Answer: Sebaceous adenoma

Histology: The tumor is comprised of cells in a nested arrangement within a sparsely cellular fibrotic background that lacks a lymphoid component. The tumor cells have a finely vacuolated cytoplasm. The vacuoles coalesce to form macro-vacuoles and micro-cystic spaces. The cells lack pleomorphism, and mitotic figures are not observed. A mucicarmine stain (not shown) is negative for mucin production.

Discussion: The unexpected demonstration of sebaceous differentiation in a primary salivary gland tumor occurs in the rare sebaceous lymphadenoma and even rarer sebaceous adenoma. They are believed to arise from sebaceous glandular elements that are found in up to 10% of parotid glands and intraparotid lymph nodes. The key defining element, of course, is the presence of epithelial cells demonstrating sebaceous differentiation in the form of finely vacuolated cytoplasm. Sebaceous adenoma differs from sebaceous lymphadenoma by the absence of a prominent lymphoid stroma.

Because of the squamoid appearance of the cells and the occasional presence of scattered mucinous cells, the differential diagnosis often includes mucoepidermoid carcinoma. A mucin stain is helpful in confirming that the cytoplasmic vacuoles do not represent intracellular mucin. The parotid gland is a frequent site of metastatic sebaceous carcinomas of the eyelids, but the malignant nature of these metastases is manifest by the presence of cytologic atypia, necrosis, and an undifferentiated cellular component, and a supportive clinical history.

Sebaceous adenoma of the parotid is a benign tumor that does not recur following complete surgical removal. There are a few reports in the literature of malignant transformation, but this is an exceedingly rare occurrence.