Boron neutron capture therapy (BNCT) is a binary treatment modality that selectively kills cancer cells. It is based on the nuclear reaction that occurs when a thermal neutron is captured by a 10B atom, resulting in particles with high linear energy transfer (LET). These LET particles (alpha particle and 7Li nuclei) have ranges that are roughly equal to the size of a human cell. Clinical trials of BNCT that utilised 10B-para-boronphenylalanine (L-BPA) as the boron delivery agent have been reported for the treatment of recurrent head and neck cancers 1− 4. Historically, BNCT was performed using neutrons generated from a nuclear reactor. An accelerator-based neutron source is becoming more popular, as it offers several proven advantages over a nuclear reactor. The world’s first accelerator based neutron source for clinical BNCT was designed and developed by Sumitomo Heavy Industries, in collaboration with Kyoto University BNCT research group 5,6. This accelerator was used in a clinical trial treating recurrent or locally advanced head and neck cancer between 2016 and 2018 7. The same type of accelerator system was installed in September 2016 at the Kansai BNCT Medical Center of Osaka Medical and Pharmaceutical University. On March 11, 2020, the Japanese Ministry of Health, Labor and Welfare approved the system as a novel medical device for manufacturing and selling an accelerator BNCT system (NeuCure® System) and the dose calculation program (NeuCure® Dose Engine). BNCT has been approved for coverage under the national health insurance system for unresectable, locally advanced, and recurring cancer of the head and neck region as of June 2020. Currently, the center has treated over 60 head and neck patients using the NeuCure® System along with Steboronine® (10B enriched borono-L-phenylalanine) produced by STELLA PHARMA corporation.
For BNCT, it is preferable to bring the patient as close to the beam port as possible to keep the treatment time short, since neutrons scatter through the air and the intensity drops. This is important as the Steboronine® is only approved for infusion for a total of 3 hours (the infusion rate for the first two hours is 200 mg/kg/h and the last hour is 100 mg/kg/h). The neutron irradiation is performed when the infusion rate is dropped to 100 mg/kg/h, so the maximum irradiation time (while the BPA is being infused) is limited to 1 hour. However, bringing the patient close to the beam port is not easy. This is because the current system only has a single fixed horizontal beam line, and the patient needs to move toward the neutron beam port (unlike conventional radiotherapy, where the gantry rotates around the patient). This is troublesome, particularly for patients with cancer near the hypopharynx area, as the shoulders get in the way when trying to bring the patient close to the beam port. Inevitably, this produces an air gap of around several centimetres between the beam port and the patient surface, and in some cases greater than 10 cm. This air gap increases the treatment time and the exposure to unnecessary parts of the body.
This paper investigates an improvement in the beam collimator design that can be easily adapted to the current system to address the issues mentioned above.