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Hadron therapy is increasing worldwide for treating some radio-resistant tumours due to its more advantageous depth dose deposition, less lateral spread and better sparing of healthy tissues close to the irradiated target as compared to photon or electron beams. Besides these favourable physical properties, ion beams offer potential biological advantages over protons making them even more suitable for the treatment. The enhancement of the relative biological effectiveness (RBE) of fast carbon ions is due to the high ionization density along the penetration depth. Furthermore, the ion interaction with tissue causes the fragmentation of the projectile and of the target along the penetration path, which results in a complex radiation field. Therefore the radiation quality (i.e. particle type and energy of the radiation field) varies significantly with depth of the irradiated volume. A characterization of the radiation biological effectiveness of the clinical beam in terms of measurable physical quantities at the subcellular scale could be useful for optimizing treatment plans. Microdosimetry can be useful for this purpose, because it studies the probability distributions of the imparted energy when a single ionizing particle crosses a micrometre sized site (the size of a chromosome). Tissue-Equivalent gas proportional counters (TEPCs) are the reference microdosimeters in experimental microdosimetry. The microdosimetric measurements can be used to assess the RBE of the radiation by linking the physical parameters with the corresponding biological response [1]. Due to the high particle fluence rate of therapeutic beams only miniaturized TEPCs of the order of 1 mm3 can be employed to minimize the signal pile-up effects when exposed to these high intensity beams. The mini TEPC, designed and built at INFN Legnaro laboratories, has successfully measured in low-LET therapeutic proton beams in the past [2]. However, the assumption that this device could also be used in carbon beams is not straightforward since high-LET radiation can give rise to large electronic avalanches, which can produce some distortions on the measured microdosimetric spectra. The aim of this research, in collaboration with the group of INFN-LNL, is to contribute to the development of mini-TEPCs for measuring the radiation quality of charged particle beams namely carbon ions. To this end, the response of the mini TEPC was characterized mainly with experimental measurements in known radiation fields but also with the general-purpose Monte Carlo code FLUKA. All the physical parameters affecting the measured microdosimetric spectrum such as gas multiplication characteristics, the calibration procedure, the detector's geometry, the simulated site size and the gas filling type have been carefully studied during this project [3]. Then, the first microdosimetric measurements with the mini TEPC at the Italian therapeutic carbon-ion beam (Centro Nazionale di Adroterapia Oncologica, CNAO) were performed with monoenergetic carbon ions [4] proving its feasibility to measure the radiation quality at various depths in a water phantom.
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Chiriotti, S., Microdosimetry of hadron therapy beams using mini tissue-equivalent proportional counters. PhD thesis. Université catholique de Louvain (Belgium) (2015).
Conte, V., Colautti, P., Chiriotti, S., Moro, D., Ciocca, M., Mairani, A. Mini TEPC Microdosimetric Study of Carbon Ion Therapeutic Beams at CNAO. New Journal of Physics (2016). Under preparation
