A Monte Carlo simulation of fast neutron beams used for radiotherapy: I. Dose deposition by a collimated (d, t) beam

A Monte Carlo code (NCF) has been developed to calculate the neutron vector flux emerging from a collimator of given composition and geometry and is applied here to the collimator of a Marconi Avionics Hileton (d, t) facility using up to 107 neutron histories. For variations in the collimator geometry and in the polythene, tungsten and boron carbide content, the calculations verified that the original Hiletron collimator was well optimised for high-energy neutrons in the field, that the neutrons backscattered into the field from the surrounding shielding are significant only in the 0-0.5 MeV energy group and that the induced activation of a collimator differentially loaded with tungsten will allow its use with a source intensity of 10 13 s-1. A second code (MCCND) was used to calculate the dose deposited in a tissue-equivalent phantom using as input the neutron vector flux output from the collimator code NCF. The authors' calculations predict that a rather larger fraction of total dose comes from heavy recoil events than is suggested by some, but not all, of the previous calculations. The authors believe that previous experimental measurements have also tended to underestimate the high LET contributions.