Our concept of boron neutron capture therapy (BNCT) is selective destruction of tumor cells using the heavy-charged particles yielded through 10B(n, alpha)7 Li reactions. To design a new protocol that employs epithermal neutron beams in the treatment of glioma patients, we examined the relationship between the radiation dose, histological tumor grade, and clinical outcome. Since 1968, 183 patients with different kinds of brain tumors were treated by BNCT; for this retrospective study, we selected 105 patients with glial tumors who were treated in Japan between 1978 and 1997. In the analysis of side effects due to radiation, we included all the 159 patients treated between 1977 and 2001. With respect to the radiation dose (i.e. physical dose of boron n-alpha reaction), the new protocol prescribes a minimum tumor volume dose of 15 Gy or, alternatively, a minimum target volume dose of 18 Gy. The maximum vascular dose should not exceed 15 Gy (physical dose of boron n-alpha reaction) and the total amount of gamma rays should remain below 10 Gy, including core gamma rays from the reactor and capture gamma in brain tissue. The outcomes for 10 patients who were treated by the new protocol using a new mode composed of thermal and epithermal neutrons are reported.