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Mammary Neoplasia in the Rat following High-Energy Neutron Irradiation

Division of Radiation Biology, Department of Radiology [J. L. M.], Department of Pathology [R. C. H.], and Department of Biostatistics [R. E. F.], Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298

Young female Sprague-Dawley rats were exposed to single doses of 35-MeV (maximum-energy) neutrons. During the 11 months following exposure, 2 of 31 unirradiated rats each developed a single fibroadenoma. Over the same time period, a total of 42 breast tumors were observed in 39 of 135 irradiated rats. Tumors originating from sites other than the breast included: lymphocytic-type lymphosarcoma, osteogenic sarcoma, myxosarcoma, fibrosarcoma, and angiosarcoma. Analysis of the relationship between the number of breast tumors per rat and radiation dose, in this and other neutron experiments, indicates that the number of tumors per rat is dependent upon (dose)'' where n is slightly less than unity. When the data were modified to account for the number of breasts at risk, the dose response approached a straight line (i.e., the exponent, n, approached 1.0). These findings are consistent with a model of carcinogenesis in which a single high-linear-energy transfer particle traversing a single cell can cause a carcinogenic alteration. The relative biological effectiveness of the 35-MeV neutrons compared to -rays for breast tumor induction was found to be 4.3 based on the slopes of the linear responses to both types of radiation. With the best-fit curves for both neutrons and -rays, the relative biological effectiveness increased from 5.0 at 40 rads of neutrons to 13.8 at 2.5 rads of neutrons.

¹ To whom requests for reprints should be addressed, at Medical College of Virginia, Box No. 31, 1200 E. Broad Street, Richmond, Va. 23298.

Received 10/28/76. Accepted 5/ 6/77.