A Monte Carlo simulation of fast neutron beams used for radiotherapy: II. Cyclotron based beams
Smith FA., Hill MA., Meek JH.
For part I see ibid vol.34, p.965-82, 1989. The codes NCF and MCCND which have been previously tested on the (d, t) beam are applied here to unfiltered cyclotron based neutron beams using 16 and 50 MeV deuterons and 55 MeV protons on Be targets having a range of thicknesses. The collimator material is assumed to be high density polythene having the dimensions necessary to give a circular field equivalent in area to 7 cm*7 cm at the phantom surface with values of SSD that are commonly used in clinical practice. The phantom is again assumed to be standard man tissue equivalent. Calculations were made of the dose per neutron, the depths of the maximum and 50% maximum dose, the ratio of gamma to total dose, the dose: LET distributions and the dose- and track-averaged LET. The averaged LETs were found to be mostly constant with depth, except in the build-up region where, in the case of the higher energy beams, a significant reduction takes place between the surface and the dose maximum. In the distribution of dose between three broad LET regions, the authors' calculations show that the (d, t) beam has the highest fraction in the range 88-240 keV mu m-1 with the highest mean energy beam, p(55)Be(10), showing the highest fraction in the heavy recoil region, 240-1250 keV mu m-1.