A New Strategy for Range Verification in Proton Therapy: the Coaxial Approach

Range verification during proton therapy via prompt gamma-rays is a technique that has reached clinical translation. Initial research studies on a small number of patients were conducted during the last years by three independent hospitals, using gamma-ray detectors behind heavy collimators, located perpendicular to the beam axis. However, it remains unsure whether the size and weight of these prototypes will allow their applicability on a widespread scale. To address this pitfall, a new method without collimation was recently proposed: the monitoring of prompt gamma-rays with a single detector, in an orientation coaxial to the proton beam, behind the treated area. By solely counting the number of detections per spot, one would be able to identify range deviations with respect to the treatment plan. With this compact and affordable method, the integration in the treatment room would be facilitated compared to the collimated counterparts. Nonetheless, this novel orientation entails unexplored challenges, namely high count rates and large neutron background in forward direction. We report on initial developments of a demonstrator detection system specifically tailored to cope with up to 10 million counts per second. It comprises a cerium bromide scintillator coupled to a photomultiplier tube and a fast digitizer. First experimental tests show the ability of acquiring continuous waveforms at 1.25 GSPS without any dead time during a time span typical of a clinical treatment field, which in turn allows for a sophisticated decomposition of pileup events. Dedicated photomultiplier supply electronics able to sustain high count rate variations have been designed with the help of behavioral circuit simulations and is being manufactured and tested with controlled light sources.