Dosimetric evaluation of light ion beams for spatially fractionated radiation therapy: a Geant4 Monte Carlo study

Spatially fractionated radiation therapy (SFRT) is an approach that spares healthy tissue compared to conventional radiation therapy. Light ions have also more advantages over heavy charged particles and X-ray beams including physical and radiobiological aspects. The composition of SFRT, particularly minibeam radiation therapy (MBRT), with these privileges could improve the therapeutic index. Monte Carlo simulations were performed by Geant4 (Geant4-11.0.1) to evaluate the radiation of broad beam, single, and arrays of proton and light ion minibeams in a water phantom. Several minibeam sizes and center-to-center (ctc) distances were selected. The contribution of different secondary species, peak and valley doses, peak-to-valley dose ratio (PVDR), and Bragg peak-to-entrance dose ratio (BEDR) was studied. Light ion minibeams have higher PVDR in normal tissues, more BEDR, and even were broadened more slowly compared to protons in same ctcs. Reduced lateral scattering for heavier ions than protons creates sharper peaks and lower valley doses. A higher ctc (3.5 mm) can enhance normal tissue-sparing due to its higher PVDR and cause lower contribution of secondary fragments, but dose conformity is more complicated in the target region for heavier ions. This non-uniformity can be reduced when ctc decreases (1.2 mm), while perfect normal tissue-sparing could not be achieved. Although the contribution of nuclear products is enhanced with atomic number of incident ions, dominant dose deposition occurs at deeper depths in valleys. These results highlight that light ions fulfill advantageous dose profiles and might be good candidates for MBRT.