PURPOSE: The aim of this paper is to investigate the limits of LET monitoring of therapeutic carbon ion beams with miniaturized microdosimetric detectors. METHODS: Four different miniaturized microdosimeters have been used at the 62 MeV/u 12C beam of INFN Southern National Laboratory (LNS) of Catania for this purpose, i.e. a mini-TEPC and a GEM-microdosimeter, both filled with propane gas, and a silicon and a diamond microdosimeter. The y-D (dose-mean lineal energy) values, measured at different depths in a PMMA phantom, have been compared withLET¯D (dose-mean LET) values in water, calculated at the same water-equivalent depth with a Monte Carlo simulation setup based on the GEANT4 toolkit. RESULTS: In these first measurements, no detector was found to be significantly better than the others as a LET monitor. The y-D relative standard deviation has been assessed to be 13% for all the detectors. On average, the ratio between y-D and LET¯D values is 0.9 ± 0.3, spanning from 0.73 ± 0.08 (in the proximal edge and Bragg peak region) to 1.1 ± 0.3 at the distal edge. CONCLUSIONS: All the four microdosimeters are able to monitor the dose-mean LET with the 11% precision up to the distal edge. In the distal edge region, the ratio of y-D to LET¯D changes. Such variability is possibly due to a dependence of the detector response on depth, since the particle mean-path length inside the detectors can vary, especially in the distal edge region.

Belgian Nuclear Research Centre SCK·CEN Boeretang 200 2400 Mol Belgium

EBG MedAustron Marie Curie St 5 2700 Wiener Neustadt Austria

INFN Laboratori Nazionali del Sud via S Sofia 62 95123 Catania Italy; ELI Beamlines Za Radnici 835 Dolní Břežany 252 41 Czech Republic

INFN Laboratori Nazionali del Sud via S Sofia 62 95123 Catania Italy; ELI National Physical Laboratory CMES Medical Radiation Science Teddington UK

INFN Laboratori Nazionali di Legnaro viale dell'Università 2 35020 Legnaro Italy

INFN Sezione di Padova via Marzolo 8 35131 Padova Italy; University of Padova Physics and Astronomy Department via Marzolo 8 35131 Padova Italy

INFN Sezione di Roma Tor Vergata via della Ricerca Scientifica 1 00133 Roma Italy; University of Roma Tor Vergata via del Politecnico 1 Roma 00133 Italy

Politecnico di Milano Dipartimento di Energia via La Masa 34 20156 Milano Italy; INFN Sezione di Milano via Celoria 16 20133 Milano Italy

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