Validation of nuclear models in Geant4 using the dose distribution of a 177 MeV proton pencil beam

TL;DR

This study validates the nuclear interaction models in Geant4 by comparing simulated dose distributions of a 177 MeV proton pencil beam with experimental data, confirming accurate modeling over a wide range.

Contribution

It provides a comprehensive validation of Geant4 nuclear models for proton therapy dose calculations using experimental data.

Findings

Excellent agreement over five orders of magnitude in dose distribution

Normalizations of depth dose curves slightly lower than experiment, possibly due to positioning errors

Neutron production model verified in distal region

Abstract

A proton pencil beam is associated with a surrounding low-dose envelope, originating from nuclear interactions. It is important for treatment planning systems to accurately model this envelope when performing dose calculations for pencil beam scanning treatments, and Monte Carlo (MC) codes are commonly used for this purpose. This work aims to validate the nuclear models employed by the Geant4 MC code, by comparing the simulated absolute dose distribution to a recent experiment of a 177 MeV proton pencil beam stopping in water. Striking agreement is observed over five orders of magnitude, with both the shape and normalisation well modelled. The normalisations of two depth dose curves are lower than experiment, though this could be explained by an experimental positioning error. The Geant4 neutron production model is also verified in the distal region. The entrance dose is poorly modelled, suggesting an unaccounted upstream source of low-energy protons. Recommendations are given for a follow-up experiment which could resolve these issues.