The nature and extent of positive evidence associated with animal carcinogens vary widely, yet present regulatory policy does not permit adequate discrimination among the many carcinogenic substances. Most are treated as if they pose equal potential risk to humans, and this is not consistent with the available data. Without knowledge of carcinogenic mechanisms, the evaluation of responses in intact mammalian surrogates best reflects the potential levels of human risk. An example of a scoring system is proposed by which animal carcinogens are ranked according to the most relevant toxicological evidence derived from animal and genotoxicity studies. Different classes of animal carcinogens could thus be recognized and would permit several regulatory options and provide a means to establish priorities for public and scientific concerns.
Neoplastic and nonneoplastic lesions in untreated (C57BL/6N x C3H/HeN)F1 (B6C3F1) mice used as controls in carcinogenesis tests were tabulated and evaluated. The most common neoplasms in 2,543 male mice were hepatocellular adenomas and carcinomas. In 2,522 female mice, common tumors were lymphomas, leukemias, pulmonary adenomas and carcinomas, hepatocellular adenomas and carcinomas, and pituitary adenomas. The risk of developing most neoplasms increased with the age of the mouse. Hepatocellular carcinomas metastasized in 12% of the animals with these tumors. Other than lymphomas and leukemias, few other tumors metastasized. Nonneoplastic lesions included cystic hyperplasia of the uterus, nephritis, ovarian and uterine cysts, inflammatory lesions of the lung, mineralization in the brain, and focal hyperplasias in several tissues. The focal hyperplasias in lung and pituitary, adrenal, and thyroid glands were suggestive of the early stages of neoplasia. Comparative aspects of lesions in aging mice and their interpretation in carcinogenesis tests are discussed.
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