Spontaneous skin neoplasms in aged Sprague-Dawley rats.

A total of 93 tumors of the epidermis, its appendages, and dermis were observed in 1,433 (717 males, 716 females) rats employed in oncogenicity studies over a 2-yr period. Mammary gland neoplasms will be reported separately. Fifty-seven (61.3%) were epithelial with 49 in males and 8 in females. Keratoacanthoma was the most frequent epithelial neoplasm in males (22) followed by squamous cell carcinoma (11) and papilloma (5). Sebaceous gland neoplasms seen in males (5) included both adenomas (3) and carcinomas (2). In males, there were also 3 trichoepitheliomas, 1 pilomatricoma, 1 basal cell tumor, and 1 malignant melanoma. Of the 8 epithelial neoplasms in females, there were 3 squamous cell carcinomas, 2 keratoacanthomas, and 1 each basal cell tumor, malignant melanoma, and trichoepithelioma. There were 21 mesenchymal neoplasms in males and 15 in females. The most frequent neoplasm was fibroma (7 males, 8 females) followed by lipoma (7 males, 4 females) and fibrosarcoma (4 males, 3 females). One male had a liposarcoma and 2 males each had hemangioma. The total neoplasm incidence of 70/717 (9.8%) in males and 23/716 (3.2%) in females showed that skin neoplasms were 3 time more common in males than in females. Epithelial neoplasms of the skin were 6 times more common in males than in females. Males were more than twice as likely to have epithelial rather than mesenchymal skin neoplasms whereas the reverse was seen in females.