Design parameters for an epithermal neutron field for an accelerator-based source of neutrons for boron neutron capture therapy are developed. The parameters that are developed incorporate predicted biological effects in patients’ heads. They are based on an energy-spectrum-dependent neutron normal-tissue relative biological effectiveness and the treatment planning methodology of Gahbauer and his coworkers, which includes the effects of dose fractionation. The neutron field optimization parameters are evaluated for two epithermal neutron fields resulting from an accelerator-based neutron source with two different moderator assemblies. For the two moderator assemblies and moderator thicknesses evaluated, the D2O-Li2CO3 moderator assembly is superior to the BeO-MgO moderator assembly. The absorbed-dose delivered to the tumor for the D2O-Li2CO3 moderator assembly is larger than that for the BeO-MgO moderator assembly for almost all tumor depths. The treatment times for the D2O-Li2CO3 moderator assembly are slightly longer than for the BeO-MgO moderator assembly. However, for a 10-mA proton current, the treatment times for both are reasonable.