Monte Carlo-based study on signal and background characteristics of BNCT-SPECT systems

BackgroundBoron Neutron Capture Therapy (BNCT) is an effective targeted radiotherapy technique, but real-time monitoring of boron dose distribution during treatment remains a critical challenge. The strong neutron-gamma mixed radiation field in BNCT creates a high-background environment, significantly limiting the practical application of BNCT-SPECT systems based on detecting 478 keV prompt gamma rays from the 10B(n,α) 7Li reaction.PurposeThis study aims to develop an efficient simulation approach that quantitatively analyzes the signal and background contributions to the pulse height spectrum (PHS) of BNCT-SPECT detectors, thereby improving computational efficiency and accuracy.MethodsA Monte Carlo-based Benchmark geometry model of the BNCT-SPECT system was firstly established, including a simplified head phantom, tumor, collimator, detector, shielding structure, and treatment room. Then, four different gamma ray sources contributing to the detector's 478 keV energy channel were identified based on their physical generation processes: 478 keV gamma rays from 10B(n,α) 7Li reactions, 2.223 MeV gamma rays from 1H(n,γ) 2D reactions, prompt gamma rays from neutron capture in detector materials, and stray gamma rays from neutron capture in other materials. Finally, a stepwise calculation method was developed to generate intermediate source terms for each contribution using variance reduction techniques, and the PHS for each source was calculated separately and then superimposed to obtain the total spectrum.ResultsSimulation results show that the total count rate in the 478 keV energy window is 6.83 s -1 (Counts per Second, CPS), with 53% (3.63 s -1) originating from the 10B(n,α) 7Li reaction, including 29% (2.01 s -1) from tumor tissue and 24% (1.62 s -1) from normal brain tissue. The primary background contribution of 33% (2.28 s -1) comes from Compton scattering of 2.223 MeV gamma rays generated by the 1H(n,γ) 2D reaction, with approximately 71% of this background penetrating through the collimator walls via oblique incidence. Neutron-induced gamma rays in detector materials contribute 2% (0.12 s -1), while other stray gamma rays contribute 12% (0.8 s -1). The stepwise calculation method reduces computational time from days to hours while maintaining comparable statistical accuracy, with the 478 keV energy channel count rates from direct calculation and stepwise calculation being 6.66 s -1 and 6.83 s -1, respectively.ConclusionsThe simulation calculation method for PHS for BNCT-SPECT detectors proposed in this study can distinguish between signal and background sources, enables quantitative analysis of pulse height spectra from different γ sources, providing a reference for further practical applications.