Computer simulation of neutron capture therapy

Abstract

Analytical methods are developed to simulate on a large digital computer the production and use of reactor neutron beams f or boron capture therapy of brain tumors. The simulation accounts for radiation dose distributions in tissue produced by fast neutrons and by neutron capture reaction products such as gamma rays, C -particles, protons, and heavy particles. These techniques are applied to optimize the effectiveness of the M.I.T. Reactor Medical Therapy Facility through a survey of the effects of neutron filters and of modifications to the beam collimation system. Neutron beams reflected from thin slabs of hydrogenous materials are shown to have an improved ability to effectively irradiate a deep tumor without destroying normal tissue above it because relatively few fast neutrons are reflected. Considerable improvements in thermal neutron distribution in tissue are shown to result from surrounding the head with a neutron-reflecting annulus to reduce lateral neutron leakage. A new numerical solution is obtained for the problem of neutron transport in finite thickness slabs with isotropic scattering. Gaussian quadratures are used to evaluate the neutron transport integral equations, yielding transmission, absorption, and reflection probabilities, and fluxes, as a function of collision number. Collision history correlations are devised which use only five paraeters to predict the fate of neutrons incident on an infinite slab having arbitrary thickness and neutron cross sections. A very fast multigroup neutron spectrum calculation is developed by combining collision history correlations with single-collision group transfer probabilities to directly obtain transmission and reflection matrices for multi-slab shielding problems.