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

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Abstract
Build-up and central axis depth-dose curves were obtained for d[deuteron](50) + Be, p[proton](42) + Be and d + T [tritium] neutron beams. Measurements carried out with the collimator opening covered with a layer of Pb showed that for all 3 neutron beams the entrance dose is approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams will be approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves were established for d(50) + Be neutrons at 129 cm SSD [source-surface distance], for p(42) + Be neutrons at 125 cm SSD and d + T neutrons and 60Co .gamma.-rays at 80 cm SSD. The 50% dose values in a water phantom were at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm, respectively, for field sizes of approximately 15 cm .times. 20 cm. Insertion of a 6 cm thick nylon filter in the p(42) + Be beam increased this value from 12.0 to 13.5 cm. The .gamma.-component for the d + T neutron beam was higher than for the cyclotron beams.

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