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6 RADIATION DOSE, RADIATION PROTECTION AND THEIONISING RADIATIONS REGULATIONS
Overview
This chapter discusses the issues of radiation dose, radiation protection and the
relevant aspects of United Kingdom legislation as they apply to bone densitometry
investigations, concentrating in particular on the technique of dual X-ray
absorptiometry (DXA). DXA scanning gives very low radiation dose to patients, and
this is an important factor in its popularity as a method of measuring bone mineral
density (BMD). An understanding of dose is useful for reassuring patients who may
be anxious about radiation risks, while the recording of individual patient exposure is
a requirement of the Ionising Radiations Medical Exposure (IRMER) Regulations,
and detailed information about dose is needed to complete the new Central Office for
Research Ethics Committees (COREC) application form for research studies. The
introduction of fan-beam DXA systems increased dose compared with older pencil-
beam machines and for certain models (the Lunar Expert in particular) required a re-
evaluation of the radiation protection of staff operating equipment. Finally, this
chapter reviews the steps required to implement the Ionising Radiations Regulations
in departments providing a DXA scanning service.
Radiation Dose in DXA
Studies of radiation exposure to patients from DXA scans have confirmed that the
doses involved are small compared with most other radiological investigations using
ionising radiation.1-18
However, clinicians requesting scans and staff performing them
should be aware that any exposure to radiation carries a potential risk. With
diagnostic examinations this is generally very small, and especially for DXA scans
where radiation levels for some types of equipment are so low that they are difficult
to measure.
The radiation hazards involved in the diagnostic use of X-rays are carcinogenesis
(the induction of cancer by exposure to radiation), and in men and women with child
bearing potential, an increased risk of diseases caused by genetic abnormalities
occurring in their future children.19
Both these hazards are examples of the
2
stochastic effects of radiation. The word stochastic means a process governed by
the laws of random chance. For radiation this means that in any individual there is a
small chance (with a risk that increases with dose) that exposure to X-rays will have
a harmful effect. A familiar example of a process governed by the laws of chance is
winning the National Lottery. Most people playing the Lottery fail to win anything at
all, and by analogy an overwhelming majority of patients having X-ray examinations
do not come to any harm from exposure to radiation. However, just as there is a
certain tiny chance of winning the Lottery, there is similarly a very small chance that
exposure to X-rays may cause cancer in an unlucky individual.
Unlike the Lottery where the exact chances of winning can be mathematically
calculated, it has not proved so easy to quantify the risks of radiation induced cancer.
This is because cancer is a relatively common disease with many other causes
unrelated to radiation. Except in a few special circumstances, such as the studies of
the survivors of the atomic bombings of the Japanese cities of Hiroshima and
Nagasaki during the Second World War,19
these other causes of cancer are much
more common and prevent us from detecting the tiny number of extra cases caused
by radiation. The few circumstances in which has been possible to quantify the
radiation risk all involve high doses of radiation much larger than those used in
diagnostic X-ray examinations. It is therefore necessary to extrapolate the risks from
these high doses down to the much lower doses used in medical practice, and in
general we do this by assuming that the risk increases in proportion to the dose.
Less is known about the risks of causing genetic disease in children of people
exposed to radiation because no study of a human population has ever detected this
effect. However, we know from experiments with plants and insects that such effects
do exist and in principle must occur in human beings too.
Table 1 lists some risks in life comparable to the risk of developing cancer after
receiving a radiation dose of 1 microsievert (1 Sv). This is comparable to the dose
received by a patient having a spine and hip DXA examination on a GE-Lunar
Prodigy system.
3
Table 1
Some activities carrying a risk of death comparable to receiving an effective dose of 1 Sv *
___________________________________________________________________
Exposure to natural background radiation for 4 hours
Smoking one-tenth of a cigarette
Travelling 3 miles by car
Travelling 15 miles in an airliner
Rock climbing for 5 seconds
Canoeing for 20 seconds
Working in a factory for half a day
Being a woman aged 30 for 60 minutes
Being a woman aged 40 for 20 minutes
Being a woman aged 50 for 8 minutes
Being a woman aged 60 for 3 minutes
Being a woman aged 70 for 1 minute
___________________________________________________________________
* The effective dose for a spine and hip DXA examination is 1.4 Sv on a GE-Lunar Prodigy system and 7.5 Sv on a Hologic Discovery (see Table 4). Data for other risks in life are taken from E E Pochin 28
Units of Radiation Dose
All accurate measurements of radiation dose are made with the help of ionisation
chambers. These measure the amount of electric charge released in a given volume
of air when the ionisation chamber is placed in the X-ray beam. The intensity of an X-
ray beam measured in this way is called exposure, and is measured in units called
roentgens (symbol, R). Manufacturers' specification sheets for DXA systems often
give figures for the X-ray beam intensity expressed in milliroentgens (mR) (1 mR
equals one thousandth of a roentgen). This is a simple measurement to make
because it only involves performing scans of an ionisation chamber. However, it is
not a particularly useful measurement of the radiation risk to patients.
4
Measurements of exposure express the effect of the radiation on air. To find the
radiation dose to the human body, we need to relate this to the effect in tissue. The
first step in doing this is to convert the measurement of exposure in air into the
absorbed dose in the human body. Absorbed dose expresses the radiation dose to
the body in terms of the energy absorbed from the X-ray beam per unit mass of
tissue. The idea behind this is that the more energy that is absorbed from the
radiation beam by the body the more damage is done to the tissue. On the molecular
scale, this damage results in broken atomic bonds in biologically important molecules
such as DNA. The unit of absorbed dose is the gray (symbol, Gy), and 1 Gy is an ab-
sorbed dose of 1 joule of energy per kilogram of tissue. One gray is a relatively large
dose of radiation, and in diagnostic radiology absorbed dose is usually expressed in
milligray (mGy) (1 mGy equals one thousandth of a gray). With DXA systems, dose
is often so low that a more convenient unit is the microgray (µGy) (1 Gy equals one
millionth of a gray). Because the air in an ionisation chamber consists of atoms with
a similar atomic number to the atoms in soft tissue, measurements of exposure in air
expressed in mR are readily converted into absorbed dose in soft tissue expressed
in µGy with a factor that varies only slightly with the X-ray photon energy. For X-rays
generated at 100 kVp, the conversion factor for lean tissue is 9.2 µGy/mR.4
An important measurement usually expressed as absorbed dose is the entrance
surface dose. This is the absorbed dose to the skin at the point where the X-ray
beam enters the patient's body. Some figures for entrance surface dose for widely
used DXA systems are listed in Table 2. As the X-ray beam passes through the pa-
tient's body the radiation is attenuated due to the absorption and scattering of the X-
ray photons. Internal organs therefore receive progressively lower doses the more
shielded they are by overlying tissues, and therefore the average dose to the human
body from the X-ray beam is considerably smaller than the entrance surface dose.
Because absorbed dose measures purely the energy transferred by the radiation to
the tissue, it turns out not to be the most appropriate way of expressing the biological
effects of the radiation such as the risk of cancer induction. The problem is that there
are several different types of ionising radiation, including alpha rays, beta rays,
gamma rays, X-rays and neutrons. For the same absorbed dose, some types of
5
Table 2
Measurements of entrance surface dose (ESD) for different DXA systems. The scan modes are identified by either the X-ray tube current or the scan time. ___________________________________________________________________
DXA System Scan Mode ESD (Gy)___________________________________________________________________
GE-Lunar DPX Medium (0.75 mA) 10
GE-Lunar Prodigy Thin (0.75 mA) 10
GE-Lunar Prodigy Standard (3.0 mA) 40
GE-Lunar Prodigy Thick (3.0 mA) 80
GE-Lunar Expert-XL 2 mA fast (12 sec) 320
GE-Lunar Expert-XL 5 mA fast (12 sec) 800
Hologic QDR-1000 Quick mode (4 min) 30
Hologic QDR-1000 Performance mode (8 min) 60
Hologic Discovery Express mode (10 sec) 100
Hologic Discovery Fast mode (30 sec) 150
Hologic Discovery Array mode (60 sec) 300
___________________________________________________________________
radiation such as alpha rays and neutrons are more likely to cause harm that others
such as X-rays. The biological harm caused by radiation is therefore expressed as
the equivalent dose (sometimes also referred to as the radiation weighted dose),
which is derived from absorbed dose by multiplying by the radiation weighting factor:
Equivalent dose = Absorbed Dose Radiation Weighting Factor (1)
The radiation weighting factor is a measure of the relative ability of each particular
type of ionising radiation to do biological damage. This ability to cause harm is
measured relative to X-rays. Thus for X-rays the radiation weighting factor equals 1.0
6
by definition, and the equivalent dose is always numerically equal to the absorbed
dose. For those types of radiation that are inherently more hazardous than X-rays
such as alpha rays and neutrons it is important to include the radiation weighting
factor to accurately express the risks involved. To differentiate between measure-
ments of equivalent dose and those of absorbed dose, the former are quoted in units
called sieverts (symbol, Sv). As with the gray, 1 sievert is a relatively large dose.
Hence in diagnostic radiology, equivalent dose is usually expressed in millisievert
(mSv) (1 mSv equals one thousandth of a sievert). For DXA scans, dose is often so
low that a more convenient unit is the microsievert (µSv) (1 Sv equals one millionth
of a sievert).
Even when we express dose measurements as equivalent dose, we have still not
arrived at a way of measuring dose that is particularly useful for expressing the
radiation risk to patients. The problem is that different types of X-ray examinations
(for example, DXA scans, chest X-rays and skull X-rays) involve exposure of different
parts of the human body, and different organs have different sensitivities to the
harmful effects of radiation. The final step to arrive at a way of measuring radiation
dose that reflects the real radiation risk to patients is to convert the equivalent dose
figures for each organ into the effective dose. This is done by multiplying the
equivalent dose for each organ by a tissue weighting factor proportional to the
sensitivity of that organ to the stochastic effects of radiation, and summing over all
the organs exposed:
Effective Dose = EDT · wT (2)T
where EDT and wT are respectively the equivalent dose and tissue weighting factor
for the Tth
organ. The advantage of using effective dose is that the radiation risk from
any type of radiological investigation can be summarised in a single figure: thus we
can directly compare the radiation hazard to the patient from a DXA scan with a
chest X-ray, a CT scan of the abdomen, a radionuclide bone scan, or any other
investigation that uses ionising radiation.
7
The tissue weighting factors wT are chosen so their sum is unity, i.e.:
wT = 1 (3)Tj
With this in mind it can be seen from Equation 2 that if every organ in the body is
uniformly exposed to the same equivalent dose, the effective dose would equal this
uniform dose. Thus effective dose can be defined as the uniform dose to the whole
body that carries the same stochastic risks to the patient as the given radiological
investigation. Like equivalent dose, effective dose is also expressed in units of
sieverts.
Table 3
Tissue weighting factors from ICRP-60 19
___________________________________________________
Tissue Type Weighting factor (wT)___________________________________________________
Ovaries 0.20
Bone marrow (red) 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Bladder 0.05
Breast 0.05
Liver 0.05
Oesophagus 0.05
Thyroid 0.05
Skin 0.01
Bone surfaces 0.01
Remainder 0.05 *
___________________________________________________
* The following 10 tissues are included in the remainder withweighting factors 0.005 each: adrenal, brain, upper largeintestine, small intestine, kidney, muscle, pancreas, spleenthymus, uterus.
8
The above scheme for calculating effective dose was established by the International
Commission on Radiological Protection (ICRP) and is explained in greater detail in
ICRP Publication 60.19
The list of tissue weighting factors published by the ICRP is
summarised in Table 3. The larger the value of the weighting factor the more
sensitive the organ is to the harmful effects of radiation. Thus X-ray examinations
that involve exposure of the trunk and abdomen are inherently more hazardous
because they include organs such as lung, stomach, colon and bone marrow that
have high tissue weighting factors. In contrast X-ray examinations that involve
exposure of the extremities are less hazardous because they include organs such as
skin, muscle and bone that have low weighting factors. The weighting factors listed in
Table 3 replaced an earlier set published in 1977 20
and are based on updated
information about cancer induction by radiation. In 2005 the ICRP proposed a further
revision of the tissue weighting factors, the main differences being a decrease in the
factor for the gonad dose expressing the genetic risk, and an increase in the factor
for breast tissue reflecting the breast cancer risk.21
Patient doses from DXA
Effective dose is the preferred method of specifying patient dose from DXA
investigations because it relates directly to the radiation risk involved. A number of
studies have published figures for Hologic1,3,4,5,10,12,13,15,17,18
and GE-Lunar2,6,7,8,9,11,13
DXA systems and results are summarised in Tables 4 to 6. The most common
method of estimating the effective dose is to scan a human shaped phantom
containing thermoluminescent (TLD) dosimeters to measure the organ doses and
then calculate the effective dose using Equation 2.
There is general agreement that the effective dose for a spine and hip DXA
examination is very small. For current models of DXA scanner it is generally between
1 µSv and 10 µSv depending on the make, model and scan mode used. Effective
doses for the GE-Lunar Prodigy system (1.4 µSv for spine and hip using the
Standard scan mode) are lower than for the Hologic Discovery (7.5 µSv using the
Express scan mode). Figure 1 shows these DXA doses compared with some other
frequently performed X-ray examinations.22 One useful way of explaining the doses
9
given by different types of X-ray examination is to compare them in terms of the
length of time one must be exposed to natural background radiation to give the same
dose. We are all unavoidably exposed to natural sources of radiation, for example
cosmic rays from outer space and naturally occurring radioactive isotopes in the
environment and our own bodies. Together these give a cumulative effective dose of
about 2.5 mSv per year, equivalent to 7 Sv a day. Some more complicated
examinations such as CT scans give the equivalent of several years exposure to
natural background radiation, while a spine and hip DXA examination gives the
equivalent of 1 day or less.
Figure 1: Comparison of the effective dose to the patient from a spine and hip DXA examination on a GE-Lunar Prodigy (using the Standard mode) and a Hologic Discovery (using the Express mode) with some other common radiological examinations. Doses for other procedures are taken from Wall and Hart22, Kalender3
and the Administration of Radioactive Substances Advisory Committee Notes for Guidance.
10
Table 4
Effective doses for DXA spine and hip examinations in adults for different makes, models and scan modes. 6,9,11,18
___________________________________________________________________
DXA Model Scan mode PA Spine (Sv) Hip (Sv)___________________________________________________________________
GE-Lunar DPX Medium 0.2 0.15
GE-Lunar Prodigy Thin 0.2 0.15
GE-Lunar Prodigy Standard 0.8 0.6
GE-Lunar Prodigy Thick 1.6 1.2
GE-Lunar Expert 5 mA fast 59.0 56.0
Hologic QDR1000 Quick 1.3 0.9
Hologic QDR1000 Performance 2.6 1.8
Hologic Discovery Express 4.4 3.1
Hologic Discovery Fast 6.7 4.7
Hologic Discovery Array 13.3 9.3
___________________________________________________________________
Effective doses for spine and hip examinations performed on the GE-Lunar Prodigy
and the Hologic Discovery are summarised in Table 4 together with figures for some
older models of DXA scanner. The lowest dose is for the Lunar DPX, an older type of
pencil-beam machine for which a spine scan gives a dose of 0.2 µSv. At the other
extreme, doses for the Lunar Expert fan-beam system are more than 100 times
greater than the DPX with a dose of 60 µSv for a spine scan,7,8,13
equivalent to
around 3 chest X-rays. The higher doses for examinations on the fan-beam models 4,6,10,11,13,16,18
may relate in part to the improvement in image resolution. Some
centres also perform whole-body DXA scans and Table 5 lists effective dose figures
for this examination.
11
Table 5
Effective doses for DXA total body examinations in adults for different makes and models. 4,9,11,18
___________________________________________________________________
DXA Model Total body (Sv)___________________________________________________________________
GE-Lunar DPX < 0.1
GE-Lunar Prodigy 0.1
GE-Lunar Expert 75.0
Hologic QDR1000 4.6
Hologic Discovery-A 4.2
Hologic Discovery-W 8.4
___________________________________________________________________
Relatively few studies have been published of the radiation dose to children from
paediatric DXA examinations.9,17,18 If children are scanned using the ordinary adult
scan modes they will receive a higher dose than adults because their thinner bodies
means that their internal organs receive less protection from the attenuation of X-
rays by overlying tissue. Also, if the scan area used for paediatric scans is the same
physical area as for adults a relatively larger proportion of the child’s body is exposed
to the X-ray beam.18 Table 6 lists some DXA doses in children that can be compared
with the adult figures in Tables 4 and 5. The software on GE-Lunar systems includes
paediatric scan modes that keep child doses low by using a lower X-ray tube current.
Njeh et al. studied the effective dose to 5- and 10-year-old children scanned using
the paediatric modes on the Lunar DPX-L system9 and found figures only marginally
greater than for adult scans.6 Hologic machines do not have paediatric scan modes
and as a result spine and hip doses are several times larger for children compared
with adults. The Hologic spine and hip data in Table 6 assume that the appropriate
scan length is set in children so that scan acquisitions are not allowed to run for the
default adult scan length. For paediatric scans performed on the Hologic Discovery
the radiation dose should be kept as low as possible by using the Express mode
rather than the Fast or Array modes, and by making sure that the scan is stopped
12
before it runs for the adult scan length.18 It is worth noting that even if the effective
dose is the same for children and adults (as with GE-Lunar models), children still
face a risk of injury that is up to 3-times higher than adults.19 This is because they
have their full life expectancy before them, and because growing tissues are more
sensitive to the harmful effects of radiation, and the calculation of effective dose
does not take these factors into account.
Table 6
Effective doses for paediatric DXA examinations for a 5 y old and a 10 y old child
___________________________________________________________________
DXA System Scan mode Child aged 5 y Child aged 10 y___________________________________________________________________
GE-Lunar DPX (a) Spine 0.28 Sv 0.20 Sv
Total body 0.03 Sv 0.02 Sv
Hologic Discovery (b) Express spine 9.1 Sv 7.1 Sv
Express hip 7.4 Sv 5.9 Sv
Total body 5.2 Sv 4.8 Sv
___________________________________________________________________
(a) Data from Njeh et al. 11
(b) Data from Blake et al. 18
Patient dose from DXA scans of the peripheral skeleton are exceptionally low. Lewis
et al. reported an effective dose of 0.07 µSv for a forearm scan on a Hologic QDR-
1000,4 while Patel et al. found a similar figure for forearm studies performed on the
Osteometer DTX-200 peripheral DXA system.14
Doses for peripheral DXA scans are
particularly low because: (1) the thinner thickness through the limb compared with
the trunk means that the X-ray beam intensity required is less; (2) the scan area is
generally smaller than for spine and hip examinations; (3) the scan area does not
generally include any of the more radiosensitive tissues with higher weighting factors.
13
Occupational Doses from DXA
The occupational dose to DXA scan operators arises because as the X-ray beam
passes through the patient’s body some X-ray photons are scattered out of the beam
and irradiate the whole room including the operator. Allowing for factors such as how
close the operator is seated to the patient during scanning, there is a relationship
between patient dose and operator dose because the more X-rays pass through the
patient’s body the more scattered photons are produced. Because patient dose is so
low DXA is a relatively safe technique for operators too, and those scanner models
that give lower dose to patients will generally also give lower dose to the operator.
Several studies6,10,11,16,23
have evaluated the occupational dose to staff performing
DXA investigations. In the United Kingdom, under the revised Ionising Radiations
Regulations (IRR 1999) the maximum permitted annual dose for a non-classified
radiation worker is 6 mSv.24
Assuming a working year of 2000 hours (8 hours/day 5
days/week 50 weeks/year) this corresponds to an average dose of 3 µSv/hour in
the work area. If dose rates around the DXA scanning table approach this limit, then
the work area should be defined as a Controlled Area. In this circumstance the
Ionising Radiations Regulations require the employer to appoint a suitably trained
member of staff as the Radiation Protection Supervisor, staff doses should be
monitored using a film badge, and Local Rules should be written that outline safe
methods of working. For members of the public (i.e. people not working with
radiation) the annual dose limit set by the regulations is 1 mSv/year. If this lower limit
is translated into the work place, 1 mSv/year corresponds to 0.5 µSv/hour in the work
area. Experience with monitoring of occupational dose levels in Radiology and
Nuclear Medicine Departments shows that in practice this lower figure of 0.5
µSv/hour is readily achievable for the large majority of hospital staff working with
radiation. There is therefore no reason why the occupational dose of staff involved
with a relatively safe technique like DXA scanning should exceed 1 mSv/year.
Results of monitoring dose at a distance of 1 metre from the centre of the scanning
table for different models of DXA systems are shown in Figure 2 where they are
compared with the Controlled Area limit (equivalent to 3 Sv/hour) and the lower
figure of 0.5 Sv/hour set by the dose limit to members of the public. The data shown
14
in Figure 2 assume that the operator is scanning 2 patients an hour for the pencil-
beam machines and 4 patients an hour for the fan-beam systems (4000 to 8000
patients a year, which represents a relatively high work load). For the two pencil-
beam systems (GE-Lunar DPX and Hologic QDR1000), the dose to staff is extremely
low even with the operator sat as close as 1 metre from the patient during scanning.
For fan-beam devices, however, the dose is higher and if the number of patients
approaches the full capacity of the system, occupational exposures may approach
the dose limit of 3 Sv/hour set by the Ionising Radiations Regulations. The simplest
way to reduce the occupational dose is to take advantage of the inverse square law
and position the operator further away from the patient during scanning. With the
operator sat 2 metres from the patient, the occupational dose is one-quarter of that
shown in Figure 2, and this is the working arrangement recommended for
osteoporosis centres using fan-beam DXA machines.
Figure 2: Comparison of the time averaged scatter dose to an operator positioned 1 metre from the centre of the scanning table for different models of DXA scanner. Results show the mean equivalent dose per hour assuming that 2 patients/hour are scanned on the pencil-beam systems (GE-Lunar DPX and Hologic QDR-1000) and 4 patients/hour on the fan-beam systems (GE-Lunar Prodigy, GE-Lunar Expert-XL and Hologic Discovery). Regulatory limits were taken from the United Kingdom revised Ionising Radiations Regulations
24 (Environmental dose data from Patel et al.
10,23).
15
Radiation Safety Checks at Installation of a New DXA System
If you are installing a new DXA scanner or replacing an older machine, it is important
to inform your local Radiation Protection Adviser early in the planning process.
Advice should be sought on where to site the scanner and the writing of local rules
on radiation safety. Although very compact room designs are possible (Figure 3),
these are not desirable unless the patient workload is very light. With the room layout
shown in Figure 3, the operator is about 1 metre from the patient during scanning. As
discussed above, with older pencil-beam models such as the Lunar DPX
occupational doses were extremely low and a compact room design such as this was
acceptable. With fan-beam systems, however, greater care is needed with room
layout to ensure the safety of staff and rooms should be large enough to ensure that
the operator’s console can be placed a least 2 metres from the patient. If this is not
possible then it may be advisable with some DXA models to consider the installation
of a lead-plastic radiation barrier to protect the operator.
Figure 3: Room layout from the manufacturer's data sheet for the Hologic Discovery.
16
After installation and before any patients are scanned, measurements should be
made of the dose from scattered radiation (to protect the operator) and entrance
surface dose (to protect the patient). An electrical safety check should also be made.
Because of the low levels of radiation involved, writing Local Rules is straightforward.
Some points that should be included are listed in Table 7. Note that in the United
Kingdom all staff operating medical equipment delivering ionising radiation must
have an adequate knowledge of the hazards of radiation and safe working practice.25
For radiographers this requirement is met as part of their professional training, but
staff from other professional backgrounds should attend an IRMER training course.
Table 7
Some Radiation Protection Requirements for Bone Densitometry___________________________________________________________________
When planning a new installation, the Radiation Protection Adviser should be consulted
If staff operating equipment do not have the appropriate professionalqualification they must attend an IRMER course
When not in use, equipment producing X-rays should be protected againstmisuse by switching off, locking, and placing the key in a secure place
Operators should never expose themselves to the primary X-ray beam
During a scan, only the patient should be within the Controlled Area limit *
The PC monitor and operator's desk should be placed well outside theControlled Area limit.* For pencil beam axial DXA systems this means atleast 1 metre away, and for fan-beam systems at least 2 metres away.If this is not possible, a radiation barrier should be installed.
___________________________________________________________________
* The Controlled Area is the region within which the time averaged dose rate exceeds 3 Sv/hour
17
The Ionising Radiations Regulations (IRR 1999 and IRMER 2000)
The Ionising Radiations Regulations (IRR 1999) and the Ionising Radiations (Medical
Exposure) Regulations (IRMER 2000) were introduced in the United Kingdom in
1999 and 2000 respectively,24,25
replacing older regulations that have applied since
the 1980's. This section reviews the ways in which the new regulations should be
implemented in a department providing a bone density scanning service.
A major reason for introducing the IRR 1999 regulations was to give effect to the
lower dose limits for radiation workers and members of the public recommended in
ICRP Publication 60.19 Annual dose limits for non-classified workers were reduced
from 15 mSv to 6 mSv for staff and from 5 mSv to 1 mSv for members of the public.
As emphasised above, scatter dose from bone densitometry equipment is mostly
very low, and the majority of operators should receive doses well below the new
limits. However, these changes emphasise the need for care in the operation of
some types of fan-beam DXA systems to ensure that operator dose is well below the
new Controlled Area limit (Figure 2). For equipment giving higher doses it will be
necessary to define the scanning table and the immediate surrounding area as a
Controlled Area. Advice should be sought from the hospital Radiation Protection
Adviser, who if necessary can help with performing radiation measurements and
advise on drawing up appropriate Local Rules and appointing a Radiation Protection
Supervisor.
The new IRMER regulations 25
came into effect in 2000 and replace the older
POPUMET regulations that required staff without the relevant professional training in
the use of radiation to attend a Core of Knowledge course. The revised regulations
also lay down requirements for appropriate training, and in addition require staff to
undertake continuing professional development to demonstrate that their knowledge
is up-to-date. As mentioned above, staff operating a DXA scanner without previous
professional training in the use of ionising radiation (for example, a nurse) are
required to attend a suitable course. This will need to include aspects of radiation
protection relevant to their duties as an operator (see below) as well as their specific
area of practice (in this case, bone densitometry).
18
A central aspect of the IRMER 2000 regulations is that they set down a requirement
for the employer (usually an NHS Trust) to provide a framework for the safe use of
radiation. The duties of the staff involved are divided into three roles, Practitioners,
Referrers and Operators. The Practitioner (often the clinician in charge of the Unit)
has a duty to ensure that the diagnostic information derived from each examination
justifies the risk entailed in exposing the patient to radiation and should scrutinise
and authorise all scan requests. In practice for straightforward procedures such as a
bone density scan, the duty of authorising scan requests can be delegated to the
Operator provided this is done under a written protocol. This might specify, for
example, that scan requests are accepted provided that they meet the Royal College
of Physicians Guidelines for the diagnostic use of bone densitometry.26 Referrers
have a duty to provide sufficient clinical information about a patient on the request
form to enable the Practitioner to justify the scan. If this information is not provided
the request card should be returned to the referrer. The Operator (usually a
Radiographer or Technologist) has a duty to perform the scan in a safe manner for
the patient. These duties include following a set procedure for patient identification,
making enquiries of patients of child bearing age whether they might be pregnant,
and performing studies in a safe manner according to the written procedures of the
Unit. The points included above are summarised in Table 8. Further information can
be found in articles in professional journals and on the Department of Health web
site.27
19
Table 8
Suggested action points for implementing the revised Ionising Radiations Regulations 24,25,27
___________________________________________________________________
IRR 1999
Consult your Radiation Protection Adviser and review whether it is necessaryto define a Controlled Area around your DXA scanner
Review your Local Rules to ensure they are up-to-date and appropriate. Ensurethat a member of staff is appointed to the role of Radiation Protection Supervisor
Ensure that proper instrument QC procedures are in place for scanning equipment
IRMER 2000
Ensure that all members of staff operating DXA scanning equipment have either an appropriate professional qualification or have attended an appropriate course
Ensure that the members of staff fulfilling the role of Practitioner and Operator are defined and that all requests for DXA studies are authorised against a set of appropriate criteria
Ensure that proper procedures are in place for patient identification and forenquiring of patients of child bearing age whether they might be pregnant
Ensure that the types of study being performed are covered by written procedures for the operator
Ask your Medical Physics Department to measure the radiation output of yourscanner and ensure that it is within limits for the make and model
___________________________________________________________________
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References
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