Analytical Models of Neutron Spectral Fluence, Kerma and Absorbed-Dose for Proton Therapy

Abstract

Purpose: The accurate prediction of stray neutron dose has become increasingly important as it increases the risk of second cancer development after proton therapy. Previously reported analytical models predicted the quantity dose equivalent, which includes physical and biological considerations but does not explicitly take into account material dependence and variation in the radiation quality. The purpose of this study was to investigate the feasibility of an analytical model of absorbed dose to water from stray neutrons in proton therapy. Methods: To calculate neutron absorbed dose and kerma in water, the authors developed analytical models of neutron spectral fluence and kerma coefficients, used neutron spectral fluence and absorbed dose data from Monte Carlo simulations, and evaluated neutron kerma coefficients from the literature. Results: The analytical model predictions of absorbed dose to water agreed relatively well with that from Monte Carlo simulations. On average, the percentage difference between the analytical model and Monte Carlo simulations was 49 percent for absorbed dose to water for the proton beam energies ranging from 120 MeV to 250 MeV. Conclusion: The results suggest that it is feasible to analytically model absorbed dose to water from stray neutrons with good accuracy. A potentially important advantage of this fluence-based approach is that it provides the ability to take material dependence into account and helps with characterization of radiation quality.