Monte Carlo derivation of beam quality correction factors in proton beams: a comparison of Geant4 versions

Abstract Objective. In the revised version of the TRS-398 Code of Practice (CoP), Monte Carlo (MC) results were added to existing experimental data to derive the recommended beam quality correction factors ( k Q ) for ionisation chambers in proton beams. While part of these results were obtained from versions v10.3 and v10.4 of the Geant4 simulation tool, this paper demonstrates that the use of a more recent version, such as v11.2, can affect the value of the k Q factors. Approach. The chamber-specific proton contributions ( f Q ) of the k Q factors were derived for four ionisation chambers using two different versions of the code, namely Geant4-v.10.3 and Geant4-v11.2. A comparison of the total absorbed dose values is performed, as well as the comparison of the dose contribution for primary and secondary particles. Main results. Larger absorbed dose values per incident particle were derived with Geant4-v11.2 compared to Geant4-v10.3 especially for dose-to-air at high proton beam energies between 150 MeV and 250 MeV, leading to deviations in the k Q values up to 1%. These deviations are mainly due to a change in the physics of secondary helium ions for which the significant deviations between the Geant4 versions is the most stringent within the entrance window or the shell of the ionisation chambers. Significance. Although significant deviations in the MC calculated f Q values were observed between the two Geant4 versions, the dominant uncertainty of the W air values currently allows to achieve the agreement at the k Q level. As these values also agree with the current data presented in the TRS-398 CoP, it is not possible at the moment to discriminate between Geant4-v10.3 and Geant4-v11.2, which are therefore both suitable for k Q calculation.