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Gold nanoparticles (GNPs) have shown a potential as a radiosensitizer in radiotherapy. The radiosensitization effect is thought to be linked to the increased dose deposition around the GNP. Monte Carlo simulations have been implemented for the calculation of the dose distributions around the GNPs and have been used for the calculation of the dose enhancement. They have also been imported to radiobiological models to predict biological endpoints. This work assessed the implications of different physical interaction models on the dose distribution and dose enhancement of GNPs surrounded by water under proton irradiation. The Penelope and Livermore physical interaction model implementation of the Geant4 simulation toolkit were compared considering the following parameters: i) GNP size, ii) proton energy, and iii) alternative physics model parameters in the gold or water medium. We found that neither the dose distribution nor the dose enhancement is sensitive to the model selection after the first 100 nm from the GNP surface. Within the first 100 nm the Livermore models calculated a higher dose, attributed to a higher production of low energy secondary electrons inside the GNP.

Geant4 is a multipurpose Monte Carlo simulation tool for modeling particle transport in matter. It provides a wide range of settings, which the user may optimize for their specific application. This study investigates GATE/Geant4 parameter settings for proton pencil beam scanning therapy. Methods: GATE8.1/Geant4.10.3.p03 (matching the versions used in GATE-RTion1.0) simulations were performed with a set of prebuilt Geant4 physics lists (QGSP_BIC, QGSP_BIC_EMY, QGSP_BIC_EMZ, QGSP_BIC_HP_EMZ), using 0.1mm-10mm as production cuts on secondary particles (electrons, photons, positrons) and varying the maximum step size of protons (0.1mm, 1mm, none). The results of the simulations were compared to measurement data taken during clinical patient specific quality assurance at The Christie NHS Foundation Trust pencil beam scanning proton therapy facility. Additionally, the influence of simulation settings was quantified in a realistic patient anatomy based on computer tomography (CT) scans. Results: When comparing the different physics lists, only the results (ranges in water) obtained with QGSP_BIC (G4EMStandardPhysics_Option0) depend on the maximum step size. There is clinically negligible difference in the target region when using High Precision neutron models (HP) for dose calculations. The EMZ electromagnetic constructor provides a closer agreement (within 0.35 mm) to