Objective. This study aims to perform a comprehensive microdosimetric characterization of a clinical helium ion beam at the Heidelberg ion-beam Therapy center, 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 was also applied to estimate the relative biological effectiveness at 10% survival (RBE₁₀). 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 distribution. The RBE₁₀ values estimated by the two solid-state detectors showed good agreement with the experimental data from clonogenic assays, within the associated uncertainties. 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.

Percentage depth–dose (PDD) distributions are fundamental to characterizing radiation beams in radiotherapy. This review provides an overview of both methods and sensor technologies for measuring PDD in photon, electron, proton, and carbon-ion beams. We summarize conventional dosimetry techniques, including water-phantom scanning with ionization chambers (cylindrical and parallel-plate) and radiochromic film, and discuss their strengths (established accuracy, calibration traceability) and limitations (volume averaging, delayed readout). We then examine emerging sensor technologies designed to improve spatial resolution, speed, and radiation hardness: multi-layer ionization chambers and Faraday cups for one-shot PDD acquisition; scintillator-based detectors (liquid, plastic, and fiber-optic) enabling real-time and high-resolution depth–dose measurements; advanced semiconductor detectors including silicon carbide diodes; as well as novel approaches such as ionoacoustic range sensing for proton beams. For each modality and detector type, we emphasize clinical relevance, measurement accuracy, spatial resolution, radiation durability, and suitability for high dose-per-pulse environments (e.g., FLASH radiotherapy). Current challenges, such as detector response in regions of steep dose gradient, saturation or recombination at ultra-high dose rates, and energy-dependent sensitivity in mixed radiation fields, are analyzed in detail. We also highlight the limitations of each technique and discuss ongoing improvements and prospects for clinical implementation. In summary, no single detector technology fully satisfies all requirements for fast, high-accuracy, high-resolution, radiation-hard PDD measurement, but the integration of emerging sensor innovations into clinical dosimetry promises to enhance the precision and efficiency of radiotherapy quality assurance.