&NA;If a sufficient concentration of the stable isotope10B is introduced into a neoplasm, radiation therapy can be effected by short‐range heavy charged particles from the disintegration of10B by slow neutrons. Brain tumors were irradiated postoperatively by Hatanaka and co‐workers in Japan using a 1 to 2 hour intraarterial infusion of10B‐enriched Na2B12H11SH (Na210B12H11SH) before exposure of the tumor‐bearing area of the brain to slow neutrons from a 100 kilowatt nuclear reactor. The clinical outcome of such boron neutron capture therapy has been favorably impressive in some patients, but its efficacy in brain tumors needs improvement. In our study, a terminally ill patient with malignant astrocytoma was infused intravenously with Na210B12H11SH for 25 hours. The postmortem distribution of10B in unfixed, frozen, tumor‐bearing brain and spinal cord tissues was studied by comparing representative cryostat sections of these specimens with neutron‐induced heavy charged particle radiographs of the same sections. Preferential accumulation of10B was observed in the tumor, with relatively little accumulation of10B in the parenchyma of the central nervous system. (Neurosurgery24:6‐11, 1989)