Spontaneous skin neoplasms in Long-Evans rats

Two hundred sixty-one neoplasms involving the skin and its appendages were observed in a total of 980 (490 of each sex) Long-Evans (LE) control rats of 7 carcinogenicity studies. All studies were 2 yr in duration and were conducted in the years 1983 through 1994. Skin neoplasms were reevaluated and classified according to current criteria and nomenclature. The various neoplasm incidences were compared to those published for Sprague-Dawley (SD), Fischer-344 (F-344), and Wistar rats. The most common neoplasms involved the mammary glands, affecting 28% of LE females. This incidence is comparable to the 31% incidence reported for SD females. Mammary gland fibroadenomas were observed in 22% of LE females compared to 19-31% of SD, 7-29% of F-344, and 4-25% of Wistar females. Mammary gland adenocarcinomas occurred in 7% of LE females in our laboratory, while incidences of 9-17% of SD, 0-2% of F-344, and 1-13% of Wistar females are reported. In LE males, the incidences of mammary gland fibroadenomas (1%) and adenocarcinomas (0.6%) were comparable to the 0-3% reported recently in other strains. As is observed in other strains, the incidence of nonmammary gland skin neoplasms in LE males (11.4%) was greater than the incidence in females (2.7%). Fibromas were the most common nonmammary gland skin neoplasm and affected 3.3% of males and 0.6% of females. The incidences of fibromas in SD, F-344, and Wistar rats range from 0-7%, 0-3.2%, and 0-12%, respectively, in males and from 0-3.3%, 0-1.1%, and 0-4%, respectively, in females. Of the 4 strains, only SD males have more nonmammary gland epithelial than mesenchymal skin neoplasms. The difference between the LE and SD males can be attributed to the criteria used to classify keratoacanthomas. Only one keratoacanthoma was observed in LE rats in this review. Reevaluation of skin neoplasms found in 980 LE control rats and comparison of the resulting incidences with those of other strains demonstrated that most skin neoplasms in LE rats have comparable incidences to those reported for SD, F-344, and Wistar rats.