Evaluation of a CdTe semiconductor-based gamma camera for real-time dose dosimetry in boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a cancer treatment that leverages the nuclear reaction between boron-10 ( ${}^{10}B$ ) and thermal neutrons to generate high-energy $\alpha$ particles and ${}^7\mathrm{Li}$ nuclei that selectively destroy cancer cells while sparing healthy tissues. However, BNCT is limited by current dosimetry methods that are incapable of monitoring boron distribution during therapy. An imaging system capable of real-time dosimetry is essential for optimizing treatment efficacy and minimizing collateral damage.

This study aimed to develop a high-resolution real-time boron dosimetry system for BNCT by employing a cadmium telluride double-sided strip detector (CdTe-DSD). The CdTe-DSD enables precise mapping of ${}^{10}B$ distribution by detecting the 478 keV prompt $\gamma$ -rays emitted during the ${}^{10}B{(n,\alpha )}^7\mathrm{Li}$  reaction.

The imaging system was constructed by integrating a CdTe-DSD with a 2-mm diameter pinhole collimator. The CdTe-DSD, comprising a 2-mm-thick CdTe semiconductor, has a sensitive area of 32 $\times$ 32  ${\mathrm{mm}}^2$ . Neutron irradiation experiments were performed at Japan Research Reactor No. 3 using various boron-containing samples, including boric acid solutions, powders, and granular boron, with boron masses ranging from 0.02 to 2.00 mg. We implemented the neutron shields using a 5-mm-thick ${}^6{\mathrm{Li}}_2{\mathrm{CO}}_3$ plate and LiF tiles to reduce the background from scattered neutrons during the measurement.

The imaging system successfully detected the 478 keV $\gamma$ -ray signal with an energy resolution of 7.3 keV at 511 keV. The reconstructed two-dimensional ${}^{10}B$ images demonstrate the capability of the CdTe-DSD to accurately map the ${}^{10}B$ distribution in the samples. Notably, we evaluated the important background contributions from the scattering neutrons in the development of the CdTe-DSD-based imaging system. When the scattering neutrons in a neutron experiment hit the CdTe-DSD, 558 keV $\gamma$ -rays of ${}^{113}\mathrm{Cd}$ were produced. Under three experimental conditions with different neutron shielding configurations, we found that enhanced neutron shielding considerably reduced the background contribution created by the ${}^{113}\mathrm{Cd}$ background signal, thereby improving the contrast-to-noise ratio (CNR) in the image quality assessment. Upon comparing the condition with minimal neutron shielding to that with maximal shielding, a 14-fold enhancement in the CNR was observed.

This study demonstrates that a CdTe-DSD-based gamma camera is a promising tool for real-time boron dosimetry in BNCT. The detector's high energy resolution and excellent spatial resolution enable precise detection of 478 keV prompt $\gamma$ -rays, facilitating accurate mapping of boron distribution during neutron irradiation. These findings support the developed system's potential to enhance BNCT treatment planning and patient-specific dose monitoring. Future research will optimize neutron shielding and explore advanced collimator designs to improve sensitivity in clinical settings.