows no more variation between active and passive dosimeters

than there is difference between the two types of approved passive dosimeter either

perpendicular to the radiation source or at 30 and 60 degrees to the source. The

only statistically significant difference in mean dose comparisons was between the

two types of passive dosimeter (section 3.2.8). The high level of agreement between

the TLD and active dosimeter results (figures 3.12a and b) suggest that differences

between the active and OSL dosimeter results are likely to be because of the OSL

dosimeters over-reporting the dose at around 1mSv. Without the high dose results

the spread of results in the Bland-Altman plot (figure 3.11b) would be much smaller.

When assessing the quality of a dosimetry service the acceptable difference be-

tween the expected result and that reported can be ±50% (Böhm et al., 1994) for

the kinds of relatively low doses experienced by radiation workers. The difference

between the results from active and passive dosimeters in the controlled experiments

was always less than 50% and usually much less than that. The spread of results

from the passive dosimeters as seen in figures 3.2.2a and b was very similar to the

spread between active and passive dosimeters as seen in figures 3.11 and 3.12. The

spread was also similar when comparing the two types of passive dosimeter (figure

4 Discussion 73

3.10). The results are summarised in table 3.6. It is clear from these results that

in controlled conditions with 511keV gammas there is no greater difference between

the active and passive dosimeters than there is between the two types of approved

passive dosimeters or even between different individual passive dosimeters of the

same type. The active dosimeters are at least as accurate at reporting dose as the

passive dosimeters in these conditions.

A premises using radioactive material in WA must have at least one calibrated

dosimeter for measuring dose rates (Western Australia, 1984). When calibrating

dosimeters, settings on the dosimeter must be adjusted to attempt to bring the

measured dose rate within 10% of the actual dose rate. If no adjustments are

capable of bringing the measured dose rate within 20% of the actual dose rate the

dosimeter fails and cannot be used. Calibration certificates are provided specifying

the isotope or x-ray energies used to calibrate the dosimeter and its results compared

to the reference dose rates. In the controlled experiments we do not have a reference

dose to compare the results to, as no laboratory calibrates dosimeters to 18FDG. The

best comparison for evaluating the suitability of the active dosimeters appears to be

the comparison with TLDs, due to the large number of OSLs which failed to provide

usable results, and the apparent over-reporting of the ~1mSv results. Even in the

best comparison (figure 3.12) some of the results of the TLDs and MGPs differed

by more than 20% of the average of their results. Much of the difference between

the results are due to the spread of results from the passive dosimeters. The spread

of results from each dosimeter type is shown in figure 3.2.2, the active dosimeters

show an obvious advantage in terms of precision.

The comparison of active and passive dosimeter results from workers is far less

conclusive than the controlled experiments. The results from the active dosimeters

were almost always lower than those from the passive dosimeters except when the

passive dosimeters reported “below detectable limit”. This bias could be a result of

failure to always wear an active dosimeter when exposed. This was definitely the

case for the 2013 PET centre results as the active dosimeters were only worn in PET

and not in Nuclear Medicine. It is also possible that the large differences in the

results are due to large anisotropies in the radiation field caused by uneven shielding

and differing positions of the two badges when working. As shown in table 3.17,

not all staff wore the two badges in the same position. When working with small,

high activity sources even small distances can make large differences to exposure of

different areas. This explanation is undermined by comparing the results shown in

figures 3.16 and 3.17. Limiting the comparison to the staff who reported wearing

4 Discussion 74

dosimeters in the same location on the body did not improve the correlation between

the results of the active and passive dosimeters.

It should be noted that there are significant errors in attempting to measure an

individual’s exposure using a personal radiation monitor of any kind. In laboratory

conditions it is usually possible to measure radiation fields to within 10% of the true

value (International Commission on Radiological Protection, 1997). In the workplace

non-uniformity and uncertain orientation can change the recorded dose by a factor of

1.5 in either direction (International Commission on Radiological Protection, 1997).

In light of this, it seems that equivalence should be judged more by the experiments

in controlled conditions than the workplace comparisons.

4.3 Repeatability

When measuring any quantity it is important that the measuring instrument reports

the same result every time it measures the same quantity. The repeatability of

each type of dosimeter can be judged by the spread of results when measuring the

same radiation dose. The Bland-Altman plots in figure 3.1 show that the average

difference between results from the same type of dosimeter are smallest for the active

dosimeters. The 2σv value for the MGPs was approximately half that of the 2σv value

for the OSL and TLD results.

4.4 Limits of Detection

The purpose of personal radiation monitoring is to provide information on the dose

individual staff members are occupationally exposed to. Radiation regulations and

standards are based around the principle of keeping radiation exposure As Low As

Reasonably Achievable (the ALARA principle) (International Commission on Radi-

ological Protection, 1997). Dose reports can be examined to highlight unexpectedly

high doses in radiation workers. The data from the staff monitoring in section 3.3.1

contained no reported staff doses from TLDs below 100μSv but a large number of

reports of “below the detectable limit”. This would suggest that the true lower

limit of detection for the TLDs used in the workplace is around 100μSv. From one

monitoring period to the next an individual’s exposure could vary from 0 to 100μSv

without any indication that changes in their work practice was exposing them to

more radiation. This is clearly not the case for an active dosimetry system that

displays reading down to 1μSv at the end of each day. The primary justification for

4 Discussion 75

a dose monitoring system is in the way in which it helps to achieve and demonstrate

an appropriate level of protection (International Commission on Radiological Protec-

tion, 1997). This present study would suggest that the use of an active monitoring

system could allow better compliance with the ALARA principle.

4.5 User compliance

Even a technically perfect dosimeter can only report the dose to the worker if it

is worn during any work involving sources of ionising radiation. There is evidence

to suggest that large numbers of radiation workers in healthcare regularly fail to

wear their dosimeters (Klein et al., 2015; Padovani et al., 2011). Compliance with

the requirement to wear a passive dosimeter is difficult to enforce, relying on spot

checks and comparison of reported doses with those of similarly employed colleagues.

Many hospital radiation workers regularly report doses below the detectable limit of

passive dosimeters which means that there is no way to tell if a dosimeter has been

worn at all. Where an active dosimeter is coupled to a logging system however, it is

a simple matter of querying the database to check whether an employee has logged

in a dosimeter regularly, such records can then be compared to shift rosters or other

work records to monitor compliance. The user survey (table 3.20) suggests that

non-compliance among those working with PET radiopharmaceuticals and patients

is low, though self reporting may not be 100% reliable. Since the conclusion of the

data gathering phase of this thesis, work has been undertaken to systematically track

the logging in and out of staff in RAPID. This system allows us to better calculate

staff dose per work day and improve radiation hygiene, it also serves to highlight

non-compliance.

4.6 User Acceptance of Active Dosimeters

Active dosimeters are larger and heavier than passive dosimeters, they also require

more effort to use as they must be logged in and out every shift. Despite these

drawbacks they were accepted well by staff and preferred over passive dosimeters by

many, as demonstrated by the results of the user survey (section 3.22).

One staff member in the PET center stopped wearing the active dosimeter (she

was not obliged to wear one) as she found an alarm emitted after eight hours logged

in to be annoying (her shifts regularly exceeded eight hours). This problem could

have been overcome with changes to the settings of the dosimeter.

4 Discussion 76

Many workers responded that they found the audible feedback from the active

dosimeters to be useful and felt that it helped them to reduce their radiation exposure.

No evidence could be found to demonstrate a reduction in radiation dose after the

introduction of the active dosimeters. Any change could easily be lost in the noise

due to the variability in worker routine, dose results, patient numbers and dispensed

activity from month to month.

4.7 Approval of Personal Radiation Dosimetry Ser-

vices

The standards required of providers of personal dosimetry services vary around the

world (Cavallini et al., 1994; Marshal, 1998). The United States has had a program

of assessing service providers since the 1960s (Schauer et al., 2004), the current

testing requirements of the National Voluntary Laboratory Accreditation Program

(NVLAP) are given in HPS N13.11-2001 (Soares, 2007). In the United Kingdom the

Health and Safety Executive (HSE) also publish a set of requirements which must

be met by service providers (Health and Safety Executive, 2010). In jurisdictions

which mandate testing of service providers, batches of dosimeters exposed to known

doses are sent for analysis by the service providers. The results are examined for

compliance with a number of criteria. The US and UK both use pass/fail criteria

based on the bias in the average of the results and the standard deviation for batches

of dosimeters. The criteria used in the UK for dosimeters designed to monitor whole

body gamma exposure are shown in table 4.1. The service provider is required to

pass the initial assessment before being approved and are subject to repeat testing

every 5-7 years (Health and Safety Executive, 2010). A 2001 update to the NVLAP

performance criteria requires that no more than 10% of the dosimeters tested fall

outside the acceptance criteria for the mean of all results (Schauer et al., 2004).

This change prevents approval of services with a rate of dosimeter failure close to or

higher than 10%.

The Radiological Council of Western Australia do not mandate regular testing of

approved suppliers; service providers must request initial approval from the Council.

In the preparation of this thesis Council officers were asked about the methods used to

approve providers. The providers are asked to produce details of their measurement

methods and processes and demonstrate that measurement methods are traceable to

Australian or international standards. One of the most significant factors in gaining

approval is the provider having a national or international accreditation from one or

4 Discussion 77

Limit on Criteria (must pass all)

Bias in average of all results < 20%Standard Deviation in all results <10%

Bias in average of < 20%each group of 5 dosimeters (< 30% for any group irradiated to 1.0 mSv or less)

Standard Deviation of < 10%each group of 5 dosimeters (< 15% for any group irradiated to 1.0 mSv or less)

Table 4.1: UK HSE Pass/Fail criteria for dosimetry services for monitoring wholebody gamma exposure (Health and Safety Executive, 2010)

more of the major accreditation providers such as National Association of Testing

Authorities (NATA). NATA accreditation for personal dosimetry is in part based

on the provider meeting the standards set by the HSE, NVLAP or International

Accreditation New Zealand (IANZ).

In our controlled experiments we have no true dose values to compare our results

to so we cannot say if any of our sets of dosimeters would have passed a review to the

standards of the HSE or NVLAP. We can say however that the MGP results showed a

much smaller standard deviation that either of the passive dosimeter types. It would

be prohibitively expensive for an individual hospital to obtain NATA accreditation

as a dosimetry service provider. This may represent a significant hurdle to getting

the active dosimetry system approved by the Radiological Council as a means of

obtaining the legal dose record.

4.8 Standards for Personal Radiation Monitors

National standards relating to radiation protection and detection tend to be based

largely on international standards published by bodies such as the International

Atomic Energy Authority (IAEA), the International Commission on Radiation Pro-

tection (ICRP) and the International Electrotechnical Commission (IEC) (Voytchev

et al., 2011). The IEC has a sub-committee (45B) focused on radiation protection

instrumentation which has published standards for both active personal dosimeters

(IEC 61526 also known as ISO 4037) and passive integrating dosimetry systems (IEC

62387-1). The two IEC standards contain a list of criteria for the dosimetry systems

including the dosimeter reading and reporting equipment. The criteria include the

minimum energy range, and minimum and maximum dose and dose rate levels, over

which a dosimetry system must function. Type testing is not included in IEC 62387-

1; it is only concerned with system properties and states that absolute calibration

4 Discussion 78

should be performed as part of routine testing (Voytchev et al., 2011). Comparing the

requirements between these two standards show that the requirements for the passive

dosimeters are more stringent, with a broader energy requirement; 12 keV–7 MeV for

passive dosimeters and 80 keV–1.5 MeV for active dosimeters when measuring Hp10

for gamma exposure. This is not to say that active dosimeters cannot meet the same

standard as the passive dosimeters, but that they do not have to in order to satisfy

IEC 61526 Edition 3. Boziari and Hourdakis (2007) and others have demonstrated

that some active personal dosimeters show good energy independent response down

to 50keV, while with others the response drops sharply below 65keV making them

unsuitable for some applications.

The results of tests of a number of active personal dosimeters against aspects of

IEC 61526 have been published. Texier et al. (2001) tested the energy response of

seven different active dosimeters including the DMC 2000S, one of the dosimeters

used in this study. It was found that the majority conform to the standard but many,

including the DMC 2000S, have a poor response below 50 keV. Using the x-ray and

gamma energies suggested in IEC 61526, angular response was tested for two types

of active dosimeter by Suliman et al. (2010) and very little variation was found with

rotation which is in agreement with the results in section 2.4.6.

4.9 Calibration of APDs

Unlike passive dosimeters, active dosimeters may be in use for more than 10 years

(see section 4.12.3). There is potential for their performance to degrade over time,

or be altered by calibration or changes to settings. If the active dosimetry system

were to provide the legal record of occupational dose this problem would need to be

overcome through regular calibration. This could either be performed in-house, if a

suitable source and measurement apparatus is available, or by an approved service

provider against a source traceable to a national standard. This requirement for

calibration would place an extra workload and/or expense on the center using APDs.

4.10 Record Keeping and Data Analysis

Having a single database containing all the dose data for your staff has advantages

and disadvantages. Whatever dosimetry system is used as the legal record of worker

dose there is a legal requirement for data retention; in Western Australia personal

dosimetry records can not be disposed of without the explicit permission of the

4 Discussion 79

Radiological Council (Western Australia, 1984). A single database provides a single

point of failure if the database is not backed-up, posing a great risk of loss of data.

This is counteracted by the ease with which electronic files can be backed-up and

stored, in comparison to paper records. The main advantage of a single structured

database is the ease with which data can be queried and cross-referenced. The

reports provided by external suppliers have moved from paper to electronic files.

The files provided are of changing file format and structure which makes compiling

data from multiple years difficult, error prone and time consuming. Data analysis

is much easier with a single, large structured database than a large collection of

separate files.

4.11 Incident investigation

One of the significant advantages of active over passive dosimeters is in the area of

incident investigations. With passive dosimeters dose reports much be checked for

doses above regulatory limits or internal trigger levels and investigations of unusually

high reported doses must be carried out. Any results, either monthly or quarterly,

which are above the pro-rata annual limit (20mSv/year whole body dose) must

be reported to the Radiological Council in Western Australia (Western Australia,

1984). The delays in data collection and reporting with passive dosimeters, and the

inability to pinpoint the day a high dose occurred, can cause significant problems in

investigating and reporting on high doses.

Active dosimetry systems on the other hand can provide immediate notification

of unusually high exposure or dose rates. This acts to both to reduce exposure and

pin-point where and when the exposure occurred. If a high monthly dose is shown

without a record of a high exposure incident there will be a record of chronic, above

average exposure. Whether the exposure is acute or chronic, the dose record can be

a significant help when investigating and comparing work practices and technique

with those of other staff. A day by day record of received dose and maximum dose

rate can be a significant help in attempting to reduce radiation dose to workers.

4.12 Economic Comparison

Active dosimeters are required by the state regulator in Western Australia for cy-

clotron production of radiopharmaceuticals. This means that both active and passive

dosimeters are required by legislation. There is an obvious economic advantage to

4 Discussion 80

removing the requirement for passive dosimeters if active dosimeters can be demon-

strated to provide adequate protection alone. In other work areas where there is no

requirement for active monitoring, whatever the technical advantages of an active

dosimetry system it is unlikely to be adopted if it is prohibitively expensive when

compared to passive dosimetry.

This section is not intended as a full economic assessment of the available dosime-

try systems, but is intended to give an overview and impression of the costs involved.

As with any costings projecting years into the future, a number of assumptions about

future prices and equipment reliability must be made, based on past information.

A comparison of the costs of passive and active dosimetry systems is a comparison

of a small but ongoing cost, against a larger upfront cost, with the outlay going

forward being a requirement for equipment replacement. Any dose monitoring system

will also require staff time to manage, which can be non-trivial.

4.12.1 Costs of Passive Dosimetry

For each wear period, either one or three months, a dosimeter must be supplied and

returned for each member of staff. There is a delay between ordering and receiving

dosimeters, so usually a large department will have a standing order for a number

of spare dosimeters, to be assigned to new members of staff or to cover for lost or

damaged dosimeters. There is a fixed cost for each dosimeter supplied (excluding

control dosimeters). If any badge is not returned within a designated time, for

example within 1 month of the end of the wear period, it is considered lost and an

additional lost badge charge is levied. Some suppliers also charge a delivery fee, a flat

fee independent of the number of badges delivered. The cost per year of the passive

dosimetry system shown in table 4.2c are simply 12 times the unit cost, multiplied

by the number of staff and thus represent the minimum cost with no lost badges

or delivery fee. The dollar values given in the costings in table 4.2a are indicative

values based on 2015 figures quoted to staff in Medical Technology & Physics by

three of the main service providers in Australia. There is no significant difference

between the cost of TLD and OSL badges.

There is a great deal of time spent by staff receiving, checking, distributing,

collecting, packaging and returning dosimeters each wear period when there are

large numbers of dosimeters. Time is also spent making changes to orders due to

staff changes and processing the invoices to pay for each shipment. For a large

department with regular staff changes, managing communication with the supplier

can become a significant time burden and expense.

4 Discussion 81

4.12.2 Costs of an Active Dosimetry System

An active dosimetry system requires a number of components as explained in 2.3.

While the costs of a passive system scale roughly linearly with the number of workers;

due to the requirement for one or more logging stations and a central database, this

is not the case for an active system. Due to relatively large capital outlay, any

calculation of cost per worker per year is strongly dependant on the number of

workers and the lifetime of the various pieces of equipment. The costs per year in

table 4.2c are calculated by summing the the unit costs of the required equipment

divided by its predicted lifetime.

As with the passive dosimetry service there will be a need for monitoring staff

doses and following up on results. Far less time is spent in the occasional battery

change and replacement of faulty dosimeters than is spent in the routine handling of

large numbers of passive dosimeters. There would be the additional staff overhead

of report production, but this is a largely automated process.

4.12.3 Lifetime of MGP Active dosimeters

One of the important economic considerations is the lifetime of the dosimeters. The

date of purchase and failure of every MGP dosimeter used at Sir Charles Gairdner

Hospital was obtained to assess the typical lifetime of an active dosimeter of this

type.

4 Discussion 82

Figure 4.1: Lifetime of MGP dosimeters

It can be seen in figure 4.1 that the typical lifetime of the MGPs which have

failed is around 10 years. There are however three dosimeters still in use after 11

years and two more still active after 13 years. For this simple analysis a value of

10 years was chosen. The logging stations and PC used to run the database were

replaced in 2016 so a lifetime of 4 years was used.

4.12.4 Comparison of costs per year

With any set of reasonable assumptions it is clear from table 4.2c that for large

groups (> 20) an active dosimetry system can cost less per year than a passive

system. The difference in costs will however be outweighed easily by any change

in staff time allocated to managing the dosimetry system, analysing results and

performing investigations. As noted in section 4.11 investigation of high exposure

should be simpler and more effective with an active dosimetry system reducing the

duration of investigations and improving their effectiveness. Alternative dosimetry

systems may prove more cost effective in the future. Instadose dosimeters are a

4 Discussion 83

Unit cost (AU$)

Passive dosimeter 15MGP active dosimeter 800

Logging Station 3000Database PC 2000

Software 500

(a) Indicative unit costs for active and passive dosime-try systems in 2015 AUD

Lifetime (Years)

MGP active dosimeter 10Logging Station 4

Database PC 4

(b) Expected Lifetime of active dosimetry system com-ponents

Total staff requiring monitoring 5 12 12 25Staff requiring monitoring simultaneously 5 12 10 20

Cost (AU$/year)

Passive Dosimetry 900 2160 2160 4500Active Dosimetry 1775 2335 2175 2975

(c) Averaged cost per year of active and passive dosimetry systems for different usergroup sizes. Costs exclude staff costs in managing the system.

Table 4.2: Economic Comparison of Active and Passive Dosimetry

4 Discussion 84

small electronic personal dosimeter without a screen which can report doses to a

central database via the web when plugged into a PC. They are currently cost

competitive with passive dosimeters and their cost could fall with greater uptake of

the technology.

4 Discussion 85

4.13 Legislative issues in Western Australia

Every state has its own legislation regarding the use of ionising radiation and the

protection of personnel working with it. In Western Australia the relevant legislation

is The Radiation Safety Act 1975 and the Radiation Safety (General) Regulations

1983. The Act and Regulations are administered by a statutory body called the

Radiological Council (Western Australia, 1975).

The Act and Regulations were written before the advent of active personal dosime-

ters and because of this the wording of the Regulations relating to the requirements

for personal dosimetry could pose an impediment to the replacement of passive

dosimeters with active ones even if active dosimeters are deemed technically suitable

or even superior. There is nothing in the Regulations that specifically prohibits the

use of active dosimeters, the means by which dose information is obtained is not

described at all.

Section 25 of the Regulations relate specifically to personal monitoring devices,

in particular this section refers to use of “an approved personal monitoring device”

and the use of “the services of radiation monitoring organizations that have been

approved”. A list of approved organizations can be obtained from the Radiolog-

ical Council website, all of the approved organizations provide passive monitors

(Radiological Council of WA, 2010).

It is possible to request approval from the Radiological Council for a new ser-

vice provider but the approval of active monitors would be a significant shift from

current practice. At present an approved supplier supplies the badges and is re-

sponsible for reading them and reporting the doses. The approved suppliers are

either large laboratory organizations who have obtained internationally recognised

quality accreditations or their agents. With the active dosimetry system discussed

in this thesis the organization producing the reports is the organization employing

the workers, this could be seen by the Council as a conflict of interest. If the Council

were amenable to approving individual workplaces as approved personal radiation

monitoring service providers it would mean that each workplace would have to apply

individually. Applying for approval could prove a significant administrative bur-

den which could discourage centres from switching despite the advantages of active

dosimeters. Having to assess a large number of applicants for approval would also

place additional work on the Council and its Officers.

One type of electronic dosimeter, the Instadose (www.instadose.com), has been

approved by the Radiological Council. This dosimeter differs significantly from the

active dosimeters discussed in this thesis in that it offers no direct feedback. Its main

4 Discussion 86

advantage is that it can be read at any time by plugging it into a PC with the correct

software installed; the dosimeter does not need to be returned to the supplier. The

dose data is stored by the supplier on remote servers which receive updates from the

software. Account managers can access and assess the doses recorded through the

software. The use of such a system may overcome some of the issues with passive

systems, but it does not offer the full range of advantages of an active dosimetry

system and would still require RAPID workers to wear two dosimeters.

Chapter 5

Conclusion and Future Work

5.1 Conclusions

There seems to be no technical reasons why the active dosimetry system described in

this work could not be used to provide the occupational dose record for staff working

with PET radiopharmaceuticals and patients. Controlled experiments demonstrated

that the ability of the active dosimeters to measure radiation dose from 18FDG was at

least as good as that of the passive dosimeters currently in use at Sir Charles Gairdner

Hospital. The agreement between the active dosimeters in the same experiments

was better than that between the same type of passive dosimeters.

The controlled experiments demonstrated a problem with the reliability of the

passive dosimeters, with many results being significantly lower than expected. In

a real life situation this could lead to under reporting of occupational doses. Both

the controlled experiments and the staff dose results showed that the actual lower

limit on detection for the passive dosimeters is greater than that stated by the

suppliers. Over a year the unreported doses from passive dosimeters could total

1mSv and opportunities for dose reduction could be missed. If the project were to

be repeated it would be sensible to use a larger number of passive dosimeters to

gather better statistics and attempt to gather data for all dosimeter types over the

dose range of interest. When designing the experiments such a large failure rate was

not anticipated, in particular the failure of multiple dosimeters in the same batch.

The nature of gathering workplace data meant that few conclusions could be

drawn from comparing active and passive dose results for staff. The correlation

between the dose results for active and passive dosimeters was far from clear, though

it did seem to improve over the course of the monitoring. A large proportion of

the results were below the detection limit of the passive dosimeters and so gave no

87

5 Conclusion and Future Work 88

meaningful data. There was an obvious overall lower dose recorded by the active

dosimeters. Gathering more informative workplace data would require a change to

work practices such that workers wore the active dosimeters whenever they were at

work and more supervision to ensure that there were not periods where they were

not being used. The problems with doses below 100μSv not being recorded by the

passive dosimeters are always going to result in a smaller pool of useful comparisons.

With one exception, staff reported that they were comfortable using the active

dosimeters and many stated a preference for the active over passive dosimeters.

For a medium to large workforce (20 or more radiation workers) there should be

no economic disincentive to switching to an active dosimetry system if a long term

(multi-year) view is taken.

The major hurdle is legal approval. Active dosimetry systems have been approved

in some jurisdictions but the only system approved for use in Western Australia uses

dosimeters which simply store data and report it back to a central service without

relying on local data storage and report production. No currently approved personal

dosimetry systems give instant feedback of dose or dose rate, which is one of the

major advantages of active personal dosimetry.

5.2 Future Work

The number of passive dosimeters which reported doses well below those expected in

the controlled experiments suggests that further work be carried out to determine how

significant this problem is. A wider survey involving larger numbers of dosimeters,

exposed to a range of known doses, should be carried out. Such a survey is probably

the remit of state and national regulatory bodies. A program of regular assessment

similar to the US model (Böhm et al., 1994) may be required to assure that personal

dosimetry services are providing the expected results.

The next obvious step for this work is to apply to the Radiological Council

for approval of the active personal dosimetry system. If approval can be obtained

for use in the RAPID and/or Nuclear Medicine areas then attention could turn

to other areas in the hospital who might wish to use an active dosimetry system.

Radiation Oncology, Radiology and Cardiovascular Medicine all have large numbers

of radiation workers. Each area has the potential for significant exposure either

through routine work or accident situations and may benefit from the feedback from

active dosimetry.

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Appendix A

User Experience Survey

94

If you have any questions please contact Steve on x1450 or [email protected]

Personal Radiation Monitoring Questionnaire

This questionnaire is intended to gather data as part of a project to determine whether

electronic personal dosimeters can replace passive dosimeters (TLDs) for staff working

with PET radiopharmaceuticals. The data you provide will help determine whether

SCGH will pursue the replacement of TLDs with electronic dosimeters. The introduction

of electronic dosimetry will only happen if it is technically appropriate, satisfies legal

requirements and is desirable to the staff of SCGH.

Your Name …………………………………………………………………..

Your name will not be included in any publications; it is collected for data analysis only.

Are you a…

Technologist [ ] Nurse[ ] Physician[ ] Radiochemist[ ] Other[ ]

How long have you been using passive dosimeters (TLDs, OSLs, film badges)?

Less than 3 months [ ] 3-6 months [ ] 6-12 months [ ] longer than 12 months [ ]

How long have you been using the electronic dosimeters?

Less than 3 months [ ] 3-6 months [ ] 6-12 months [ ] longer than 12 months [ ]

How would you rate the ease of use of TLD badges and MGP Electronic dosimeters?

Very Easy Complex/Difficult

TLD 1 2 3 4 5

Electronic 1 2 3 4 5

How comfortable is it to wear the dosimeters?

Very Comfortable Difficult/Uncomfortable

TLD 1 2 3 4 5


Where do you normally wear the dosimeters? (mark with an X)

TLD Electronic

Please continue overleaf

If you have any questions please contact Steve on x1450 or [email protected]

How often do you check your results?

TLD Every month [ ] Most months [ ] Sometimes [ ] Hardly ever [ ] Never [ ]

Electronic Every day [ ] Most days [ ] Sometimes [ ] Hardly ever [ ] Never [ ]

How much do you trust the results?

Completely Not at all

TLD 1 2 3 4 5


Roughly how often do you forget to use a dosimeter?

TLD Never [ ] Hardly ever [ ] Sometimes [ ] Once per month [ ] Once per week [ ]

Electronic Never [ ] Hardly ever [ ] Sometimes [ ] Once per month [ ] Once per week [ ]

How useful are the results or feedback from the dosimeters in monitoring and reducing

your exposure? Very Useful No use at all

TLD 1 2 3 4 5


Given the choice would you rather…

use just the TLD [ ] use just the Electronic Dosimeter [ ] use both [ ]

Please add any other comments…

Appendix B

Example Dose Reports

97

SURNAME FIRSTNAME

DOSIMETRIC SUMMARY

Name : SURNAME

First Name : FIRSTNAME From : 01/07/2015 to : 31/07/2015

Year : 2015 Dose Hp(10)G / Hp(10)N / Hp(0.07)

Month : 7

01/07/2015 5 / 0 / 0 µSv

02/07/2015 8 / 0 / 0 µSv

03/07/2015 28 / 0 / 0 µSv

06/07/2015 4 / 0 / 0 µSv

08/07/2015 11 / 0 / 0 µSv

09/07/2015 14 / 0 / 0 µSv

10/07/2015 15 / 0 / 0 µSv

13/07/2015 9 / 0 / 0 µSv

14/07/2015 7 / 0 / 0 µSv

15/07/2015 17 / 0 / 0 µSv

16/07/2015 2 / 0 / 0 µSv

17/07/2015 9 / 0 / 0 µSv

20/07/2015 10 / 0 / 0 µSv

21/07/2015 6 / 0 / 6 µSv

22/07/2015 9 / 0 / 12 µSv

23/07/2015 7 / 0 / 0 µSv

27/07/2015 10 / 0 / 0 µSv

28/07/2015 6 / 0 / 0 µSv

30/07/2015 17 / 0 / 18 µSv

31/07/2015 10 / 0 / 0 µSv

Sum : 204 / 0 / 36 µSv

Total dose for month : 7

204 / / 36 µSv

Total dose for year : 2015 787 / 0 / 264 µSv

SURNAMETotal dose for requested dates: / / µSv

Licensed to : MED TECH / SCGH

19/08/2015 1 / 49

the suitability of active personal dosimeters as the legal dosimeter for pet radioisotope workers

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