Neoplastic and Non-Neoplastic Proliferative Mammary Gland Lesions in Female and Male Guinea Pigs: Histological and Immunohistochemical Characterization

Mammary tumors occur in female and male guinea pigs. However, published data on their histology and sex predispositions are limited. Histologically, we examined proliferative mammary lesions of 69 female and 48 male pet guinea pigs. Lobular hyperplasia was observed only in females (<i>n</i> = 50). Benign tumors included simple adenomas (<i>n</i> = 20), adenolipomas (<i>n</i> = 3) and intraductal papillary adenomas (<i>n</i> = 5). All except two intraductal papillary adenomas occurred in females. Most malignancies were tubulopapillary and solid carcinomas (<i>n</i> = 54), and intraductal papillary carcinomas (<i>n</i> = 13). These were diagnosed more frequently in male (<i>n</i> = 41) than in female (<i>n</i> = 26) guinea pigs. The carcinomas of males had higher mitotic counts than those of females (<i>p</i> = 0.05). Three carcinosarcomas developed in adenolipoma, and one arose in adenoma. Results show that the mammary tumor classification of dogs and cats can be applied to guinea pigs. However, some tumors (adenolipoma, metaplastic carcinoma) are unique to guinea pigs and shared with laboratory rodents and humans, respectively. Benign tumors may undergo malignant progression. Male guinea pigs appear predisposed to ductal-associated and malignant tumors. Data suggest that male guinea pigs represent an animal model for human male breast cancer.