1. Adapting A Clinical Medical Accelerator For Primary Standard Dosimetry
Radiotherapy Section, ARPANSA
Duncan Butler, Andrew Cole, Ramanathan Ganesan, Peter Harty, Jessica Lye, Chris Oliver, Viliami Takau, David Webb, Tracy Wright

2. Talk Outline Radiotherapy Calibration
Current Practice (60Co)
Australian Primary Standard
Using a medical linac for calibrations
Elekta Synergy Linac – technical
Beam Monitoring System
QA – Beam energy, flatness and symmetry
Monte Carlo Model
Direct Calibration Service

3. Radiotherapy About 150 radiotherapy linacs in Australia
50,000 patients per year treated
Large doses (~80 Gy over 6 weeks)
How do we know the dose is right?

4. Calibration of Radiotherapy Ionisation Chambers at ARPANSA
Linac output is measured with ionisation chamber (by physicists in the clinic)
At ARPANSA, these chambers are calibrated at 60Co
– Calibration Factor, ND,W is determined
For clinical MV linac beams, kQ factor is applied

5. External beam radiotherapy
Hospitals calibrate their linac output for radiotherapy with a dosemeter
Electrometer
Ionization chamber

6. Australian Primary Standard
Primary standard is a detector of radiation
Australian Primary Standard
60Co
Monte Carlo

7. Formalism used in the calibration as per IAEA TRS-398
DwQ (zref) = MQ NDCo kQCo with DwQ (zref) - the dose in the users beam quality Q at reference location zref MQ - the corrected ionisation chamber reading NDCo - the absorbed dose to water factor for Cobalt as provided by the calibrating laboratory kQCo - a correction for beam quality difference between Cobalt and the user’s beam ( from IAEA TRS-398 for standard chambers) Estimated relative standard uncertainty of DwQ (zref): 1.5% (k=2)
Energy correction, because
chamber calibrated at 60Co

8. Elekta Synergy Linac Direct Calibration ARPANSA Linac with calorimeter
Typical Therapy Linac
Direct Calibration

9. Technical specifications
Model
Elekta Synergy Platform
Configuration
Power Source
Accelerator
Length
Electron energy
Linear accelerator
Magnetron
Travelling wave
2.5 m
4 – 25 MeV

10. Advantages of using a linac instead of 60Co
Practical/technical reasons – reduces chance of errors
Absolute accuracy (Gy is a Gy) / reduced uncertainty
Patient dose consistency (Australia = US = Europe)

11. 1. Technical Reasons 60Co versus linac MV photons: 60Co
MV linac photons
Spectrum
Two gammas
plus scatter
Bremsstrahlung
Energy
1.3 MeV
1 – 25 MeV
Ave absorbed dose rate
mGy/s
60 mGy/s
Peak dose rate
10 mGy/s
60,000 mGy/s
Type
Continuous
Pulsed
Depth
5 cm
10 cm

12. 2. Reduced uncertainty using a linac (J/kg)
A. Consider ND,w,Q – the calibration factor of ion chamber for clinic beam Q
Co-60 uncertainty in ND,w
u (%)
ARPANSA ND,w,Co-60
0.4
TRS-398 energy correction factor kQ
1.0
Combined standard uncertainty
1.1
MV uncertainty in ND,w
u (%)
ARPANSA ND,w,Qo
0.5
Effect of spectral difference Qo vs Q
0.3
Combined standard uncertainty
0.6

13. 3. Consistency of patient doses
Dose prescriptions come from overseas publications
Australian patient doses need to be consistent with these doses
(regardless of whose doses are more accurate in the absolute sense)
It turns out that using the linac will make Australia closer to the US

14. Problems with using a Linac for primary standard dosimetry!
Need constant beam output monitoring
Beam profile and energy not to change
High accuracy MC model is required for primary standard corrections

15. 1. Beam output – Implemented an External Beam Monitor
Courtesy of Ganesan Ramanathan and Peter Harty
PTW 786 thin-window transmission chamber

16. Comparison: linac internal monitor : 2561 chamber : External Beam Monitor

17. Comparison of internal and external monitors

18. Effect of External Beam Monitor
The external beam monitor is able to reduce the effect of variations in the linac beam output to <0.2%.
The use of the external beam monitor reduces the overall uncertainty in a direct linac calibration.

19. 2. Regular QA Measurements
Courtesy of Andrew Cole
the linac using a sun nuclear daily qa3 ™ ion chamber and diode array.
Beam Characteristic
Ratio of nominal value (%)
σ (± %)
Photon
Electron
Output
100.23
100.16
0.33
0.32
Energy
99.82
100.25
0.66
0.41
Averaged beam variation for Photon and Electron modes.

23. Courtesy Tracy Wright
and Jessica Lye
3. MC Linac Model
Graphite calorimeter – primary standard for absorbed dose to graphite Dg
Dg measured in calorimeter but we need Dw for chamber calibrations
Use ratio of calculated doses ([Dw/Dg]MC)
But is MC accurate enough?

24. Modelling conversion ratio+ – + –
Dw = Dg(meas) x [Dw/Dg]MC
[Dg]MC
[Dw]MC

25. Linac model BEAMnrc/DOSXYZnrc user codes
Electron beam incident on target
All components included

26. Matching a model for primary standards
PDD on central axis match with high accuracy
Recombination correction not constant with depth
PDD matched in two phantom materials
Profiles and horns less important
Criteria
Typical tolerance
Ok for primary standards?
Central region local diff
< 2% 
Penumbra local diff
< 10%
1 mm DTA
Out of field global diff
< 3.5%
PDD local diff beyond dmax
< 1.5 – 2% 

27. PDD matching
Linear fit  difference gradient
y = x – 0.015

28. MC model End Result = [Dw/Dg]MC and uncertainty Source of uncertainty
Combined uncertainty
(%)
Combined uncertainty due to geometry
0.20
Combined uncertainty due to MC model
0.24
Combined statistical uncertainty in Dw/Dg
0.10
Combined relative standard uncertainty in the 6MV [Dw/Dg]MC ratio (k=1)
0.33

29. International comparison of linac dose BIPM.RI(I)-K6
6 MV
10 MV
18 MV

30. Calibration Steps – Field trial now operational
Calorimetry
MC conversion to absorbed dose to water
Calibration of reference chamber
Calibration of user chamber

31. Conclusions
Beam monitoring system reduces output fluctuations to less than 0.2% - suitable for primary standard work.
Standard clinical QA methods used to ensure energy and profile remain constant.
Monte Carlo model of ARPANSA linac used to obtain Dw for calibration procedure.
International comparison shows success of method.
Direct megavoltage calibrations now available with an ND,w,Q uncertainty of 0.6% (k=1)

32. THANK YOU CONTACT ARPANSA
Special thanks to:
Peter Harty
Ganesan Ramanathan
Andrew Cole
Tracy Wright
For use of their slides
THANK YOU
CONTACT ARPANSA
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