Atlas and histologic classification of tumors of the rat mammary gland

Studies performed in experimental animal models have demonstrated that mamm ary cancer is a complex multistep process that can be induced either by che micals, radiation, viruses, or genetic factors. Rodent models have been use ful for dissecting the initiation, promotion, and progression steps of mamm ary carcinogenesis. Chemically induced mammary tumors, such as those induce d by 7,12-dimethylbenz(a)anthracene and N-methyl-N-nitrosourea, are, in gen eral, hormone-dependent adenocarcinomas whose incidence, number of tumors p er animal, tumor latency, and tumor type are influenced by the age, reprodu ctive history, and endocrinologic milieu of the host at the time of carcino gen exposure as well as by diet and the dose of carcinogen administered. Th ere is a need to classify tumors according to their histopathological type because those characteristics have implications in the interpretation of ex perimental data. In the classification presented here we attempt to provide a working framework for diagnosis of the type of lesions found in the mamm ary glands of rats treated with chemical carcinogens or radiation and to cl arify criteria for establishing the basic biological characteristics of tum ors.