Microdosimetry investigation of target fragments in proton therapy, assessing the impact of the detector type

Objectives - Proton therapy is increasingly used in oncology due to its enhanced biological effectiveness and precision in targeting tumour tissues. Accurately characterising the radiation quality is, therefore, fundamental. However, the role of fragments in proton therapy remains understudied, mainly due to their low frequency of occurrence and limited impact on the dose. The fragments generated, though, interact differently with biological tissues and may substantially affect the biological response. This work characterises the fragments field generated by a typical clinical proton beam using microdosimetry. Approach - Experiments and Monte Carlo simulations were carried out for this purpose, using a 70.2 MeV proton beam moderated by a lung-equivalent material. Main results - Fragments contributed around 2% to y F and over 60% to y D , underscoring their relevance. The detector structure critically influenced their measurement, highlighting the necessity of detector-specific interpretation of spectra when used for biological modelling. Significance - Our results stress the importance of fragment-aware approaches for accurately assessing radiation quality and predicting biological outcomes in proton therapy.