Detection and discrimination of neutron capture events for NCEPT dose quantification

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 10B or 157Gd-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 10B and 157Gd, 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 300×300×300 mm3 cubic PMMA targets were irradiated by 4He or 12C ion beams with a spread out Bragg peak (SOBP) depth range of 60 mm; one target is homogeneous while the others include 10×10×10 mm3 neutron capture inserts (NCIs) of pure 10B or 157Gd located at the distal edge of the SOBP. The arrival times of photons and neutrons entering a simulated 50×50×50 mm3 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 (RTF) was calculated; for targets with 10B and 157Gd NCIs, the detector materials which resulted in the highest RTF were cadmium-shielded CdTe and boron-shielded LSO, respectively. The optimal irradiation period for both carbon and helium ions was 1 µs for the 10B NCI and 1 ms for the 157Gd NCI.