Removal-diffusion theory for calculation of neutron distributions in BNCT

The routine use of Boron Neutron Capture Therapy (BNCT) for the radiation treatment of humans will require the planning of an acceptable RBE dose distribution for each patient. The first step in the calculation of a dose distribution is the determination of the neutron flux distribution. Neutron flux distributions for BNCT have been calculated by the Monte Carlo simulation and the Discrete Ordinates techniques. Use of removal-diffusion theory to calculate neutron distributions for BNCT offers some advantages over these techniques including, most notably, the relatively short amount of computer time necessary to calculate a neutron flux distribution. This work is an examination of the potential use of removal-diffusion theory to adequately describe the neutron flux distribution in a human head by first considering the calculation of the neutron flux distribution for the simpler geometry of a rectangular parallelopiped. Diffusion theory for the calculation of neutron flux distributions is usually developed as an approximation to transport theory. The assumptions that are made for steady-state conditions include: the angular neutron flux is at most linearly dependent on the scattering angle cosine; all the neutrons in an energy group are the same; and, neutron sources are isotropic. In removal-diffusion theory, source neutrons are transported in straight lines until they suffer their first collision. The resulting spatial and energy distribution of the once-collided neutrons is then used as the neutron source for a subsequent diffusion calculation