Uncertainty limits of range verification in proton therapy by means of prompt gamma-ray timing (PGT)

Range verification during clinical treatments is a key for further improving the precision and for reducing the normal-tissue toxicity of radiotherapy with proton beams. In spite of the breakthrough achieved with IBA’s knife-edge slit camera, capable of imaging single beam spots in pencil-beam scanning (PBS) treatment fractions delivered to cancer patients, there are ongoing activities aiming at systems distinguished by lower expense, lower weight, easier integration in the therapy facility, and potentially higher precision. In this context, OncoRay’s Prompt Gamma-Ray Timing (PGT) technology has been further explored with the focus on quantifying uncertainty contributions in potential clinical applications. Experimental data acquired in the treatment room of the University Proton Therapy Dresden (UPTD) during single pencil beam delivery to a polymethyl methacrylate (PMMA) target with arbitrary air cavities have been carefully analyzed. Besides the limited number of events the instable phase relation between proton-beam bunches and accelerating RF turned out to be the weakest point at the given facility. Technical means for monitoring this phase relation at a time scale of split seconds without touching the medical beamline are being developed and will be discussed in the paper. Altogether PGT could provide range verification with 3 mm accuracy at PBS spot level, if (at least) eight PGT detection units are deployed. Technically even a larger number of detectors could be arranged around the nozzle, which would further reduce the uncertainty.