Section 6 -

Cystic and Mucinous Tumors (Dr. Powers) William C. Faquin, M.D., Ph.D. Celeste N. Powers, M.D., Ph.D.

Clinical History: A 58 year-old man with a 2.0 cm non-tender, enlarging left parotid gland mass. An FNAB was performed. Cytologic Findings: The specimen was hypocellular and cystic, consisting of abundant background mucin, scattered lymphocytes and macrophages, and occasional clusters of cohesive epithelial cells. The epithelial cells were cytologically bland with oval nuclei, indistinct nucleoli, and a small amount of dense waxy cytoplasm giving the cells an overall squamoid appearance. Rare cells admixed with the squamoid epithelial cells contained intracellular mucin and had a signet ring appearance. A mucicarmine stain performed on the cell block material confirmed the presence of intracellular mucin. Based upon these cytologic findings a diagnosis of low-grade mucoepidermoid carcinoma was made. The lesion was excised via parotidectomy, and histologic examination confirmed the diagnosis of low-grade mucoepidermoid carcinoma. Section 6 - Figure 1 Section 6 - Figure 2 Discussion: Low-grade Mucoepidermoid Carcinoma Mucoepidermoid carcinoma is the most common salivary gland malignancy in children and adults and the most common malignancy of the major and minor salivary glands. The cytology of these tumors is variable depending upon the grade of the tumor, but low-grade forms are the most frequently encountered. Low-grade mucoepidermoid carcinoma is also the most common cause of a false-negative cytologic diagnosis, in part because the lesion is cystic and aspirates may yield only cyst contents, and partly because the epithelial cells are bland, and the mucin-containing cells can easily be misinterpreted as histiocytes or muciphages. Cytologically, low-grade mucoepidermoid carcinoma is characterized by a combination of bland cohesive squamous cells, intermediate cells, and mucus cells in a background of cystic mucinous material. These lesions can be extremely challenging, and when possible, material for a cell block preparation should be obtained. The latter can be quite useful since ancillary marker stains for keratins and for intracellular mucin will help to solidify the diagnosis. Section 6 - Figure 3 Low-grade mucoepidermoid carcinoma Cytologic features of low-grade mucoepidermoid carcinoma: Mucus-containing epithelial cells Epidermoid cells (have squamous features) Intermediate cells (low N/C ratio) Mucinous background Histologic Features of Low-Grade Mucoepidermoid Carcinoma: Low-grade mucoepidermoid carcinomas are predominantly cystic tumors lined by a combination of epidermoid, intermediate, and mucinous (goblet) cells. A dense fibrous stroma is often present, and sometimes, a dense lymphocytic infiltrate may be present as well. The tumor often invades with a broad almost pushing-type advancing front. Section 6 - Figure 4 Low-grade mucoepidermoid carcinoma showing a cystic pattern with epidermoid and mucinous epithelial cells in a dense fibrous stroma with lymphocytes. Differential Diagnosis of Mucus-Containing Cysts: The differential diagnosis of non-neoplastic and low-grade tumors where mucus-containing cells predominate includes a mucocele (a pseudocyst because it lacks an epithelial lining), ductal mucinous metaplasia (mucus retention cyst) in sialolithiasis, mucinous metaplasia in Warthin tumor or pleomorphic adenoma, and low-grade mucoepidermoid carcinoma. The key to distinguishing low-grade mucoepidermoid carcinoma from benign cysts is to search for the characteristic 3 cell types, and especially the combination of mucus-containing epithelial cells and squamoid or intermediate cells. Sometimes a definite diagnosis is not possible, especially when cell block material is not available or only cyst contents are aspirated. Aspirates of mucoceles are hypocellular and contain histiocytes, mucinous background material, scattered inflammatory cells, and sometimes crystals. Section 6 - Figure 5 Mucocele Differential Diagnosis of mucinous cysts: Mucocele Mucus retention cyst Low-grade mucoepidermoid carcinoma Warthin tumor with mucinous metaplasia Pleomorphic adenoma with mucinous metaplasia