Microdosimetry of hadron therapy beams using mini tissue-equivalent proportional counters

Carbon therapy is increasing worldwide for treating some radioresistant tumours due to its better conformal dose distribution and increased relative biological effectiveness (RBE) in comparison to conventional, photon-based, radiotherapy. The enhancement of the RBE using carbon ion beams is due to the increasing density of ionization (high linear energy transfer (LET)) around the particle track along the penetration depth. Due to the strong dependence of the LET on the kinetic energy of the charged particles and the nuclear interactions, the radiation quality (i.e. particle types and their energy spectra) of these particle beams varies significantly within depth of the irradiated volume. Therefore, a complete characterization of the radiation quality of clinical beams in terms of measurable physical quantities which are related with the biological response at the subcellular scale may lead to a more accurate determination of the RBE and thus more accurate treatment plans. In particular, microdosimetry can be useful for this purpose. In this scientific context, the aim of this PhD research is to study the current issues of experimental microdosimetry for its application to hadron therapy beams such as those found in carbon therapy. The feasibility of miniaturized Tissue-Equivalent gas proportional counters (mini TEPCs), designed and built at INFN Legnaro laboratories, have been investigated for measuring the microdosimetric spectra in a therapeutic carbon ion beam.