Effect of hyperthermia on epithelial microneoplastic cell populations induced by irradiation of rat skin.

Two groups of male rats of the Charles River CD stock received a dose of 1,600 rad beta-radiation (700 rad/min) on the skin of the dorsum. Two months later, the site of irradiation of one of the groups was treated with hyperthermia at 44 degrees C for 2.5 minutes. A third control group received only the hyperthermic treatment. Over 90% of the animals in the 2 irradiated groups developed skin tumors (benign and malignant epithelial) at the irradiated site. There was no significant difference between these 2 groups in incidence of animals with tumors, incidence of tumors, distribution of tumor types, or rate of tumor appearance. The incidence of animals with tumors in the control group was less than 4% at any time.

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.

Association of malignancy with rapid growth in early lesions induced by irradiation of rat skin.

Epithelial lesions induced by irradiation of rat skin were studied to determine a) the relationship of malignancy to dose, b) the types of lesions and circumstances leading to overt malignancy, and c) the growth rates of lesions progressing to malignancy versus those of lesions remaining benign. High doses of radiation were shown to be associated with the production of epidermal cancers, the maximum yield being obtained at 6,400 rads. Conversely, a peak yield of noncancerous lesions was obtained at 1,600 rads. This association between malignancy and high dose was consistent for cancers evolving from warts, cysts, and chrnoic ulcers. Although the proportion of warts among the induced lesions was much higher than that of the cysts or chronic ulcers (76, 14, and 10%, respectively), the likelihood of warts becoming cancerous was substantially lower (14, 23, and 21%). The combined data for all doses showed that the latency period of the epidermal cancers was significantly (P = 0.015) shorter than that of the benign tumors. Rapid growth rates were observed for warts, cysts, and chronic ulcers progressing to overt cancer, and these did not overlap at any point on the growth scale with rates for benign tumors. This finding suggested that the potential for malignant development had been established early in the carcinogenic process, very likely at induction.

Tumor-promoting activity of cigarette tar in rat skin exposed to irradiation.

A threefold increase (P=0.006) in the number of skin tumors induced by beta irradiation was demonstrated in rats after surface applications of cigarette tar. The effect appeared due to a tumor-promoting activity of the cigarette tar rather than to an additive tumorigenic effect of the two agents.