Dose distributions in a human head phantom for neutron capture therapy using moderated neutrons from the 2.5 MeV proton-Li-7 reaction or from fissionof U-235

The feasibility of neutron capture therapy (NCT) using an accelerator-based neutron source of the Li-7(p,n) reaction produced by 2.5 MeV protons was i nvestigated by comparing the neutron beam tailored by both the Hiroshima Un iversity radiological research accelerator (HIRRAC) and the heavy water neu tron irradiation facility in the Kyoto University reactor (KUR-HWNIF) from the viewpoint of the contamination dose ratios of the fast neutrons and the gamma rays. These contamination ratios to the boron dose were estimated in a water phantom of 20 cm diameter and 20 cm length to simulate a human hea d, with experiments by the same techniques for NCT in KUR-HWNIF and/or the simulation calculations by the Monte Carlo N-particle transport code system version 4B (MCNP-4B). It was found that the Li-7(p,n) neutrons produced by 2.5 MeV protons combined with 20,25 or 30 cm thick D2O moderators of 20 cm diameter could make irradiation fields for NCT with depth-dose characteris tics similar to those from the epithermal neutron beam at the KUR-HWNIF.