Investigation of the effects of the step size of Geant4 electromagnetic physics on the depth dose simulation of a small field proton beam

Dose distribution, especially the Bragg peaks or percentage depth doses (PDDs) in a small proton field, are strongly influenced by the field size. Simulation is problematic because of the lack of lateral proton equilibrium. The aim of this study is to investigate the influence of the step size of Geant4 on the simulation of the depth dose, and to suggest the best combination of parameters for small-field proton simulation. In this work, the emstandard_opt3, emstandard_opt4 and emlivermore models (with or without lateral displacement models and guard volume) were examined for the wobbling proton beams in a water phantom. The PTSim based on Geant4 was used to simulate the transport and interaction of protons. These simulations were first benchmarked by large-field measurements and then implemented in various small-field simulations in order to investigate the effects of various physical parameters on the depth dose distribution under conditions of non-equilibrium protons. The comparison between measurement and simulation data was carried out on small fields, such as a circular field with a diameter of 10 mm and a square field of 30 × 30 mm2 for three different proton beam energies. The results showed that, at most, the dose deviation between G4EmStandardPhysics models and measurement was up to 9.7%, 11.1% and 14.48% in the plateau area located at about half the beam range for 110, 150 and 190 MeV, respectively. Small-field proton simulations showed that Geant4 v10.04.p02, emstandard_opt3 multiple Coulomb scattering models and with-Guard Volume offer the identical PDD compared to measurements.