Dose distributions of clinical fast neutron beams

Abstract

A review is given of dosimetric properties of sixteen clinical fast neutron beams. The central axis depth dose, gamma-ray contribution, beam profile and dose build-up have been compared for about equal field sizes. The depth in the phantom, at which the central axis total n+γ dose has been reduced to 50% of its maximum value, varies between about 8 cm and 14 cm, depending mainly on the neutron-producing reaction and SSD employed. The gamma-ray contribution to the total n+ γ dose in the phantom shows large discrepancies between the different machines. The average cyclotron values are somewhat lower than the comparable average data, at equal depth, for d+T neutron sources. The penumbral width, measured at 10 cm depth in a phantom, is decreasing with increasing average energy of the neutrons. The edges of the useful beam are less sharp for the d+T neutron generator than for the cyclotron beams. Measurements of the build-up of the dose below the surface show large variations between the different machines, even for comparable neutron energy spectra. Due to the relatively high RBE of the short range particles, the skin-sparing properties of all neutron beams will probably not differ considerably and are more or less comparable to those of electron beams.