A Neutronic Study of an Accelerator-Based Neutron Irradiation Facility for the Treatment of Superficial Melanoma by Boron Neutron Capture Therapy

Previously, a neutronic study of an accelerator-based epithermal neutron irradiation facility (AENIF) for boron neutron capture therapy (BNCT) was performed using three-dimensional Monte Carlo transport calculations. The major components of the AENIF are a radio-frequency quadrupole proton accelerator, a ⁷Li target, and a moderator assembly. Neutrons are generated by bombarding the ⁷Li target with 2.5-MeV protons. The neutrons emerging from the ⁷Li target are too energetic to be used for BNCT and are moderated as they traverse the moderator assembly to the patient. The design of a moderator assembly for an AENIF for the treatment of glioblastoma is reviewed, and this design is compared with the design of a moderator as sembly for an accelerator thermal neutron irradiation facility (A TNIF) for the treatment of superficial melanoma. The ATNIF moderator assembly consists of a 50-cm-high × 30-cm-diam cylinder of D₂O, surrounded on its top and sides by a 40-cm-thick graphite reflector. This moderator assembly creates, at the surface of a large phantom at its irradiation port, a boron absorbed dose rate of (3.2 ± 0.2) cGy/(min · mA), for a tumor ¹⁰B concentration of 24 µg of¹⁰B per gram of tissue. For a single-session dose equivalent of 40 Sv to the tumor, the treatment time is 13 min for a 30-mA proton beam. With different moderator assemblies, a 30-mA, 2.5-MeV proton accelerator can be used to treat both superficial and deep lesions from melanomas and gliomas.