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

M. C. Joiner, R. L. Maughan, J. F. Fowler and J. Denekamp
Radiation Research
Vol. 95, No. 1 (Jul., 1983), pp. 130-141
DOI: 10.2307/3576078
Stable URL: http://www.jstor.org/stable/3576078
Page Count: 12
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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-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 $D_{2}-D_{1}$ 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 below 3 Gy. These data were used to construct quasi-survival curves for epidermal cells. The X-ray data conform to an $\alpha D+\beta D^{2}$ model, whereas the neutron data indicate a predominant α component, with the initial response appearing linear from zero to 5.0 Gy. Beyond this dose, however, a potentially reparable β component of neutron damage is detectable, with the same value of β as for X rays. Thus it is the greatly increased efficiency of the α component relative to X rays, rather than an absence of a shoulder after neutrons, that gives rise to the low $D_{2}-D_{1}$ values at low doses and to the high RBE. The ratio of α components for X rays and neutrons would indicate a limiting RBE of 7.2 at X-ray doses below 1.0 Gy.

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