Detection and discrimination of neutron capture events for NCEPT dose quantification

Abstract

Neutron Capture Enhanced Particle Therapy (NCEPT) boosts the effectiveness of particle therapy by capturing thermal neutrons produced by beam-target nuclear interactions in and around the treatment site, using tumour-specific [Formula: see text]B or [Formula: see text]Gd-based neutron capture agents. Neutron captures release high-LET secondary particles together with gamma photons with energies of 478 keV or one of several energies up to 7.94 MeV, for [Formula: see text]B and [Formula: see text]Gd, respectively. A key requirement for NCEPT's translation is the development of in vivo dosimetry techniques which can measure both the direct ion dose and the dose due to neutron capture. In this work, we report signatures which can be used to discriminate between photons resulting from neutron capture and those originating from other processes. A Geant4 Monte Carlo simulation study into timing and energy thresholds for discrimination of prompt gamma photons resulting from thermal neutron capture during NCEPT was conducted. Three simulated [Formula: see text] mm[Formula: see text] cubic PMMA targets were irradiated by [Formula: see text]He or [Formula: see text]C ion beams with a spread out Bragg peak (SOBP) depth range of 60 mm; one target is homogeneous while the others include [Formula: see text] mm[Formula: see text] neutron capture inserts (NCIs) of pure [Formula: see text]B or [Formula: see text]Gd located at the distal edge of the SOBP. The arrival times of photons and neutrons entering a simulated [Formula: see text] mm[Formula: see text] ideal detector were recorded. A temporal mask of 50-60 ns was found to be optimal for maximising the discrimination of the photons resulting from the neutron capture by boron and gadolinium. A range of candidate detector and thermal neutron shielding materials were simulated, and detections meeting the proposed acceptance criteria (i.e. falling within the target energy window and arriving 60 ns post beam-off) were classified as true or false positives, depending on their origin. The ratio of true/false positives ([Formula: see text]) was calculated; for targets with [Formula: see text]B and [Formula: see text]Gd NCIs, the detector materials which resulted in the highest [Formula: see text] were cadmium-shielded CdTe and boron-shielded LSO, respectively. The optimal irradiation period for both carbon and helium ions was 1 µs for the [Formula: see text]B NCI and 1 ms for the [Formula: see text]Gd NCI.