Cystic and Mucinous Lesions: Mucocele and Low-Grade Mucoepidermoid Carcinoma

Cystic lesions are estimated to account for up to 8% of all salivary gland masses. They represent a wide range of salivary gland pathology from the non-neoplastic mucocele, salivary duct cyst, and sclerosing polycystic adenosis, to benign tumors such as cystadenoma and Warthin tumor, to malignant cystic tumors, including low-grade mucoepidermoid carcinoma and cystadenocarcinoma. In some cases, cystic degeneration occurs in what are ordinarily noncystic tumors such as pleomorphic adenoma and basal cell adenoma, and some tumors have cystic variants such as the papillary cystic subtype of acinic cell carcinoma. In the parotid gland, cystic lesions also include the difficult differential diagnosis of lymphoepithelial cysts, and metastatic cystic squamous cell carcinoma to an intraparotid lymph node. Of all of these entities, the most commonly encountered differential diagnostic problem is the cytologic distinction between mucocele and low-grade mucoepidermoid carcinoma. This chapter will focus foremost upon this common diagnostic problem, followed by an examination of other differential diagnostic entities.