Answer: Extravasational mucocele

Histology: A portion of a minor salivary gland lobule is expanded by a lobular collection of epithelioid cells with cytoplasmic clearing. The cells are uniformly distributed in a loose myxoid stroma. A alcian blue stain demonstrates the presence of stromal mucin. By immunohistochemistry, the lesional cells are positive for CD68, and they are negative for AE1/AE3, p63, S100 and actin.

Discussion: A mucocele (mucus extravasation phenomenon) results from severance of a duct draining a minor salivary gland lobule. As a result, mucin spills into surrounding tissues. Unlike a mucus duct cyst that forms as a consequence of duct obstruction, the pool of mucin is not confined by a duct epithelium. The collection of mucin tends to elicit a zone of granulation tissue including infiltration by histiocytes. In this particular case, the epithelioid appearance of the histiocytes along with their cytoplasmic clearing gives the morphologic impression of a minor salivary neoplasm. The differential diagnosis of clear cell tumors of the salivary glands is lengthy, but almost always includes mucoepidermoid carcinoma, acinic cell carcinoma, myoepithelioma/myoepithelial carcinoma, and clear cell adenocarcinoma. Immunohistochemical stains confirm the histiocytic nature of the lesional cells and dismiss the possibility of an epithelial or myoepithelial neoplasm. Appropriate management of mucoceles consists of surgical excision including removal of the source of the mucin. If the minor salivary gland is not removed in its entirety, it will continue to drain into the surrounding tissues and the mucocele will reform.