Distal edge determination precision for orthogonal prompt gamma imaging: a comprehensive simulation and optimization of the detection system

Orthogonal prompt gamma imaging (O-PGI) is a promising technique for range monitoring during proton therapy. We report results of a simulation study analyzing the precision which is possible to reach with O-PGI in determination of the position of the distal edge of the Bragg peak. The proton beam (single pencil beam, 130 MeV, 10 ns bunch period, 108 protons in total) is interacting with a polymethyl methacrylate (PMMA) phantom, which is a cylinder of 200 mm in diameter and length. The prompt gammas are collimated with a multi-slat collimator and detected using a combination of yttrium aluminum perovskite (YAP) scintillators, installed in the collimator apertures, and light sensors. Two scintillator packing schemes, with one and with two scintillator plates per aperture, are considered. The collimator configuration (the septal thickness, aperture and height), resulting in the best precision, is determined using two methods of detector optimization. Precision of about 2 mm (full width at half maximum) in the edge position determination is demonstrated.