The response of mouse skin to irradiation with neutrons from the 62 MV cyclotron at Clatterbridge, U.K

Acute skin reactions on mouse feet were used to measure the effect of 62 MeV p-Be neutrons from the cyclotron at Clatterbridge, U.K. The results were compared with the response to 16 MeV d-Be neutrons from the cyclotron at Hammersmith, 4 MeV d-Be neutrons from the van de Graaff accelerator at the Gray Laboratory, and 250 kVp X-rays. For equivalent skin reactions, 9-16% more dose was needed with p(62)-Be neutrons compared with d(16)-Be neutrons. The top-up studies indicated that this figure might rise to approximately 14-32% at very low doses of neutrons, the value depending on the method of analysis of the data. Relative to X-rays, the RBE for p(62)-Be neutrons was 1.6±0.02 for a single X-ray dose of 30 Gy, rising to 2.9±0.04 for an X-ray dose per fraction of 4.6 Gy given 16 times. The full-course fractionation data and the top-up data together indicate an extrapolated limiting RBE at vanishingly small doses per fraction of 4.2-4.8 depending on the method of analysis. The X-ray data were well-fitted by a linear-quadratic (LQ) model of dose-fractionation, with α/β=8.6±1.5 Gy. The LQ model also provides a fairly good description of the neutron responses, α/β being large (>24) reflecting predominantly linear underlying dose-responses for all the neutron beams. This in turn reflects the small variation observed in the relative effectiveness between the 3 neutron beams with changes in dose per fraction. 29 refs.; 6 figs.; 3 tabs