The RBE for mouse skin irradiated with 3-MeV neutrons: single and fractionated doses

Early skin reactions on the feet of mice were measured after irradiation with 240-kVp X rays or with neutrons from a 4-MV Van de Graaff accelerator. The results are compared with previous experiments using cyclotron-produced neutrons. Single doses, or 2, 5, or 9 equal fractions, were given to measure the RBE and repair capacity for neutrons and X rays over the neutron dose per fraction range from 1.0 to 13.0 Gy. The RBE increased with decreasing dose per fraction, and these data could be fitted with a straight line on a log-log plot. The RBE was 4.6 at 1.0 Gy and fell to 2.1 at 13.0 Gy. These RBE values are significantly higher than those from cyclotron-produced 7.5-MeV neutrons, especially at low doses per fraction. The repair capacity was calculated from a comparison of the single-dose and fractionated data over a skin reaction range from 0.6 to 2.4. The D2 - D1 value was 5-10 Gy for X-ray doses up to 18 Gy and less than 1.5 Gy for 7 Gy of neutrons. This reduced repair capacity after neutrons was even more apparent with further fractionation, becoming close to zero for neutron doses below 3 Gy. These data were used to construct quasi-survival curves for epidermal cells. The X-ray data conform to an alpha D + beta D2 model, whereas the neutron data indicate a predominant alpha component, with the initial response appearing linear from zero to 5.0 Gy. Beyond this dose, however, a potentially reparable beta component of neutron damage is detectable, with the same value of beta as for X rays. Thus it is the greatly increased efficiency of the alpha component relative to X rays, rather than an absence of a shoulder after neutrons, that gives rise to the low D2 - D1 values at low doses and to the high RBE. The ratio of alpha components for X rays and neutrons would indicate a limiting RBE of 7.2 at X-ray doses below 1.0 Gy.