Mammary oncocytoma in a female dog: Histochemical, immunohistochemical and ultrastructural findings

Abstract Oncocytic cells are characterised by an abundant, eosinophilic cytoplasm, filled with numerous mitochondria. Oncocytic proliferative lesions are rarely reported in the mammary glands of dogs. In this study, the second documented case of mammary oncocytoma in a dog is presented. Histopathological evaluation using haematoxylin and eosin staining revealed a proliferation of well‐organised cell clusters, lacking basement membrane invasion. The cells exhibited a round shape with abundant, eosinophilic and granular cytoplasm, consistent with oncocytoma. The diagnosis was confirmed through positive phosphotungstic haematoxylin histochemical staining, immunohistochemical labelling for mitochondrial antibody in most of the neoplastic cells, and ultrastructural examination via transmission electron microscopy, which revealed a cytoplasm densely packed with altered mitochondria features that are not present in apocrine cells, besides their morphological similarity. The significance of diagnosing oncocytic lesions lies in distinguishing them from morphologically similar tumours, such as apocrine neoplasms.