Microdosimetry with a mini-TEPC in the spread-out Bragg peak of 148 MeV protons

A new miniaturized Tissue Equivalent Proportional Counter (mini‒TEPC) with cylindrical sensitive volume of 1.0 mm in diameter and height, and with external diameter of 1.53 cm, has been developed to work without gas flow. With this mini counter we have measured the radiation quality of the 148 MeV energy-modulated proton beam at the radiobiological research line of the Trento Proton Therapy Centre. The irradiation field had a diameter of about 6 cm, a modulation width of 2.5 cm and a range of about 13.5 cm. At four specific depths, the response function of the new microdosimeter has also been compared to that of a commercial spherical microdosimeter, the LET-1/2” Spherical TEPC produced by Far West Technology. Afterwards, measurements were performed with the mini‒TEPC at nine precise positions across the Spread-Out Bragg Peak (SOBP).