Ceruminous Gland Carcinoma with Malignant Proliferation of Acinar Epithelial and Myoepithelial Components in a Dog

A 6-year-old, castrated male Shiba-inu displayed a mass showing invasive growth in the external auditory canal of the left ear. Histologically, the tumor mass predominantly comprised spindle-shaped neoplastic cells, arranged in solid nests admixed with irregular tubulo-acinar structures, lined by cuboidal to polygonal neoplastic cells. Neoplastic cells had extensively infiltrated into deep tissues. No bone formation or chondroid matrix was observed in the tumor stroma. Both types of neoplastic cells showed nuclear atypia and high mitotic activities. Luminal epithelial elements were immunohistochemically positive for pan-cytokeratin and cytokeratin 8. Spindle-shaped cells showed intense immunoreactivity for vimentin, pan-cytokeratin and α-smooth muscle actin, indicating that spindle-shaped cells retained the original histological characteristics of myoepithelial cells. These findings were consistent with a diagnosis of ceruminous gland carcinoma with malignant proliferation of acinar epithelial and myoepithelial tumor cells, which rarely occurs in dogs. Following resection with wide surgical margins, no recurrence or metastases have been observed as of 6 months postoperatively.