Prompt-gamma tomographic imaging for range verification in pencil beam-scanning proton therapy

This study aimed to develop a prompt-gamma imaging system to verify the range of proton beams. We designed an optimized collimation system and calculated the proton range using the GATE nuclear-medicine simulation toolkit (v. 9.1). An initial system, which was devised using protons without including specific equipment models, was employed to assess proton beams with energy ranges of 80, 90, 100, 110, and 120 MeU. The gamma-camera system design does not interfere with the nozzle structure of the treatment equipment. It consists of a pinhole collimator and a BGO scintillator and has a 100 x 100-pixel array with a 0.5 mm x 0.5-mm pitch. Next, we constructed an entire system using 12 gamma camera systems for prompt-gamma detection. The pinhole collimator was optimized with respect to the 4.4-MeU prompt-gamma energy, considering the detection efficiency and full width at half maximum. Finally, the prompt-gamma rays at 2.2, 4.4, and 6.1 MeU were comparatively analyzed using an in-house program. Based on the simulation results, the pinhole collimator was optimized to a 1-cm height and 3-mm diameter. The peak of the 4.4-MeU prompt gamma and the reconstructed peak point matched within 1 mm for most simulated energies. These results showed that our newly proposed imaging system is capable of accurately monitoring the proton range using specific prompt-gamma energies.