Investigating the use of

GafChromic® EBT3 films for

clinical beams output audits

Wamied Abdel-Rahman, PhD, FCCPM, DABRConsultant / Chief Radiation Oncology Physicist

Ibrahim Abdelkreem, MScMedical Physicist

Saleh Bamajboor, MSc, MIPEMHead Radiation Therapy Physics

1

07- 09 Nov 2017, 9th Saudi Medical Physics Society Conference, KFMC, Riyadh, Saudi Arabia

This work originated through the Saudi

Medical Physics Society - Radiation

Oncology Physics Group

2

Radiotherapy process

3

Assessment of

patientDecision to

treat Treatment

protocol Positioning and

immobilization Imaging,

contouring,

and simulation

Treatment

planning

Treatment Info.

transferPatient set-up

and plan

verification

Treatment

delivery and

monitoringPost treatment

follow-up

Introduction

• Quality assurance is a corner stone for radiotherapy process.

• Many radiotherapy departments participate in beam output

audits services available throughout the world.

• The auditing bodies may offer their services nationally,

regionally, or internationally.

• In some situations, participation of a radiotherapy centre in

an auditing program is part of credentialing for clinical trials.

4

International

Atomic

Energy

Agency

• Clinical photon beams.

• Based on TLDs.

• Irradiation in a water phantom

under full scattering

conditions.

• Available free of charge.

5

KFSH-D output audit results

history

94%

96%

98%

100%

102%

104%

106%

2008 2009 2011 2013

Measure

d / S

tate

d

Year

600 CD - 6 MV

2300 CD - 6 MV

2300 CD - 18 MV

6

Imaging and

Radiation

Oncology

Core(Radiological Physics Center)

• Used to credential centres

participating in RTOG, GOG,

NCCTG, and NSABP clinical

trials.

• TLDs (past) and OSLD

(present).

• Photon, electron, and proton

beams.

• Free for participating centers.

7

Postal dosmitry

audit in Japan

• Radio-Photoluminescence Glass

Dosimeter (RGD) GD-302M

• Equivalent dosimetric characteristics to

TLDs

• Reduced fading compared to the TLDs

• Has been used to audit photon beams

output for reference condition and

wedged fields since 2008.

• Developed postal dosimetry audit

system using RGD for IMRT.

Advances in Cancer Therapy, Edited by Hala Gali-Muhtasib,

ISBN 978-953-307-703-1,

Radiotherapy

Centres in Saudi

Arabia

• Currently, 14 centres in operation.

• 5 more centres are planning to

open in the next 5 years.

• Many centres are expanding:

- more modern machines

- more exotic beams (Cyber, Tomo,

protons)

• Increase in number of patients!!

9

Objective of this work

Can we establish a simple test to audit

clinical beam outputs throughout the

Kingdom?

10

Materials

and

methods

11

GafChromic® EBT3

• GafChromic® EBT3 is a

radiochromic film that consists

of a single layer of radiation-

sensitive organic microcrystal

monomers between two thin

polyester layers with a

transparent coating.

12

GafChromic® EBT3

• No processing required (self

developing).

• Insensitive to light.

• Sensitive to UV.

• Negligible energy dependency

in the mega-voltage range

13

GafChromic®

EBT3

• Darkness of film increases

with absorbed dose.

unexposed

exposed

14

Transmission measurement

Light sourcesensitometer

Film

Intensity I

15

Optical densityunexposed

exposed

16

T =I

I0

OD = log

10

I0

I

Images are split into

Red, Green, and Blue

components and colour

values are read using

ImageJ software.

Available from:

http://imagej.nih.gov

17

Phase I: Calibration of

GafChromic® EBT3 films

18

Scanner

Epson 10000XL desktop flat-

bed document scanner.

• Transmission mode scanner

• 300 dpi

• 48-bit RGB

19

Unexposed

Exposed

20

Signal is obtained from

averaging the intensity

over 5 regions within

the film.

1

2

3

4 5

~2.5 cm

~2.0

cm

21

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 100 200 300 400

OD

Dose (cGy)

Dose response curve for EBT3 film

22

Used in this work

Film calibration

(irradiation)

• Standard set-up in Solid Water™

• 100 cm SSD

• 10×10 cm2 field size

• depth = zmax

SS

D =

100 c

m10×10 cm2

zmax

source

film

23

Film calibration

(Parametrization)

• An in-house GUI program was developed

for parametrizing the 3 models used to

describe the calibration curve of EBT3

films.

• The software was written using XOJO -

known previously as RealBasic -

programming environment software

(XOJO Inc., Austin, TX, USA).

24

Program interface

25

Film calibration

(Parametrization)

The EBT3 films calibration curve was modelled using the following

relationship:

where b, c, and n are parameters determined from the fit.

The above equations was used with:

1) n = 2

2) n = 3

3) n is variable

26

Dose = bi(OD)+ c i(OD)n

0

100

200

300

400

0 0.1 0.2 0.3

Dose t

o w

ate

r (c

Gy)

OD

b = (627 ± 89) cGy

c = (1495 ± 126) cGy

n = 1.81 ± 0.16

Calibration curve for a GafChromic® EBT3 film in 6 MV beam

(Red channel)

27

Calibration

curves for

GafChromic®

EBT3 films

• Calibration obtained for 2 EBT3 film batches using 7 clinical beams

- 6 MV (3 beams)

- 15 MV (2 beams)

- 18 MV (2 beams)

• Data fitted models:

0

100

200

300

400

0 0.1 0.2 0.3

Dose w

ate

r (c

Gy)

OD

28

Dose = b i(OD) + c i(OD)n

n = 2

n = 3

n = var .

ì

íï

îï

650

750

850

950

1 2 3 4 5

b (

cG

y)

BEAM

1200

1400

1600

1800

1 2 3 4 5

c (

cG

y)

BEAM

bavg = (849 ± 6) cGy

bavg = (737 ± 5) cGy

cavg = (1626 ± 20) cGy

cavg = (1406 ± 24) cGy

Coefficients for fitted model with n = 2

Date in Red is for film batch # 2

Date in Blue is for film batch # 1

29

800

900

1000

1100

1 2 3 4 5

b (

cG

y)

BEAM

Coefficients for fitted model with n = 3

bavg = (979 ± 4) cGy

bavg = (894 ± 3) cGy

3000

3500

4000

4500

1 2 3 4 5

c (

cG

y)

BEAM

cavg = (3902 ± 50) cGy

cavg = (3454 ± 59) cGy

30

Date in Red is for film batch # 2

Date in Blue is for film batch # 1

500

600

700

800

900

1000

1 2 3 4 5

b (

cG

y)

BEAM

Coefficients for fitted model with n = var.

bavg = (931 ± 10) cGy

bavg = (702 ± 13) cGy

1000

1800

2600

3400

4200

1 2 3 4 5

c (

cG

y)

BEAM

cavg = (1536 ± 90) cGy

cavg = (1515 ± 40) cGy

1.5

2.0

2.5

3.0

3.5

1 2 3 4 5

n

BEAM

navg = (2.25 ± 0.07)

navg = (1.91 ± 0.04)

31

Date in Red is for film batch # 2

Date in Blue is for film batch # 1

Coefficients for fitted model (2 add.

Beams)

500

600

700

800

900

1 2 5 6 7

b (

cG

y)

BEAM

1200

1400

1600

1800

2000

1 2 5 6 7

c (

cG

y)

BEAM

1.50

1.75

2.00

2.25

2.50

1 2 5 6 7

n

BEAM

cavg = (1513 ± 53) cGy

navg = (1.91 ± 0.07)

bavg = (716 ± 24) cGy

Film batch # 2

Parameter n = 2 n = 3 Variable n

b (cGy) 849 ± 6 979 ± 4 931 ± 10

c (cGy) 1406 ± 24 3454 ± 59 1515 ± 40

n 2 3 2.25 ± 0.07

Film batch # 1

Parameter n = 2 n = 3 Variable n

b (cGy) 737 ± 5 894 ± 3 702 ± 13

c (cGy) 1626 ± 20 3902 ± 50 1536 ± 90

n 2 3 1.91 ± 0.04

33

Fitted model coefficients

Phase II: Construction of a postal

phantom

34

Phantom material

• Cheap material

• light in weight

• Easily machined

• Water equivalent

• Rigid

Lucite

ρ = 1.18 g/cm3

35

Element Composition by wight (%)

H 8

C 60

O 32

Determination of

water equivalent

depth

• Monte Carlo methods (EGSnrcMP

code) were used to determine the

radiological depth in Lucite equivalent

to 10 cm water.

• dosxyznrc used code with 30×30×30

cm3 water and lucite phantoms.

• Beam spectra for Co-60, 10 MV, 15

MV, and 20 MV were used.

• Equivalent depth depended on energy

and ranged between 9.0 cm (Co-60)

to 9.5 cm (20 MV).

SS

D =

100 c

m

10×10 cm2

10 c

m

source

10×10 cm2

zsource

waterlucite

36

Dose to water

Postal phantom

• Material: Lucite (ρ = 1.18 g/cm3)

• Dimensions: 4×4×20 cm3

• Weight < 0.4 kg

• cost ~600 SAR (not including

material)

37

Determination of postal

phantom correction

factors fpostal

• Monte Carlo methods (EGSnrcMP)

was used to determine a phantom

correction factor that includes:

- reduction in scattered dose

- attenuation differences between

water and Lucite.

• Beam spectra for Co-60 and

15 MV, and full linac modelling for

6 MV and 18 MV beams.

SS

D =

100 c

m

10×10 cm2

10 c

m

source

10×10 cm2

8.9

cm

source

waterlucite

beam fpostal

Co-60 0.905

6 MV 0.920

15 MV 0.941

18 MV 0.964

38

Dose to water

Phase III: Testing the postal

phantom in clinical beams

39

Postal

phantom

(irradiation)

SS

D =

100 c

m

10×10 cm2

8.9

cm

source

lucite

• 300 MU irradiation

• 100 cm SSD

• 10×10 cm2 field size

• 15 clinical beams were used for

testing the phantom.

film

40

SSD calibration

SS

D =

10

0 c

m10×10 cm2

source

10×10 cm28

.9 c

m

source

waterlucite

zmax

Provided by the centre

Calibration (SSD output)

Measured output

(calculated with MC)

Stated output

41

PDD(10 cm,10 ´10 cm2,100 cm)

fpostalPostal irradiation setup

300 MU

SAD calibration

SS

D =

10

0 c

m

10×10 cm2

source

10×10 cm2

8.9

cm

source

waterlucite

zmax

Calibration (SAD)

Postal irradiation setup

300 MU(calculated with MC)

42

10×10 cm2

source

water

Stated output

Provided by the centre PDD(10 cm,10 ´10 cm2,100 cm)

fpostal

ISF =100

100+ zmax

æ

èç

ö

ø÷

2

Measured / statedBatch

BEAM n = 2 n = 3 Variable n

A 1.05 ± 0.02 1.04 ± 0.02 1.04 ± 0.03

1

B 1.03 ± 0.01 1.02 ± 0.01 1.02 ± 0.02

C 1.02 ± 0.03 1.01 ± 0.03 1.02 ± 0.04

D 0.99 ± 0.05 0.98 ± 0.05 0.98 ± 0.06

E 1.00 ± 0.02 0.99 ± 0.02 1.00 ± 0.02

F 0.99 ± 0.03 0.98 ± 0.03 0.98 ± 0.03

G 0.99 ± 0.02 0.98 ± 0.02 0.98 ± 0.02

H 1.04 ± 0.02 1.05 ± 0.02 1.06 ± 0.03

2

I 1.00 ± 0.02 1.02 ± 0.02 1.02 ± 0.03

J 1.01 ± 0.01 1.01 ± 0.01 1.02 ± 0.02

K 1.01 ± 0.03 1.02 ± 0.03 1.03 ± 0.04

L 1.02 ± 0.03 1.03 ± 0.03 1.04 ± 0.03

M 1.02 ± 0.03 1.03 ± 0.02 1.04 ± 0.03

N 0.99 ± 0.03 1.00 ± 0.03 1.01 ± 0.03

O 1.01 ± 0.03 1.02 ± 0.03 1.02 ± 0.03

Results

43

Summary

• The use of GafChromic® EBT3 films for photon beam output

audits was investigated.

• Calibration curves for two batches of the EBT3 films were

established using clinical beams (three 6 MV beams, two 15 MV

beams, and two 18 MV beams).

• A miniature lucite phantom was designed and built for postal

beam output auditing.

• The EBT3 films with the miniature lucite phantom were tested in

15 clinical beams.

44

Conclusions

• GafChromic® EBT3 film is a practical dosimeter to be

used for dose validation in the mega-voltage beams.

• The EBT3 exhibits a negligible energy dependance but

on the other hand they are not linear dosimeters.

• The performance of the EBT3 postal phantom are very

satisfactory for conducting beam output audits.

45

Future work

• Include more radiotherapy centres in the study. More

beams to calibrate the films and more beams to test the

performance of the phantom.

• Improve films handling (enclose films in small black

envelopes).

• Advertise our work for national clinical trials.

• Electrons beams????

• Extend the work to audit a full radiotherapy process.

• and many more ……………..

46

Acknowledgements

• Khalid Hussain

• Sameer Mango

• Ibrahim Arif

• Adel Ghuloom

• Abdelaziz Al-Aradi

• Mukhtar Al-Shanqity

• Saad Al-Delaijan

47

SMPS - ROP Group