First report of malignant adenomyoepithelioma in a guinea pig (Cavia porcellus).

Mammary cancer is one of the most common neoplasms in women and female dogs, and it has also been found to occur with significant frequency in guinea pigs (Cavia porcellus). A 4-year-old female guinea pig presented with a firm, irregular shaped mass, measuring 6.6 cm in the largest axis, in the right mammary gland. Cytological examination suggested mammary carcinoma. Based on these results, a right inguinal mammectomy was performed. Histological examination revealed a biphasic neoplastic proliferation in the hypodermis consisting of epithelial cells in cribriform, papillary, tubule and nest arrangements, and malignant myoepithelial cells arranged in multidirectional bundles and nests, with marked anisocytosis and anisokaryosis. Immunolabelling was positive for GATA3 and Ckpan in the epithelial component and positive for p63 in the myoepithelial component. After 71 days, the patient developed severe dyspnoea and agitation, and chest radiography showed several rounded structures distributed multifocally throughout the lung parenchyma, the largest of them measuring 0.9 × 0.6 cm. Macroscopic and histological examinations revealed multifocal metastases in the lung parenchyma, multifocal suppurative pneumonia and severe diffuse pulmonary oedema, confirming the cause of death as respiratory failure due to lung metastases. A metastatic malignant adenomyoepithelioma was diagnosed on the basis of the macroscopic, histological and immunohistochemical findings.