Spontaneous mammary neoplasms in guinea pigs: 85 cases (2006-2022)

A scoping search of the literature review found 30 clinical or clinical research articles on spontaneous mammary neoplasms in guinea pigs between 1908 and 2023. Despite the many reports, only 4 described tumor biological behavior and/or the presence of metastases. This retrospective study collected data on the tumors found in guinea pigs' mammary/inguinal regions. Both mammary and mesenchymal (lipomatous) neoplasms were seen. Survival time after diagnosis of a neoplasm was evaluated. For mammary neoplasms, risk factors, such as age and sex, and prognostic factors, such as neoplasm malignancy and the effect of surgery on survival, were investigated. Medical records of guinea pigs with mammary/inguinal neoplasms between 2006 and 2022 from 3 Parisian veterinary referral hospitals and 1 French diagnostic pathology laboratory were reviewed. Eighty-five (77%) guinea pigs had mammary neoplasms, and 25 (23%) had lipomatous neoplasms. A logistic regression model highlighted that males have significantly increased odds of malignant mammary neoplasms than females, while females (85%) are often found to have benign mammary neoplasms. Malignancy was significantly associated with aging. Older animals had greater odds of having malignant neoplasms than young animals. The mean age at diagnosis for mammary/inguinal lipomas was 3.0 years, while a benign mammary neoplasm was 3.1 years, and a malignant mammary neoplasm was 4.2 years. The survival prognosis was not influenced by the histopathological diagnosis of malignant versus benign neoplasm or the surgical technique (mastectomy vs lumpectomy) to treat the mammary neoplasm. The median survival time after diagnosis of a mammary neoplasm was 1.2 years, and for an inguinal lipoma was 3.0 years. Twenty-three percent of neoplasms in the mammary/inguinal region are lipomatous. Male guinea pigs usually have a malignant mammary neoplasm, while females mostly have a benign one. Malignancy is typically seen in older animals. In this dataset, survival time was unaffected by malignant versus benign neoplasm diagnosis or surgical treatment. The survival analysis results should be considered with caution in light of the small sample size.