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Purpose In proton therapy precise knowledge of the beam range is essential to guarantee the treatment’s efficacy and to avoid unnecessary toxicities. Over a fractionated course of treatment anatomical changes can severely impact the delivered dose distribution from that planned; evaluation of these changes on a fraction-by-fraction basis is essential. We report the first results of a new method to determine proton beam range in three dimensions, for pencil-beam scanning systems, with an uncertainty below 3 mm. Methods The range is determined through the reconstruction of the origin of prompt gamma (PG) rays emitted from nuclear de-excitations following proton bombardment. PG emission is almost instantaneous and is characterised by a high-production rate. The prototype system is comprised of 16 symmetrically-spaced LaBr3(Ce) detectors, in a spherical design. Initially the position reconstruction capability of the detector system was examined using Geant4 simulations. To determine the PG-rays emission positions in 3D, the information recorded by each detector is fed into a reconstruction algorithm, developed in the MATLAB environment. The development, testing and laboratory validation of the algorithm has been conducted using a sealed 60 Co source. Furthermore the algorithm has been employed to investigate, by means of Geant4 simulations, how the system performs with a proton pencil beam at different clinical energies. In the simulations a water phantom with 2-cm thick body materials slabs embedded inside has been modelled. Results Preliminary simulation results show that for an ideal detector system the reconstruction algorithm is capable of determining the source position to within 1 mm in the 3D space. The algorithm is also able of discriminating between multiple sources with a relative separation of 0.15 mm. A good agreement has been observed between the dose and the prompt-gamma distribution from a proton pencil beam impinging the different phantoms at all the employed clinical energies. Conclusions Proof-of-principle for the reconstruction algorithm with a sealed 60 Co source has been obtained. The response of the system with a clinical proton beam has been evaluated, by means on Monte Carlo simulations. The next stage is to test the system with a proton beam experimentally.

In proton beam therapy precise knowledge of the proton beam range is essential to guarantee the treatment’s efficacy and to avoid unnecessary toxicities. Unlike photon beams, protons stop inside the patient’s body, therefore a direct detection of the distal fall-off is impossible. One technique to determine the beam range is to detect the prompt gamma (PG) rays emitted from the nuclei de-exciting following proton bombardment [1] . PG emission is almost instantaneous and has a high-production rate. The aim of this project is to develop a new method, based on an optimized PG detector system, which can achieve 3D range determination with an uncertainty of no more than 2 mm. The presented method is based on the detection of discrete gamma-rays. As a first step, the position reconstruction capability of the PG detector system was examined by means of Geant4 simulations. The prototype system is comprised of 12 LaBr 3 (Ce) detectors. The information recorded by each individual detector is fed into a reconstruction algorithm to determine the gamma-ray emission point in 3 dimensions. The development of the algorithm, proof-of-principle and simulation validation, have all been conducted using a sealed 60 Co source. Our simulations demonstrate that an ideal detector system with the current reconstruction algorithm is capable of determining the source position with sub-millimetre accuracy. Having obtained proof-of-principle for the reconstruction algorithm the next stage is to investigate how implementing a realistic detector system affects the reconstruction performance. In addition, the ability of the detector system to discriminate between multiple sources in different positions is under evaluation.