BNCT filter optimisation using two- and three-dimensional discrete ordinates

An initial version of an optimisation code utilising two-dimensional radiation transport methods has been completed. This code is capable of predicting material compositions of a beam tube-filter geometry which can be used in a boron neutron capture therapy treatment facility to improve the ratio of the average radiation dose in a brain tumour to that in the healthy tissue surrounding the tumour. The optimisation algorithm employed by the code is very straightforward. After obtaining an estimate of the gradient of the dose ratio with respect to the nuclide densities in the beam tube-filter geometry, changes in the nuclide densities are made based on the magnitude and sign of the components of the dose ratio gradient, the magnitude of the nuclide densities, the upper and lower bound of each nuclide density, and the linear constraint that the sum of the nuclide density fractions in each material zone be less than or equal to 1.0. A local optimal solution is assumed to be found when the conditions are satisfied in every material zone. Three test cases which illustrate the optimisation procedure have been analysed and improvements in the dose ratio of 50% or more were obtained. (R.P.)