Oncocytic low-grade myoepithelial carcinoma ex pleomorphic adenoma - A rare case illustrating key learning points.

BACKGROUND: Oncocytic myoepithelial carcinoma ex pleomorphic adenoma neoplastic is a rare neoplastic event and may not display overt malignant radiological features. METHODS: Using routine histopathology and immunohistochemistry, we characterize a case of low-grade oncocytic carcinoma ex pleomorphic adenoma. RESULTS: The tumor arose in the left parotid gland in a 59 year old female. Computed tomography (CT) imaging demonstrated a well-defined, lobulated, enhancing lesion with relative central stellate hypoenhancement. Histologically, the tumor displayed a multi-nodular, non-destructive, invasive pattern, low mitotic activity (one mitotic figure per 10 high power fields) and a small remnant focus of pleomorphic adenoma. The neoplastic cells showed significant expression of cytokeratin 5/6, S-100 protein, smooth muscle actin and p63. CONCLUSION: Low-grade oncocytic carcinoma ex pleomorphic adenoma is a challenging histopathological diagnosis which can be established with use of immunohistochemistry, generous tumor sampling and recognition of the multi-nodular, non-destructive, pattern of invasion. In the absence of clear-cut tumor encroachment into external structures, its malignant nature may not be easily identified on pre-operative imaging.