A New Method to Reconstruct in 3D the Emission Position of the Prompt Gamma Rays following Proton Beam Irradiation

A new technique for range verification in proton beam therapy has been developed. It is based on the detection of the prompt gamma rays that are emitted naturally during the delivery of the treatment. A spectrometer comprising 16 LaBr3 (Ce) detectors in a symmetrical configuration is employed to record the prompt gamma rays emitted along the proton path. An algorithm has been developed that takes as inputs the LaBr3 (Ce) detector signals and reconstructs the maximum gamma-ray intensity peak position, in full 3 dimensions. For a spectrometer radius of 8 cm, which could accommodate a paediatric head and neck case, the prompt gamma-ray origin can be determined from the width of the detected peak with a sigma of 4.17 mm for a 180 MeV proton beam impinging a water phantom. For spectrometer radii of 15 and 25 cm to accommodate larger volumes this value increases to 5.65 and 6.36 mm. For a 8 cm radius, with a 5 and 10 mm undershoot, the sigma is 4.31 and 5.47 mm. These uncertainties are comparable to the range uncertainties incorporated in treatment planning. This work represents the first step towards a new accurate, real-time, 3D range verification device for spot-scanning proton beam therapy.