Monte Carlo Simulation of Microdosimetric Spectra in Hadron Therapy: Mathematical Framework for Predicting Relative Biological Effectiveness in Heterogeneous Tissue Environments

Abstract

In this study, we implement a hybrid Monte Carlo scheme within the Geant4 toolkit to compute microdosimetric spectra in hadron therapy and translate them into estimates of relative biological effectiveness across tissues with differing compositions. Primary particles are transported by condensed history, while secondary electrons and nuclear fragments are followed individually, so that lineal energy distributions, together with the dose mean and frequency mean lineal energy, can be scored in a sphere of one micrometre simulated diameter that matches a tissue-equivalent proportional counter. Spectra are computed for protons, carbon ions and oxygen ions over clinical energies. Rather than presenting the transport scheme as new, we treat the study as a validation of an established methodology assembled into a single spectrum-to-RBE workflow. Simulated spectra reproduce published proportional counter measurements to within 5% in the dose-mean lineal energy for a 150 MeV proton beam and 8% for a 290 MeV per u carbon beam, with reduced chi-square values near unity. The workflow reproduces the expected rise in RBE from about 1.1 in the entrance region to 3.8 at the carbon Bragg peak. We offer the results as an openly specified reference dataset and a transparent method that hadron therapy facilities now emerging in Africa can adopt, test on their own beam lines, and extend.