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

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

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?

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

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

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

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

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

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

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)

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 1 - 10 mGy/s 60 mGy/s Peak dose rate 10 mGy/s 60,000 mGy/s Type Continuous Pulsed Depth 5 cm 10 cm

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

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

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

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

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

Comparison of internal and external monitors

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.

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.

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?

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

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

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% 

PDD matching Linear fit  difference gradient y = -0.008x – 0.015

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

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

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

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)

THANK YOU CONTACT ARPANSA Special thanks to: Peter Harty Ganesan Ramanathan Andrew Cole Tracy Wright For use of their slides THANK YOU CONTACT ARPANSA Email: info@arpansa.gov.au Website: www.arpansa.gov.au Telephone: +61 3 9433 2211 Freecall 1800 022 333 General Fax: +61 3 9432 1835