Radiobiology: A Geant4 Extended Example for voxel-based ion-beam transport and radiobiological endpoints

This work presents radiobiology, an official open-source Geant4 Extended Example providing a lightweight, modular workflow for voxel-based ion-beam transport studies in which dosimetric quantities and radiobiological endpoints are obtained within a single simulation chain.

The application models therapeutic proton and light-ion beams interacting with a voxelized water phantom, configurable via macro commands. Dose and track- and dose-averaged Linear Energy Transfer (LET) are scored voxel-wise using Geant4 reference electromagnetic and hadronic physics configurations, with LET estimates available for primaries only or for the full mixed field including charged secondaries. Radiobiological quantities are computed by coupling Monte Carlo transport to a module based on pre-tabulated linear-quadratic parameters and a Local Effect Model (LEM) implementation. Validation was performed against experimental benchmarks at INFN-LNS, including depth-dose curves measured with a Markus plane-parallel ionization chamber for 62 MeV protons and 62 MeV/u ⁴He, LET-related trends derived from MicroPlus microdosimetric spectra, and RBE estimates compared with clonogenic assay data for MDA-MB-231 cells at mid-SOBP. A phase-space write/replay capability is also introduced.

Simulated depth-dose distributions reproduced the measured range, modulation and distal fall-off for both proton and helium beams. LET-related quantities showed depth-dependent trends consistent with the MicroPlus reference spectra. The LEM/LUT-based workflow yielded RBE values in overall agreement with experimental survival data.

radiobiology fills a practical gap between comprehensive beamline-focused applications and track-structure approaches, enabling rapid and reproducible endpoint-oriented studies in voxelized phantoms.