MO-AB-BRA-07: Prompt Gamma-Ray Spectroscopy for Range Verification of Clinical Proton Beams

Purpose: We developed a pre-clinical prototype system for range verification of proton pencil-beam scanning fields. The system was evaluated using phantom treatment plans delivered with a clinical dose rate. Methods: The absolute range of proton pencil-beams was verified through an optimization procedure, which matches energy- and time-resolved prompt gamma-ray measurements with models, based on cross sections for discrete prompt gamma-ray line excitations. Phantom experiments were performed with a pre-clinical prototype detector, using treatment plans delivered with a clinical pencil-beam scanning system. The detector consisted of an actively shielded lanthanum(III) bromide scintillator. Tungsten was used to collimate the gamma-rays. To support high event rates, the detector readout featured custom amplifiers and an active voltage divider for the photomultiplier. The detector signals were acquired by fast analog-to-digital converters and processed using digital algorithms. The data acquisition was also synchronized with the pencil-beam scanning and dosimetry systems. Results: We successfully acquired prompt gamma-ray spectra during the delivery of proton pencil-beams with a clinical beam current of 2 nA at the exit of the treatment head. The number of events in the primary detector ranged from 1 x 10⁶ to 2 x 10⁶ per second. In phantom experiments, non-uniform range errors were introduced by placing strips of plastic in the beam path. The magnitudes and positions of these range errors were correctly detected in two-dimensional range maps that were generated from the measurements. With our small scale prototype, a 1.0 mm standard deviation on the absolute range required about 5 x 10⁸ protons per delivered pencil-beam. Conclusions: Prompt gamma-ray spectroscopy to verify the absolute range of proton beams was demonstrated under clinical pencil-beam delivery conditions. A 1 mm to 2 mm range verification accuracy for a field delivering 1 Gy, appears feasible with a full scale system. This work was supported by the Federal Share of program income earned on C06-CA059267, Proton Therapy Research and Treatment Center