Impacts of Geant4 hadronic physics models on secondary particle productions in proton therapy simulations

Monte Carlo simulations using the Geant4 toolkit are widely used in proton therapy to predict the dose distribution and secondary particle production. The choice of physics models used in the simulation can greatly affect the accuracy of the results. However, general hadronic models in Geant4 are not specifically tuned for medical physics regions and available experimental data are still limited. In this study, we investigated three different Geant4 hadronic physics models: BIC, BERT, and INCL++, by calculating the yields and kinematical distribution of the secondary neutron, gamma, and positron emitters as well as their incident energy dependence. The simulations were performed for a water phantom irradiated with 70 – 250 MeV proton beams. Our analysis revealed significant differences in the yields, angular, and energy distributions of emitted secondary particles between the three models. We also found a systematic underestimation of yields for the positron emitter 11C in the recent version 10.7 of Geant4. Overall, our study highlights the importance of carefully selecting a hadronic physics model for Geant4 simulations in proton therapy. Our findings also emphasize the need for a specifically tuned Geant4 hadronic model for proton therapy applications in order to consistently reproduce a wide range of important observables. More experimental data for proton-induced reactions in human tissue are critically needed to constrain and validate the suitable physics models for proton therapy simulation.