Microdosimetry at nanometric sites of charged Helium, Carbon and Oxygen beams with an advanced Tissue Equivalent Proportional Counter

The effectiveness of radiotherapy, particularly hadron therapy, is closely tied to the interactions of radiation at the cellular and sub-cellular levels. Understanding the local energy deposition of charged particles is essential for accurately predicting their biological effects. Traditional dosimetric approaches, based on absorbed dose, fail to describe the stochastic nature of energy deposition at micrometric and nanometric scales. This study investigates the micro- and nano-dosimetric properties of Helium, Carbon, and Oxygen ion beams at 62 MeV/u using an advanced Tissue Equivalent Proportional Counter (TEPC). This TEPC, designed to simulate site sizes ranging from 0.5 μm to 25 nm, was placed at various depths across the Bragg peaks of the ion beams at the INFN-LNS facility in Catania (Italy). Results show a clear dependence of microdosimetric distributions on both simulated site size and position across the depth-dose profile. Smaller site sizes shift the distribution toward higher lineal energies, especially at proximal depths, suggesting that microdosimetric spectra at nanometric scale can offer different insights on the radiation interaction with tissue. This study also underlines the role of secondary electrons and fragmentation effects, which vary with the atomic number of the ion, producing different effects for Helium, Carbon and Oxygen ions. These findings may have significant implications for improving relative biological effectiveness (RBE) models in hadron therapy. By extending microdosimetric analysis to the nanometric scale, this research provides new data for a possible improvement of the predictive accuracy of radiation-induced biological effects. The novel TEPC used in this study bridges the gap between microdosimetry and nanodosimetry, offering a more refined assessment of radiation quality.