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Boron neutron capture therapy using reactor epithermal neutron beams could effectively control in‐transit and lymph node metastases originating from a cutaneous malignant melanoma

Dear Editor, Boron neutron capture therapy (BNCT) using boronophenylalanine (BPA) is a selective radiotherapeutic approach for cancer cells and has achieved good results for treating malignant melanoma. BPA was originally developed based on melanin synthesis; BPA containing boron-10 (B) selectively accumulates in melanoma cells. Irradiation with thermal neutrons causes the B nucleus to ultimately decay into Li and alpha particles, which have a high linear energy transfer with extremely short track ranges (4 lm and 9 lm for Li and alpha particles, respectively), resulting in destruction of only tumor cells without damaging the surrounding normal tissues. We use epithermal neutron beams rather than thermal neutron beams because they have the potential to induce more efficient capture of B at the tumor depth owing to their higher energy and transformation to low-energy thermal neutrons at that location. A 57-year-old man visited our clinic with a black tumor on his left cheek (Fig. 1a). Based on a melanoma diagnosis in 2011, he underwent a wide local excision and cervical lymphadenectomy. We administrated dacarbazine-based adjuvant chemotherapy. Within a year, the tumor recurred on his left cheek; computed tomography revealed lymphadenopathy of the jugular nodes despite no detectable distant metastasis (Fig. 1b,c); because it was difficult to completely remove the recurrent metastatic lesions, we decided to use BNCT. Approval was obtained from the ethics committee of the Kyoto University Research Reactor Institute and institutional