Malignant spindle cell myoepithelioma mimicking peripheral nerve sheath tumour in canine mammary gland

Abstract Malignant myoepithelioma is characterised by neoplastic myoepithelial cell proliferation and is rare in human and canine mammary glands. This study presents a case of malignant myoepithelioma primarily composed of spindle cells mimicking peripheral nerve sheath tumours in a 7‐year‐old female dog. Microscopically, the tumour exhibited myoepithelial proliferation in a storiform pattern, with spindle cells forming concentric whorls, resembling peripheral nerve sheath tumours. Neoplastic cells were negative for various antibodies but showed positivity for calponin and p63. A high proliferative index (67.2%) supported the diagnosis of spindle cell malignant myoepithelioma. The diagnostic challenge lies in distinguishing malignant myoepithelioma from other spindle cell neoplasms. Additionally, myoepithelial cells display various appearances and their neoplastic malfunction leads to decreased expression of normal markers and contributes to a difficult diagnosis. The rarity of malignant myoepithelioma in both people and dogs emphasises the importance of comparative pathology in advancing the understanding of such a condition.