Experimental comparison of Prompt Gamma-Ray Imaging and Spectroscopy under Equalized Conditions Using a Calibrated Head Phantom

First studies on range verification in proton therapy using prompt gamma-ray spectroscopy (PGS) or imaging (PGI) have been recently conducted with patients. To benchmark the performance of each method, we designed a multi-center experiment between Massachusetts General Hospital and OncoRay with a shared anthropomorphic head phantom irradiated under equalized conditions. The treatment plan was delivered at clinical beam currents and doses in the respective proton therapy hospitals. Two gantry beam angles were used: horizontal and oblique (270° and 350°). Each field contained the same spot positions and energies as well as target volume in both institutions. The experimentally measured range was compared against the ground truth based on the reference stopping-power
map of the phantom. The absolute range error of PGS or PGI was 2.4 mm or -0.5 mm for the horizontal field and 1.3 mm or 2.4 mm for the oblique one, respectively. The ability to detect relative range deviations was assessed by introducing 2 mm and 5 mm plastic slabs on the ventral half of the horizontal field. The measured range shifts were 1.8 mm or 4.8 mm for PGS, and 2.0 mm or 4.2 mm for PGI, respectively. The designed set of experiments represents the first systematic comparison of prompt gamma-ray systems for proton range verification across different institutions using a ground-truth head phantom as reference, and effectively proposes a standard testing platform for benchmarking future proton range verification prototypes in reproducible conditions.