Morphologic features and incidence of spontaneous hyperplastic and neoplastic mammary gland lesions in Wistar rats.

A morphologic study of spontaneous proliferative lesions of the mammary gland was based on histologic examination of mammary glands from 1020 male and 1145 female albino Wistar rats aged 6 to 110 weeks. Three hundred and seventy-five mammary tumors representing an overall incidence of 33% were identified in the female groups, while the males had a total of five tumors which represented an incidence of 0.5%. Histologically, the most common types of tumors in females were: fibroadenoma (236), carcinoma (85), adenoma (40) and fibroma (8). Duct papilloma (1), lobular carcinoma (1), fibrosarcoma (1) and phyllodes tumor (1), were rare and constituted less than 2% of mammary neoplastic lesions. Mammary tumors were rare before one year of age, but increased with age thereafter. Nine percent of female rats studied also had proliferative non-neoplastic lesions that showed a mixture of benign ductular and/or lobular hyperplasia. One hundred and six of these lesions were identified, representing 22% of all grossly palpable nodules, thus stressing the importance of histologic examination of all gross mammary nodules for tumor evaluation.