Monte Carlo-derived ionization chamber correction factors in therapeutic carbon ion beams.

The accuracy of electromagnetic transport in the GEANT4 Monte Carlo (MC) code was investigated for carbon ion beams and ionization chamber (IC)-specific beam quality correction factors were calculated. This work implemented a Fano cavity test for carbon ion beams in the 100-450 MeV/u energy range to assess the accuracy of the default electromagnetic physics parameters. The Urban and the Wentzel-VI multiple Coulomb scattering (MCS) models were evaluated and the impact of maxStep, dRover, and final range parameters on the accuracy of the transport algorithm was investigated. The optimal production thresholds for an accurate calculation of fQ values, which is the product of the water-to-air stopping power ratio and the IC-specific perturbation correction factor, were also studied. The fQ correction factors were calculated for a cylindrical and a parallel-plate IC using carbon ions in the 150-450 MeV/u energy range. Modifying the default electromagnetic physics parameters resulted in a maximum deviation from theory of 0.3%. Therefore, the default EM parameters were used for the remainder of this work. The fQ factors were found to converge for both ICs with decreasing production threshold distance below 5 μm. The fQ values obtained in this work agreed with the TRS-398 stopping power ratios and other previously reported results within uncertainty. This study highlights an accurate MC-based technique to calculate the combined stopping power ratio and the perturbation correction factor for any IC in carbon ion beams.