Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy

Build-up and central axis depth-dose curves have been obtained for d(50)+Be, p(42)+Be and d+T neutron beams. Measurements carried out with the collimator opening covered with a layer of lead showed that for all three neutrons beams the entrance dose was approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams are approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves have been established for d(50)+Be neutrons at 129 cm SSD, for p(42)+Be neutrons at 125 cm SSD and d+T neutrons and 60Co γ-rays at 80 cm SSD. The 50% dose values in a water phantom are at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm respectively, for field sizes of approximately 15 cm x 20 cm. Insertion of a 6 cm thick nylon filter in the p(42)+Be beam increased this value from 12.0 cm to 13.5 cm. The gamma component for the d+T neutron beam was higher than for the cyclotron beams. (author)