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To realize the accelerator-based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron-based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron-producing reactions and moderator materials. Neutrons emitted at 90 degrees from a thick (stopping-length) Ta target, bombarded by 50 MeV protons of 300 microA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron, AlF3/Al/6LiF, and lead was chosen as moderators, based on the simulation trials using the MCNPX code. The depth dose distributions in a cylindrical phantom, calculated with the MCNPX code, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30-cm-thick iron, 39-cm-thick AlF3/Al/6LiF, and 1-cm-thick lead, provides an epithermal neutron flux of 0.7 x 10(9) [n cm(-2) s(-1)]. This results in a tumor dose of 20.9 Gy-eq at a depth of 8 cm in the phantom, which is 6.4 Gy-eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator-based BNCT proposals.

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An engineering design of an epithermal neutron generator for boron neutron capture therapy (BNCT) has been completed, which utilizes the spallation reaction by protons accelerated to 50 MeV. The critical issues for realization of the neutron generator are the mechanical structure of a target with cooling capability and its integrity under operating conditions with powers as high as 50 MeVÂ300 mA.The integrity of a target structure design has been confirmed by thermal and stress analyses with a finite element method code ANSYS. Moreover, a target replacement strategy is also studied based on a radioactivity evaluation performed by the IRACM code system.In addition to the target structure design, the neutronics design has been optimized with the Monte Carlo code MCNPX. A high epithermal neutron flux of 1:8Â10 9 cm À2 Ás À1 has been achieved at the aperture of the collimator, which allows a RBE dose of over 30 Gy-eq to be delivered to a brain tumor within 5.9 cm in phantom depth for a therapeutic time of 31 min.

No abstract