Dose distributions of clinical fast neutron beams

The dosimetric properties of sixteen clinical fast neutron beams are reviewed. 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, varied between about 8 and 14 cm, depending mainly on the neutron-producing reaction and source-to-surface (SSD) employed. The gamma-ray contribution to the total n + γ dose in the phantom showed large discrepancies between the different machines. The average cyclotron values were lower than the comparable average data for d + T neutron sources. The penumbral width, at 10 cm depth in a phantom decreased with increasing average neutron energy. The edges of the useful beam were less sharp for the d + T neutron generator than for the cyclotron beams. Measurements of the build-up of the dose below the surface showed large variations between the different machines. 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 comparable to those of electron beams. (author)