In proton therapy, most treatment planning systems (TPS) use a fixed relative biological effectiveness (RBE) of 1.1 all along the depth-dose profile. Innovative TPS are now investigated considering the variability of RBE with radiation quality. New TPS need an experimental verification in the quality assurance (QA) routine in clinics, but RBE data are usually obtained with radiobiological measurements that are time consuming and not suitable for daily QA. Microdosimetry is a useful tool based on physical measurements which can monitor the radiation quality. Several microdosimeters are available in different research institutions, which could potentially be used for the QA in TPS. In this study, the response functions of five detectors in the same 62-MeV proton Spread Out Bragg Peak is compared in terms of spectral distributions and their average values and microdosimetric RBE. Their different response function has been commented and must be considered in the clinical practice.

Centre for Medical Radiation Physics University of Wollongong 2522 Wollongong Australia

ELI Beamlines Center Institute of Physics Czech Academy of Sciences 252 41 Dolní Břežany Czech Republic

INFN Laboratori Nazionali del Sud 95125 Catania Italy

INFN Laboratori Nazionali di Legnaro 35020 Legnaro Italy

INFN Milano 20133 Milano Italy

INFN Roma2 Dipartimento di Ingegneria Industriale Università di Roma Tor Vergata 00133 Roma Italy

Politecnico di Milano Dipartimento di Energia 20156 Milano Italy

Trento Institute of Fundamental Physics and Applications 38123 Povo Trento Italy

University of Trento Dipartimento di Fisica 38123 Povo Trento Italy

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