Clinical feasibility of a commercial bolus in boron neutron capture therapy: Dosimetric evaluation and activation risk.

BACKGROUND: Boron neutron capture therapy (BNCT) is a cell-selective cancer treatment in which cancer cells are irradiated by charged particles produced by the nuclear reaction within the cells. The development of accelerator-based BNCT (AB-BNCT) has made hospital-based treatment possible, expanding its clinical applications. In AB-BNCT, epithermal neutrons penetrate tissues and slow down, with thermal neutron flux peaking at a depth of approximately 20 mm. Unlike reactor-based BNCT, which enables selection between thermal and epithermal neutron beams, AB-BNCT exclusively utilizes epithermal neutrons. Therefore, optimizing surface-dose distribution is essential for the effective treatment of superficial tumors. In conventional radiotherapy, bolus materials are used to enhance the surface dose; however, their application in BNCT remains unestablished. Although custom-made boluses, such as paraffin, hydrophilic polymer gels, and polyurethane foam, have been investigated for BNCT, their labor-intensive fabrication limits to clinical implementation. The feasibility of commercially available boluses for BNCT, particularly regarding dosimetric accuracy and activation risks, remains largely unexplored.

PURPOSE: This study evaluates the feasibility of using a commercial bolus for BNCT by assessing its impact on the thermal neutron flux, gamma-dose distribution, treatment planning accuracy, and neutron-induced activation. Additionally, a quality assurance (QA) framework for the bolus materials used in BNCT is examined.

METHODS: The feasibility of using a commercial bolus (Clearfit bolus, Fujidenolo Co., Ltd.) in the AB-BNCT field was evaluated using the NeuCure (Sumitomo Heavy Industries, Ltd.) system. Thermal neutron flux and gamma-dose rate distributions were measured on a water phantom, both with and without the bolus, using the gold activation method and thermoluminescent dosimeters. NeuCure Dose Engine (Sumitomo Heavy Industries, Ltd.), integrated with RayStation (RaySearch Laboratories), validated calculation accuracy. Activation analysis was conducted using a high-purity germanium detector, residual radioactivity was monitored with a Geiger-Müller (GM) counter, and dose rate variations were measured using a NaI(Tl) scintillation survey meter.

RESULTS: The bolus exhibited near-water equivalence in BNCT, indicating only a slight reduction (≤3%) in peak thermal neutron flux and a parallel shift in the flux distribution. The treatment planning system accurately reproduced the measured neutron- and gamma-dose distributions when the correct bolus composition was specified. Activation analysis identified primarily short-lived radionuclides, including 11C, 13N, 15O, and 125mSn, with half-lives of 20.4, 10.0, 2.0, and 9.5 min, respectively. Residual radioactivity measurements confirmed that the GM count rate decreased to 1% of its initial value within 5 h and background levels within 2 days. The scintillation survey meter recorded background gamma-dose rates within 6 h. These findings confirm that the bolus is safe for clinical reuse following a short cooling period.

CONCLUSIONS: This study is the first to comprehensively evaluate a commercially available bolus for BNCT and establish a QA framework. Three key findings demonstrate that a commercial bolus material used in radiation therapy is suitable for BNCT, offering dosimetric accuracy, precise treatment planning, and minimal activation risk. The evaluation QA framework established in this study provides basic QA items for the bolus materials used in BNCT and can be applied when introducing other new materials.