Microdosimetric measurements of a monoenergetic and modulated Bragg Peaks of 62 MeV therapeutic proton beam with a synthetic single crystal diamond microdosimeter

Purpose The purpose of this study was to investigate for the first time the performance of a synthetic single crystal diamond detector for the microdosimetric characterization of clinical 62 MeV ocular therapy proton beams. Methods A novel diamond microdosimeter with a well-defined sensitive volume was fabricated and tested with a monoenergetic and spread-out Bragg peak (SOBP) of the CATANA therapeutic proton beam in Catania, Italy. The whole sensitive volume of the detector has an active planar-sectional area of 100 mu m x 100 mu m and a thickness of approximately 6.3 um. Microdosimetric measurements were performed at several water equivalent depths, corresponding to positions of clinical relevance. From the measured spectra, microdosimetric quantities such as the frequency mean lineal energy (y over bar F), dose mean lineal energy (y over bar D) as well as microdosimetric relative biological effectiveness (RBE mu) values were derived for each depth along both a pristine Bragg curve and SOBP. Finally, Geant4 Monte Carlo simulations were performed modeling the detector geometry and CATANA beamline in order to calculate the average linear energy transfer (LET) values in the diamond active layer and water. Results The microdosimetric spectra acquired by the diamond microdosimeter show different shapes as a function of the water equivalent depths. No spectral distortion, due to pile-up events and polarization effects, was observed. The experimental spectra have a very low detection threshold due to the electronic noise during the irradiation of about 1 keV/mu m. They over bar Fandy over bar Dvalues were in agreement with expected trends, showing a sharp increase in mean lineal energy at the distal edge of the Bragg peak. In addition, a good agreement between the mean lineal energy values and the calculated average LET ones was also observed. Finally, the RBE values evaluated with the diamond microdosimeter were in excellent agreement with those obtained with a mini tissue equivalent proportional counter as well as with radiobiological measurements in the same proton beam field. Conclusions The microdosimetric performance of the tested synthetic single crystal diamond microdosimeter clearly indicates its suitability for quality assurance in clinical proton therapy beam.