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 $$^{10}$$ 10 B or $$^{157}$$ 157 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 $$^{10}$$ 10 B and $$^{157}$$ 157 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 $$300\times 300\times 300$$ 300 × 300 × 300 mm$$^3$$ 3 cubic PMMA targets were irradiated by $$^4$$ 4 He or $$^{12}$$ 12 C ion beams with a spread out Bragg peak (SOBP) depth range of 60 mm; one target is homogeneous while the others include $$10\times 10\times 10$$ 10 × 10 × 10 mm$$^3$$ 3 neutron capture inserts (NCIs) of pure $$^{10}$$ 10 B or $$^{157}$$ 157 Gd located at the distal edge of the SOBP. The arrival times of photons and neutrons entering a simulated $$50\times 50\times 50$$ 50 × 50 × 50 mm$$^3$$ 3 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 ($$R_{TF}$$ R TF ) was calculated; for targets with $$^{10}$$ 10 B and $$^{157}$$ 157 Gd NCIs, the detector materials which resulted in the highest $$R_{TF}$$ R TF were cadmium-shielded CdTe and boron-shielded LSO, respectively. The optimal irradiation period for both carbon and helium ions was 1 µs for the $$^{10}$$ 10 B NCI and 1 ms for the $$^{157}$$ 157 Gd NCI.

First experimental demonstration of real-time neutron capture discrimination in helium and carbon ion therapy

“…To demonstrate the improvement in specificity which can be achieved by adding temporal windowing, the simulation output is filtered to the time interval between 22 ns and 10 4 ns post-irradiation (which was found to be optimal in our previous publication 14 ). As most of the positron annihilation photons arrive at the detector after 10 4 ns, the area under the neutron capture peak was considered independently for the results with this timing window through a single Gaussian fit.…”

“…Our previous publication evaluated the expected temporal characteristics of neutron capture photons relative to the timing structure of the ion beam and discusses the possibility of using a combination of temporal and spectral windowing for discrimination of the neutron capture photons from other prompt gamma and delayed emissions, such as positron annihilation photons 14 . As temporal discrimination is yet to be incorporated into the prototype detector system experimentally, the observation of a measurable increase in 478 keV neutron capture photons with both helium and carbon ion beams using spectral techniques alone is a promising indication that the complete system will be effective.…”

“…In our previous Monte Carlo simulation study, we have demonstrated that discrimination between neutron capture photons and other photon or particle detection events is theoretically feasible based on a combination of spectral and temporal windowing, with the latter being performed relative to the ion beam pulse structure 14 . In this study, we experimentally quantify the neutron capture photons emitted during carbon and helium ion irradiation of a PMMA target with and without a boron-loaded insert, and compare the results to the output of an equivalent Monte Carlo simulation.…”

First experimental demonstration of real-time neutron capture discrimination in helium and carbon ion therapy

“…In our previous Monte Carlo simulation study, we have demonstrated that discrimination between neutron capture photons and other photon or particle detection events is theoretically feasible based on a combination of spectral and temporal windowing, with the latter being performed relative to the ion beam pulse structure 18 . In this study, we experimentally quantify the neutron capture photons emitted during carbon and helium ion irradiation of a PMMA target with and without a boron-loaded insert, and compare the results to the output of an equivalent Monte Carlo simulation.…”

A Monte Carlo model of the Dingo thermal neutron imaging beamline

“…Thermal neutron sources are essential for research on neutron-based radiation therapy modalities such as Neutron Capture Therapy (NCT) and Neutron Capture Enhanced Particle Therapy (NCEPT), both leveraging neutron capture events in a tumourtargeting agent with high thermal neutron capture cross-section (such as 10 B or 157 Gd) [2][3][4][5] . Ongoing research programs focus on new neutron capture agent discovery, improved clinical protocols and dosimetric and image-based treatment verification / quality assurance methods [6][7][8][9] .…”