Rat mammary tumor classification: notes on comparative aspects.

Mammary tumors (170 spontaneous and 1,613 induced with 7,12-dimethylbenz[a]anthracene) in inbred SD rats were classified histologically. The neoplasms were divided into three main categories: fibroepithelial, epithelial, and connective tissue. In the fibroepithelial category, compound tumors showing a wide range of histologic structures with variation in the arrangement of both epithelial and connective tissue elements that differ among lobules occurred in 44 (25.9%) spontaneous and 1,027 (63.7%) induced neoplasms, whereas fibroadenomas occurred in 41 (24.1%) untreated and 175 (10.9%) treated rats. Uncommon fissured tumors were found in 1 (0.7%) untreated and 6 (0.4%) treated animals, whereas carcinosarcomas were found in only 10 untreated animals. In the epithelial category, tubular adenomas occurred in 48 (28.2%) untreated and 164 (10.2%) treated animals. Cystadenomas, duct papillomas, intraductal carcinomas, and anaplastic carcinomas were found less frequently. Adenoacanthomas occurred in only 12 (0.8%) of the treated animals. Among tumors with a fibrous component only, fibromas occurred in 22 (13%) untreated and 97 (6.0%) treated animals, Fibrosarcomas were less frequent, occurring in 4 untreated (2.4%) and 54 (3.4%) treated animals.