A calculation of the energy dependence of the RBE of neutrons

The purpose of this paper is to obtain a relation for the absorbed-dose-averaged neutron relative biological effectiveness (RBE) versus neutron energy appropriate for boron neutron capture therapy (BNCT), using an empirical RBE-LET (linear energy transfer) relationship and accounting for energy deposition by recoil protons from the 1 H(n, n') 1 H reaction only. As a first step, the energy distribution of the proton fluence at a point in an infinite water medium is obtained, assuming isotropic neutron scattering on hydrogen as the source of the protons. As a second step, the corresponding LET distribution of the proton fluence is determined. As a third step, the proton RBE (for cells of human origin for a surviving fraction of 10%) as a function of LET is weighted by the product of the LET and the proton fluence LET distribution for various neutron energies E n , and then integrated over all LET to determine the neutron RBE as a function of E n . Finally, the resulting RBE as a function of E n is normalized to a clinically relevant beam-average neutron RBE for the Brookhaven Medical Research Reactor (BMRR) epithermal-neutron beam for BNCT. The resulting RBE is a maximum for a neutron energy of approximately 350 keV. Its value at this energy is 6.2.