Feasibility of Prompt Gamma Ray Detection and Imaging Using CdTe Detectors for BNCT ― Monte Carlo Study

Boron neutron capture therapy (BNCT) has experienced a renaissance in recent years, due to the availability of accelerator-based neutron sources and more effective delivery vehicles of boron-10 (10B). For successful BNCT, 10B concentration and distribution in the tumor (as well as in the critical organs) must be determined non-invasively prior to BNCT treatments. While conventional nuclear imaging (e.g., PET/SPECT), in conjunction with radiolabeled 10B, may serve for this purpose, another approach, based on the detection of prompt gamma (PG) rays from the BNC reaction, appears to be more attractive, most notably because it can be implemented within the BNCT setup without radiolabeling of 10B. The detectability of PG rays (478 keV) from the BNC reaction has been shown experimentally within the context of BNCT utilizing energy-resolving solid-state detectors. However, the feasibility of this approach under the clinically relevant conditions is currently unknown. Thus, the present work investigated this feasibility by performing Monte Carlo (MC) simulations using the simulation toolkit.MC models of various phantom cases were created to simulate epithermal neutron beam irradiation relevant to accelerator-based BNCT (a-BNCT). For benchmarking and future experimental verification, the phantom geometry (20 cm-diameter/24 cm-height cylinder) and material (PMMA) were chosen based on a published experimental study. The phantom had 3 cm-diameter/3.8 cm-height cylindrical inserts (located at 5 cm depth along the long-axis of the phantom) that contained 10B solution at different concentrations ranging from 1 - 30,000 parts-per-million (ppm). For detection of PG rays, a 23.1 cm-diameter ring detector consisting of 120 cadmium telluride (CdTe) crystals was modeled. CdTe was chosen because of its high photon counting efficiency and energy resolution. Each CdTe detector crystal (5 × 5 × 1 mm3) contained an inherent 0.1mm-thick beryllium window and a novel/custom thermal neutron filter made of gadolinium. Two billion incident neutron histories were used during each MC simulation. The detected PG ray signals were used to test both direct mapping and CT techniques for quantitative imaging.The current MC simulations showed PG ray signals from the10B-loaded tumor-mimicking cylindrical insert were above the background noise (with a 95% confidence level), down to the clinically achievable tumor 10B concentration of 30 ppm. The custom thermal neutron filter was effective in suppressing the inherent background from CdTe detectors.This MC study demonstrated the detectability of PG rays from the BNC reaction using CdTe detectors under the conditions closely mimicking realistic a-BNCT scenarios. It also showed that clinically acceptable quantitative imaging of 10B distribution could be performed utilizing the PG ray signals from the BNC reaction.H. Moktan: None. C. Lee: None. S. Cho: Research Grant; TAE Life Sciences.