Proton Therapy treatments are affected by uncertainties on the penetration depth of the beam within the patient. For this reason, real-time range control is highly desirable to deliver safer treatments. Real-time range control can be performed by imaging prompt gammas emitted along the proton tracks in the patient. Our approach uses a knife-edge slit collimator to obtain a 1-dimensional projection of the beam path on a gamma camera. The energy spectrum of prompt gammas includes energy events up to 10 MeV and the event rate on a 500 cm scintillator is tens of MHz. Standard SPECT and PET modules are not suitable for the purpose and a dedicated gamma camera was designed. The camera features a 3 cm thick LYSO crystal segmented in two rows of 20 slabs with a width of 4 mm and a height of 10 cm. The crystal is coupled to arrays of Silicon Photomultipliers, read out by dedicated electronics boards to perform both spectra acquisition at low rates and photon counting at high rates for profile reconstruction. The prototype was aimed at reaching clinical requirements. The camera was tested in the Proton Therapy Center in Prague using an anthropomorphic phantom on which realistic treatment plans were delivered in pencil beam scanning mode. For each layer of the treatment, acquired profiles corresponding to the single spots were compared to simulated profiles and the shift was retrieved. The study demonstrated that the system is actually suitable for patient treatment monitoring.
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