Geant4 electromagnetic physics model assessment for RBE-weighted dose calculation of carbon-ion radiotherapy

Abstract

Carbon-ion radiotherapy (CIRT) is being increasingly practiced owing to its advantages over conventional photon-based radiotherapy. Several CIRT-related dose calculation studies considering the biological effects of carbon ions were performed using the Geant4 Monte Carlo toolkit. For accurate RBE-weighted dose calculation in CIRT, the Geant4 physics models used in a simulation code need to be validated. Previous studies have validated the hadronic physics model of Geant4, but a validation study for electromagnetic (EM) physics models is missing. Therefore, we validated the Geant4 EM physics models for carbon RBE-weighted dose calculation. For the validation process in Geant4, the geometry used in the Microdosimetric kinetic model was irradiated by mono-energy beams between 0.1 and 1000 MeV/u of six elements (H, He, Li, Be, B, and C). The imparted dose calculated using various EM physics models is compared with the theoretical value. A G4EMStandardPhysics-based model is confirmed to result in breaking points for RBE-weighted dose calculations, whereas no such breaking points are observed when using our physics model and Geant4-DNA physics model. Because the breaking points observed in the results may induce uncorrected dose calculations, validating the EM physics models is recommended when performing carbon RBE-weighted dose calculation using Geant4.