Irradiation of the lungs of mice with 1000 or 3000 rads of γ-rays prior to the i.v. injection of viable fibrosarcoma cells increased the number of tumor nodules found in the lungs when the animals were killed 20 days later. This effect of irradiation, which was observed for 2 days but not at 1 week after irradiation, is probably not related to immunosuppression or to inactivation of macrophages. Studies with radioisotope-labeled tumor cells showed that irradiation did not affect the initial lodgment of tumor cells in the lungs but, rather, that the subsequent dissemination to other parts of the body was less in irradiated animals than in controls. It is postulated that radiation effects on pulmonary capillaries and interstitial tissues may underlie the temporary increase in vulnerability of irradiated lung to metastasis formation. This phenomenon is useful in experimental techniques in which the lung colony assay system is used, since it can be utilized to increase the efficiency of viable tumor cell detection. No reports could be found in the literature that suggested that this phenomenon occurs in clinical radiotherapy.

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This investigation was supported by NIH Research Grants CA 6294 and CA 11138 from the National Cancer Institute.

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