Objective: The ARPANSA Medical Standards Linac was installed in 2009 with the purpose of establishing a direct megavoltage calibration service, instead of relying on calibrations against ⁶⁰Co. The linac is an Elekta Synergy Platform. We calculate the error introduced when using a calibration factor from this linac on a Varian high energy platform beam at 6 MV.
Methods: One of the ARPANSA 6 MV beams was approximately matched to the Varian high energy platform 6 MV beam. The electron beam energy was adjusted to match, as closely as possible, the Percentage Depth Dose (PDD) curve and TPR_20,10. Monte Carlo models of the Varian and matched Elekta accelerator accurately predict the measured PDDs and profiles, but show significantly different energy spectra which result from differences in target thickness between the two accelerators.
Results: Monte Carlo modelling of the energy correction factor k_Q of a secondary standard NE2561 chamber shows a difference of 0.4 % between the Varian and Varian-matched Elekta accelerator. Although small, this is significantly higher than the desired accuracy of 0.1%.
Conclusions: Ionisation chambers calibrated on the ARPANSA 6 MV beam will be in error by 0.4% when used on a Varian 6 MV beam. Similar variations may occur with other manufactures. ARPANSA is investigating precise matching to a Varian beam with a custom designed flattening filter.