Which accelerator photon beams are “clinic‐like” for reference dosimetry purposes?

Abstract

Previous work has demonstrated that, for photon beam dosimetry, TPR20 10 is not an ideal beam quality specifier for all bremsstrahlung beams, especially for lightly filtered beams in some standards laboratories. This paper addresses the questions: Is TPR20 10 an adequate beam quality specifier for all modern clinical therapy ac- celerators? and When can non-clinical beams in standards laboratories be used to calibrate ion chambers or measure kQ factors as a function of TPR20 10? Based on detailed Monte Carlo simulations of Varian, Siemens, Elekta and GE (Saturn) accelerators one can conclude that TPR20 10 is an adequate beam quality specifier for all these machines in the sense that for a given value of TPR20 10, the value of stopping-power ratios is the same. It is shown that, for the heavily filtered beams used in standards laboratories, TPR20 10 is an adequate beam quality specifier. It is also demonstrated that, for a larger range of bremsstrahlung beams than previously, %dd(10)x is a good beam quality specifier for all clinical beams as well as the lightly and heavily filtered beams in some standards laboratories. A criterion, based on the measured values of TPR20 10 and %dd(10)x for the beam, is proposed for determining whether a non-clinical beam is well specified by TPR20 10. Agreement between calcu- lations for specific accelerators and measured beam quality specifiers is shown to be good, but agreement with published data for a variety of clinical accelerators is not as good. Possible reasons for the discrepancy are discussed.