Proton therapy can overcome the limitations of conventional radiotherapy due to the more selective energy deposition in depth and to the increased biological effectiveness. Verification of the delivered dose is desirable, but the complete stopping of the protons in patient prevents the application of electronic portal imaging methods that are used in conventional radiotherapy During proton therapy beta(+) emitters like C-11, O-15, C-10 are generated in irradiated tissues by nuclear reactions. The measurement of the spatial distribution of this activity, immediately after patient irradiation, can lead to information on the effective delivered dose. First, results of a feasibility study 2 of an in-beam PET for proton therapy are reported. The prototype is based on two planar heads with an active area of about 5 x 5 cm Each head is made up of a position sensitive photomultiplier coupled to a square matrix of same size of LYSO scintillating crystals (2 x 2 x 18 mm(3) pixel dimensions). Four signals from each head are acquired through a dedicated electronic board that performs signal amplification and digitization. A 3D reconstruction of the activity distribution is calculated using an expectation maximization algorithm. To characterize the PET prototype, the detection efficiency and the spatial resolution were measured using a point-like radioactive source. The validation of the prototype was performed using 62 MeV protons at the CATANA beam line of INFN LNS and PMMA phantoms. Using the full energy proton beam and various range shifters, a good correlation between the position of the activity distal edge and the thickness of the beam range shifter was found along the axial direction.
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