Monte Carlo calculations of ion passages through brain endothelial nuclei during boron neutron capture therapy

The Monte Carlo technique is used to examine the passage of ions from the (n, p) reaction in nitrogen and the (n, alpha) reaction in 10B through the nuclei of endothelial capillary wall cells of normal white matter. Contributions to the dose from different regions are evaluated, as is the distribution of doses to the nuclei for mean doses from 1 to 100 Gy. Calculation of mean dose to the nuclei is generalized for any neutron fluence and element concentration. The dose-averaged LETs for both reactions are found giving (for the particular conditions of the calculation); 60 keV/microns for the (n, p) reaction and 216 keV/micron for the (n, alpha) reaction in boron limited to the lumen, and 224 keV/micron for a uniform distribution of boron. From a consideration of 'minimal survival' and hit and dose distributions, evidence is presented which suggests that the RBE for the (n, alpha) reaction depends upon the distribution of the boron in the tissue, with the uniform distribution being less effective at cell killing than the boron restricted to the lumen for the same mean dose to the cell nuclei.