Microdosimetric study at the CNAO active-scanning carbon-ion beam

The Italian National Centre for Oncological Hadrontherapy (CNAO) has been treating patients since 2011 with carbon-ion beams using the active-scanning modality. In such irradiation modality, the beam spot, which scans the treatment area, is characterised by very high particle-fluence rates (more than 10⁵ s^-1 mm^-2 ). Moreover, the Bragg-peak is only ∼1 mm FWHM. Commercial tissue-equivalent proportional counters (TEPC), like the Far West Technologies LET-1/2 are large, hence they have limited capability to measure at high counting fluence rates. In this study we have used two home-made detectors, a mini-TEPC 0.81 mm² in sensitive area and a silicon telescope 0.125 mm² in sensitive area, to perform microdosimetric measurements in the therapeutic carbon-ion beam of CNAO. A monoenergetic carbon-ion beam of 189.5 ± 0.3 MeV/u scanning a 3 x 3 cm² area has been used. Spectral differences are visible in the low y-value region, but the mean microdosimetric values, measured with the two detectors, result to be pretty consistent, as well as the microdosimetric spectra in the high γvalue region. (authors)