Prompt gamma imaging with a multi-knife-edge slit collimator for large FOV monitoring of scanned proton pencil beams

Proton therapy is superior to conventional radiation therapy due to dose deposition sharply increasing at Bragg peak. However, an uncertainty in Bragg peak position as large as 15 mm could occur due to a variety of uncertainties in proton therapy treatment delivery. One solution is to monitor the treatment during delivery by prompt gamma imaging. Pencil beam scanning mode is one of the widely adopted modes in proton therapy treatment In this mode, two sets of magnets scan the proton beams in a 2D pattern, and therefore the emitted gamma photons are focused to a small region, and the position of this region are rapidly changing with time. This is different with conventional nuclear medicine imaging, when the gamma source are distributed over the entire patient body, and rapidly changing with time. To meet the requirement of 40 cm × 40 cm × 30 cm FOV for proton pencil beam scanning mode, we propose a prompt gamma imaging system design with a multi-knife-edge slit collimator based on the optimization of a single-knife-edge slit collimator design. Monte Carlo simulation was performed to evaluate the positioning accuracy of Bragg peak and the spatial resolution of the imaging system. The prompt gamma distribution was generated via Monte Carlo simulation of proton interaction with water in GATE. The generated prompt gamma distribution was subsequently used to evaluate the system performance. For the multi-knife-edge slits collimator-based imaging system, preliminary evaluation results show that it can provide a large FOV size of 40×40×30 cm 3 , ∼0.17% detection efficiency at FOV center, <2mm special resolution and <3 mm positioning accuracy along Bragg peak direction. Based on the preliminary results, we conclude that the proposed multi-knife-edge slits collimator-based prompt gamma imaging system design is very promising and could potentially facilitate precise proton therapy.