Enhancement of skin tumorigenesis by cigarette smoke condensate following beta-irradiation in rats.

The tumor-promoting ability of cigarette-smoke condensate (CSC) has been demonstrated in rat skin after beta-irradiation. Skin tumors from male albino Charles River CD rats (outbred Sprague-Dawley descended) were classified into 2 groups: carcinomas and other noncarcinoma tumors. A statistically significant increase (P less than 0.01) in tumor yield occurred after CSC treatment that began 2 months after irradiation. This finding confirmed our previously published pilot observation. Extension of the pilot experiment to obtain data on carcinoma yield and an experiment to observe the effects of CSC treatment beginning immediately after irradiation were performed. When CSC treatment began immediately after irradiation, the yield of noncarcinoma tumors was significantly reduced (P less than 0.01), whereas the carcinoma yield increased but statistically not significantly (P = 0.12). The increased yield of noncarcinoma tumors is attributed to a significant increase (P less than 0.01) of acute ulceration caused by CSC on recently irradiated skin. The increase in carcinoma yield resulted from an increase in the rate of conversion of noncarcinoma tumors to cancer. Carcinoma yield was also increased by CSC treatment beginning 2 months after irradiation, but the increase was not significant (P = 0.08). The lack of statistical significance for the carcinoma yields in both experiments may be ascribed to the insufficient number of cancers produced by the treatments. The relative ratios of cancer yields (1.7 and 2.5) did not differ greatly from the 2.2 ratio for the increase of noncarcinoma tumors. The possible relevance of the findings to human carcinogenesis is discussed.