Microdosimetric characterization of a clinical helium beam using a MicroPlus-Bridge detector and a diamond detector for RBE assessment

Objective: This study aims to perform a comprehensive microdosimetric characterization of a clinical helium ion beam at the Heidelberg Ion-Beam Therapy Center (HIT), assessing radiation quality along a spread-out Bragg peak (SOBP) and providing experimental data to support the biological effectiveness of helium ions in clinical applications. Approach: Microdosimetric spectra were measured at several water depths within the SOBP using two solid-state detectors: a silicon and a synthetic single-crystal diamond detector. Microdosimetric quantities, including dose-mean lineal energy (y D ) and frequency-mean lineal energy (y F ), were derived. Monte Carlo simulations with the Geant4 toolkit replicated the experimental setup to validate the measured quantities. The modified Microdosimetric Kinetic Model (m-MKM) was also applied to estimate the relative biological effectiveness at 10% survival (RBE 10 ). SAOS-2 and U2OS clonogenic assays were performed under identical irradiation conditions at the same facility to benchmark the RBE predictions. Main results: The experimental results demonstrated a progressive increase in y D values along the beam path, peaking at the distal edge of the SOBP, consistent with the simulated dose-averaged linear energy transfer (LET) distribution. The RBE 10 values estimated by the two solid-state detectors showed good agreement with the experimental data from clonogenic assays, within the associated uncertaintie. Significance: These findings underscore the utility of advanced solid-state detectors for helium beam characterization and highlight the importance of microdosimetry in improving radiobiological modeling. The results provide further evidence supporting the clinical potential of helium ion therapy, particularly for tumors requiring high precision and maximal healthy tissue sparing.