Life history and histopathology of ultraviolet light-induced skin tumors

The life history and histopathology of UV light-induced skin tumors were studied in NMR rats, outbred female Swiss mice, and Syrian golden hamsters. High intensity UV light of medium wavelengths produced hyperplasia and papillomas, as well as a dysplastic, intermediary solar keratosis-like stage, with distinct cellular atypia leading to several types of squamous cell carcinomas. High doses of UV irradiation of short duration caused scars, which developed into fibromas and fibrosarcomas composed of "light" and "dark" cells. Carcinomas with neoplastic squamous and fibrous components were uncommon; however, collision tumors with two components were occasionally seen. Angiomas and angiosarcomas with a proliferating endothelial structure were observed, but adnexal tumors, with follicular or sebaceous differentiation, and basal cell carcinomas were infrequent. Pigment cell tumors were found only rarely. The number of tumors and tumor-bearing animals at different stages of the experiment were also studied. Tumors were compared with lesions induced by chemical carcinogens in different systems. UV carcinogenesis was characterized by many tumor-bearing animals, but with a low total tumor count and a high mortality, thereby decreasing the number of animals-at-risk. The tumor types, their progression from on type to another, and the distribution of certain biologic characteristics were also analyzed. We concluded that UV irradiation is an effective tumor inducer in animal skin, and the type of tumor, its behavior, and location depend on the experimental conditions.