Purpose: Nuclear reactions induced during high-energy radiotherapy produce secondary neutrons that, due to their carcinogenic potential, constitute an important risk for the development of iatrogenic cancer. To quantify this risk, the ICRP neutron weighting factors may be applied; however, these factors are based on a pooling of experimental RBE data and are thus not representative of any given situation. We aim to build on previously-reported neutron microdosimetry studies by applying a weighted track-sampling approach for the determination of dose-mean lineal energy (yD) in order to help improve a bottom-up model of neutron RBE.
Methods: We have constructed a pipeline to explicitly calculate yD that consists of (a) the open source Monte Carlo (MC) toolkit Geant4, (b) its radiobiological extension Geant4-DNA, and (c) a weighted track-sampling algorithm. This approach was used to evaluate the yD of monoenergetic neutrons between 1 eV and 10 MeV at multiple depths in the ICRU sphere. A variety of scoring volumes representative of different biological targets were considered. To obtain a measure of RBE, the neutron yDs will be compared to those calculated for 250 kV x-rays.
Results and Conclusions: Graphs of neutron RBE as a function of neutron energy, based on yD ratios, will be shown for each situation using the combined contributions of secondary electrons, protons, and alphas. The shape of the yD graphs closely follows that of the ICRP weighting factors. Extension to nanometre-scale volumes had little effect on the shape of the graphs but altered the magnitude of the most prominent peak.