An experimental study of the effect of irradiation on the dissemination of cancer.

Stimulated by the clinical impression tha t some patients show rapid development of distant metastases for the first time shortly following surgical or irradiation treatment of their primary tumor, the surgical service of the National Cancer Institute has undertaken a series of experiments dealing with the effect of surgical excision or local irradiation of a transplantable mouse tumor on the development of distant metastases arising from this tumor. An experimental study by Schatten (5) revealed that surgical excision of two different transplantable mouse tumors results in, or is attended by, an increased number of pulmonary metastases or an increase in average size of metastases when compared with a control group. A similar experiment undertaken to study the effect of local irradiation likewise demonstrated changes in the pattern of pulmonary metastases (~). In a study involving 131 irradiated mice and 89 control mice, the irradiated animals received 8000 r locally to the "primary" tumor. When one tabulated the number of spontaneous pulmonary metastases found 3 weeks following irradiation of an intramuscular melanoma and compared this with the number of pulmonary metastases in an untreated group of mice, two effects were noted. A large group of treated animals averaged fewer metastases than did the controls, and a smaller group averaged many more metastases than did the controls. :Forty-five per cent of the irradiated group had less than three pulmonary metastases, as compared with 28 per cent in the controls; this included 19 per cent of the irradiated mice completely free of pulmonary metastases as compared with only 6 per cent of the controls. Also of great interest was the finding in eleven of 181 irradiated mice of more metastases than were found in any of their respec-