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Interaction of Urethan and Fractionated or Regional X-Radiation in Mice: Lung Tumor and Leukemia Incidence^*

( Department of Pathology, University of Minnesota, Minneapolis, Minnesota; and Biological and Medical Sciences Division, U. S. Naval Radiological Defense Laboratory, San Francisco, California)

Groups of young, adult (C57LXA)F₁ mice were treated with urethan and studied after single-dose or fractionated whole-body x-radiation, or with shielding or exposure of the thorax during radiation, or with shielding or exposure of the thorax during radiation. When a single dose (880 rad) of whole-body x-ray plus bone marrow was given 1 week or 8 weeks prior to a single dose of urethan which produced multiple pulmonary tumors in 100 per cent of control mice, the per cent mice with tumors and the average numbers of tumors per mouse were decreased.

Fractionation (100 rad x 9) of the radiation either before or after urethan reduced the average number of pulmonary tumors but not the percentage of mice with such tumors. Leukemia incidence, however, was increased whether urethan preceded or followed fractionated radiation. When the thorax was shielded and the rest of the body exposed no inhibition of pulmonary tumorigenesis was observed, but with thorax exposed and head and abdomen shielded both the percentage of animals with tumors and the average number of tumors per mouse were decreased. The results indicate an augmentation of leukemogenesis and an inhibition of pulmonary tumorigenesis by combinations of x-ray and urethan. The inhibitory effect is directly on lung.

^* This study was supported through funds provided by the Bureau of Medicine and Surgery, U. S. Navy Department. The opinions and assertions contained herein are those of the authors and are not to be construed as official or as reflecting the views of the Navy Department. A portion of this work was reported at the 55th Annual Meeting of the American Association for Cancer Research in Chicago, April 9–11, 1964.

Received 5/18/64.