Primary mucoepidermoid carcinoma of the thyroid gland: a report of six cases and a review of the literature of a follicular epithelial-derived tumor.

Primary mucoepidermoid carcinomas of the thyroid gland are uncommon tumors of low-grade malignant potential. We report six cases of thyroid mucoepidermoid carcinoma, four occurred in women and two in men with an age range of 29 to 57 (median, 46 years). The clinical presentation was that of a painless mass. Radiographic studies showed a single, solid, "cold" nodule in either the right or left lobe, or isolated to the isthmus. There was no history of a mucoepidermoid carcinoma developing in more typical locations (eg, salivary gland) in any of the patients. Histologically, the tumors were characterized by an intimate admixture of squamoid/epidermoid cells and mucocytes. The tumors were delineated but not encapsulated with prominent cyst formation and a variable amount of associated fibrosis. The squamoid component showed horny pearl formation, individual cell keratinization and/or the presence of intercellular bridges. Intracytoplasmic and luminal epithelial mucin was observed in all cases. In three of the cases a prominent eosinophilic cellular infiltrate was intimately identified within the neoplastic proliferation. One other case was noteworthy for the presence of ciliated epithelium. In all of the cases the uninvolved thyroid tissue showed the presence of lymphocytic thyroiditis. Rare foci of squamous metaplasia were observed in two of the cases. A microscopic focus of thyroid papillary carcinoma was observed adjacent to the mucoepidermoid carcinoma in one case. Immunohistochemical evaluation of the mucoepidermoid carcinoma showed the following antigenic profile: cytokeratin (five of five), CAM 5.2 (four of four), thyroglobulin (five of six), calcitonin (none of six), chromogranin (none of six), polyclonal carcinoembryonic antigen (four of four), and monoclonal carcinoembryonic antigen (none of five). Surgical excision was the treatment of choice. All of the patients reported are alive without disease (recurrence or metastasis) over periods ranging from 1 to 15 years. Based on our findings, we believe that these tumors are of low-grade malignant potential and originate from thyroid follicular epithelial cells rather than from solid cell nests of the ultimobranchial body.