The RBE for Mouse Skin Irradiated with 3-MeV Neutrons: Single and Fractionated Doses

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Abstract
Early skin reactions on the feet of mice were measured after irradiation with 240-k Vp [Kilovolt peak] 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 [relative biological effectiveness] and repair capacity for neutrons and X-rays over the neutron dose per fraction range from 1.0-13.0 Gy [Gray]. The RBE increased with decreasing dose per fraction, and the 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 0.6-2.4. The D2 - D1 value was 5-10 Gy for X-ray doses up to 18 Gy and <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 < 3 Gy. The data were used to construct quasi-survival curves for epidermal cells. The X-ray data conform to an .alpha.D + .beta.D2 model; the neutron data indicate a predominant .alpha. component, with the initial response appearing linear from 0-5.0 Gy. Beyond this dose, 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 < 1.0 Gy.