Malignant adenomyoepithelioma with papillary arrangement in the mammary papilla of a dog: Case report.

Mammary gland tumours are the most frequently diagnosed neoplasms in female dogs, involving either the mammary gland parenchyma or the mammary papilla. This is a case report of a malignant adenomyoepithelioma with papillary arrangement in the mammary papilla of a 13-year-old female Lhasa Apso, highlighting the histopathological and immunohistochemical characteristics of the tumour. Grossly, the sample consisted of a exophytic, soft to firm nodule measuring 1.0 × 1.0 cm. On the cut surface, the sample presented an irregular pattern and a cystic cavity. The cut surface of the sentinel lymph node showed a solid and heterogeneous aspect. Histologically, the mammary gland presented a neoplastic proliferation of epithelial and myoepithelial cells in a mostly papillary pattern. Myoepithelial cells exhibited elongated, manly vacuolated cytoplasm, with indistinct boundaries, moderate anisocytosis, anisokaryosis and moderate cellular and nuclear pleomorphism, with seven mitotic figures per 10 high-power fields (i.e., 2.37 mm2). The lymph node presented follicular hyperplasia. Immunohistochemical analysis of the primary tumour revealed neoplastic cells positive for pan-cytokeratin and p63, presented a Ki-67 index of 20% and were negative (score 0) for cyclooxygenase-2. Based on histomorphological and immunophenotypic findings, the mammary tumour was diagnosed as a malignant adenomyoepithelioma with a papillary pattern.