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 H-1(n, n')H-1 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. is normalized to a clinically relevant beam-average neutron RBE for the Brookhaven Medical Research Reactor (BMMR) 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.