Measurement of the depth dose profile and the 'averaged LET' for the determination of the biologically relevant dose of a therapeutic proton beam at PSI

Full text: Proton beams with 62 MeV are used for the irradiation of ocular tumors at the Paul Scherrer Institute (PSI), Switzerland. The advantage of ion beam therapy compared to photons lies in the unique depth dose profile of protons and heavy ions (Bragg Peak) enabling an accurate dose deposition in the tumor tissue. For irradiation of patients a spread out bragg peak (SOBP) is generated by use of different degraders. A relative biological effectiveness (RBE) for protons of 1,1 is commonly used for this therapy. Previous measurements with thermoluminescent dosemeters (TLDs) applying the HTR - method showed an increasing 'averaged LET' also in the plateau of the SOBP resulting in an increased biological effect in comparison to the unmodulated beam. Further investigations were performed using different kinds of TLDs with various thickness (200 μm - 900 μm) for a detailed determination of the absorbed dose and - LET profile of the beam with various methods developed at the Atominstitute. These measurements were accompanied by LET spectrometer investigations to relate the results of the HTR - method with microdosimetric parameters. First results will be discussed. (author)