Comparison of Geant4 multiple Coulomb scattering models with theory for radiotherapy protons

TL;DR

This study compares Geant4's multiple Coulomb scattering models with theoretical predictions for radiotherapy protons, revealing that the Wentzel model better matches the theory for outgoing angle distributions in water-like materials.

Contribution

It provides a theoretical validation of Geant4 MCS models, highlighting the Wentzel model's superior accuracy over Urban for specific scattering predictions.

Findings

Wentzel model outperforms Urban in angle distribution accuracy

Both models are similar in beam spreading within the target

Theoretical comparison reduces experimental uncertainties

Abstract

Usually, Monte Carlo models are validated against experimental data. However, models of multiple Coulomb scattering (MCS) in the Gaussian approximation are exceptional in that we have theories which are probably more accurate than the experiments which have, so far, been done to test them. In problems directly sensitive to the distribution of angles leaving the target, the relevant theory is the Moliere/Fano/Hanson variant of Moliere theory. For transverse spreading of the beam in the target itself, the theory of Preston and Koehler holds. Therefore, in this paper we compare Geant4 simulations, using the Urban and Wentzel models of MCS, with theory rather than experiment, revealing trends which would otherwise be obscured by experimental scatter. For medium-energy (radiotherapy) protons, and low-Z (water-like) target materials, Wentzel appears to be better than Urban in simulating the distribution of outgoing angles. For beam spreading in the target itself, the two models are essentially equal.