Development of focusing multi-knife-edge camera for proton beam range verification

Proton range uncertainties limit the ability to make full clinical use of the sharp distal falloff of the proton beam. To overcome this limitation, prompt gamma (PG) imaging system with mechanical collimation can be used in non-invasive proton beam range verification. In this study, we developed a focusing multi-knife-edge collimator to improve the counting efficiency and accuracy in detecting proton beam range. Unlike the common design of a single-knife-edge slit (SKE) collimator, the multi-knife-edge slit (MKE) collimator consists of three slits and focuses on the target detection area. To compare the performances of these two collimator designs, Monte Carlo simulation was unitized to model the cameras for measuring the PGs during proton irradiation. The simulation was performed using the GATE v9.1. The setup included a pencil beam of 100 MeV, 160 MeV, and 230 MV protons incident upon a cylindrical PMMA phantom with three different proton numbers of 107, 108, and 109. The detection PG profiles were fitted with a 3-line-segment fitting method to estimate the range of proton beams. Both cameras were aimed at the expected range depth for the corresponding proton beam energies. Experiments for range shifts were conducted by shifting the phantom between −20 mm and 20 mm along the beam axis. Results demonstrate that the MKE camera exhibits approximately twice the efficiency of the SKE camera in detecting PGs. In range shift retrieval, with the delivery of 108 protons, the MKE camera achieves submillimeter accuracy (except for incident protons at 230 MeV), while the SKE camera achieves accuracy within 2 mm. In conclusion, this study assesses the feasibility of the MKE camera and illustrates its potential application for range shift detection, contributing to precise range monitoring in proton therapy.