In our previous studies carried out on inbred rats of the Sprague-Dawley strain (L. Gross and Y. Dreyfuss, Proc. Natl. Acad. Sci. USA, 76: 5910–5913, 1979), the tumor incidence was increased following irradiation (150 rads, 5 times, at weekly intervals), from 22 to 93% in females and from 5 to 59% in males. Experiments here reported suggest that 2 consecutive total-body γ-irradiations of 150 rads each are sufficient to induce in rats the development of tumors, some malignant; 18 of 19 females (94.7%) developed tumors at an average age of 11.4 mo, and seven of the 14 males in this group (50%) developed tumors at an average age of 10.4 mo. In the second group, which received 3 consecutive γ-irradiations, 20 of 23 females (86.9%) and 5 of 13 males (38.4%) developed tumors at average ages of 9.1 and 7.5 mo, respectively. In the third group, among rats which received 4 consecutive γ-irradiations, 17 of 19 females (89.4%) and 4 of 12 males (33.3%) developed tumors at average ages of 9.4 and 10.5 mo, respectively.

The etiology of tumors either developing spontaneously or induced by irradiation in rats remains to be clarified. Our attempts to detect virus particles by electron microscopy in such tumors or lymphomas have not been successful. As a working hypothesis, we are tempted to theorize that tumors or lymphomas developing spontaneously or induced by gamma irradiation in rats are caused by latent viral agents which are integrated into the cell geome and are cell associated, i.e., not separable from the rat tumor cells by conventional methods thus far used.

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Supported by the Veterans Administration Research Service and by grants from the Cancer Research Institute, New York, NY, and Pergamon Press, Oxford, England, and New York, NY.

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©1988 American Association for Cancer Research.
1988
American Association for Cancer Research, Inc.