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).From the microdosimetric spectra the frequency and the dose-mean lineal energy values, yF and yD, were derived for both detectors. Afterwards, microdosimetric assessment of Relative Biological Effectiveness (RBE mu) has been performed by weighting the dose distribution of the lineal energy, d(y), with the Loncol's biological weighting function r(y).RBE mu has been used to calculate the RBED mu at different dose levels D. In the distal fall-off region the RBED mu(d) increases significantly, up to a factor 3. Finally, the RBED mu-weighted dose was calculated and compared to that obtained with a fixed RBE = 1.1. The use of a variable RBE to weight the physical dose leads to a biological range extension evaluated at R80 of about 2.0 mm, with respect to the R80 obtained with a fixed RBE = 1.1.The use of a mini-TEPC in proton therapy has been so far limited to 60 MeV proton beams, used for the treatment of ocular melanoma. This is the first time that a mini-TEPC was used with an higher energy proton beam of 148 MeV, and the spectra measured with the mini-TEPC were compared to those measured with the commercial FWT LET1/2 counter for the first time.These new results corroborate the usability of mini-TEPCs in proton therapy.