Nano-microdosimetric investigation at the therapeutic proton irradiation line of CATANA

Abstract A TEPC capable of simulating site sizes down to 25 nm was designed and constructed in order to fill the gap between nanodosimetry and experimental microdosimetry. In this paper, this device was proposed for assessing the quality of the therapeutic CATANA (INFN-LNS) proton beam. The detector was irradiated with a modulated 62 MeV proton beam and was placed at different depths across the spread-out of Bragg peak. For each position, several microdosimetric distributions for different simulated site size from 0.5 μm to 35 nm were acquired. The 0.5 μm spectra are in good agreement with spectra measured employing a reference TEPC in the same experimental conditions. The results show that, for bigger sites, microdosimetric distributions follow the slowing down of the primary particles, while, for smaller sites, they exhibit a bimodal shape, in particular at smaller depths in PMMA. The comparison between different simulated volumes shows an increment of the dose-averaged lineal energy with the reduction of the site. Moreover, a comparison between a ratio of the dose-averaged lineal energy for different simulates site sizes and the microdosimetric R B E shows that the smaller is the simulated volume, the better is the resembling of the microdosimetric R B E even if, the agreement is not yet fully reached for any site size. Finally, Monte Carlo simulations show that the electron energy threshold in FLUKA is too high for reproducing microdosimetric distributions for nanometric sites for proton beams.