Using backscattered thermal neutrons to monitor boron concentration during BNCT: a Monte Carlo feasibility study

Boron neutron capture therapy (BNCT) requires knowledge of patient-specific $^{10}$B concentration for accurate dose estimation, yet no established method provides real-time boron-sensitive information during irradiation. Backscattered thermal neutrons carry a $^{10}$B-dependent intensity modulation through the $^{10}$B(n,$\alpha$)$^{7}$Li reaction, documented in BNCT treatment rooms for three decades but not yet developed as a measurement signal. This paper uses Monte Carlo simulation to assess the feasibility of backscattered thermal neutrons as a measurement channel for $^{10}$B concentration. A thin $^{nat}$LiF-converter detector placed at the beam exit captures the composite forward-plus-backscatter field; differential imaging against a $^{10}$B-free baseline isolates the $^{10}$B-dependent component, quantified by the fractional reduction in the $^{6}$Li capture rate, termed Relative Detector Signal Reduction (RDSR). In homogeneous phantoms, RDSR shows linear concentration dependence ($R^2 = 0.997$) with a practical depth limit of approximately 6 cm. Edge-response analysis yields a diffusion-limited FWHM of 32-176 mm over 1-5 cm depth, with weak concentration dependence. In a voxelized patient phantom across 12 boron configurations, the $^{6}$Li capture cross-section provides intrinsic thermal neutron energy selectivity that preferentially weights the band where $^{10}$B absorption is concentrated. Region-of-interest integration achieves counting-statistics sensitivity below 10 ppm; the systematic detection floor (~22-28 ppm at $\pm$1% baseline uncertainty) identifies baseline-reference precision as the dominant constraint. The modeled detector produces limited dose perturbation (+11.6% treatment-time increase). These results establish the physical basis for a boron-sensitive backscattered neutron measurement concept in BNCT.