Thymic Mucoepidermoid Carcinomas

Background: Primary thymic carcinomas are rare malignant neoplasms, of which at least 10 different histologic variants have been described. Among these variants, thymic mucoepidermoid carcinomas morphologically similar to the homonymous salivary gland counterparts are particularly unusual; only 9 cases for which clinicopathologic features were available have been reported. Objective: To study the clinicopathologic features of primary thymic mucoepidermoid carcinoma in an effort to better define their histologic features and biologic behavior. Design: The clinical and pathologic features of 10 cases of thymic mucoepidermoid carcinoma were reviewed and compared with those of previously reported cases. Results: The patients ranged in age from 8 to 84 years (mean 49 years), with 6 men and 4 women. The initial manifestations included respiratory symptoms and weight loss. Some patients were asymptomatic. All tumors were located adjacent to residual benign thymic tissue, and 3 cases were associated with multilocular thymic cysts. Histologically, the tumors consisted of sheets, lobules and nests of squamous, mucinous and intermediate cells in densely fibrotic stroma. There were 8 low-grade cases and 2 high-grade cases. The 2 patients with high-grade tumors died 1 year after diagnosis, whereas all but 1 patient with low-grade tumors in which follow-up was available were found to be alive and well on follow-up examination. One patient with low-grade tumor but high-stage disease died after developing a local recurrence. Among the 9 cases reported in the literature, 4 cases resulted in fatal outcomes; 2 cases were high-stage disease and 2 were high-grade tumors. Conclusions: Thymic mucoepidermoid carcinomas are predominantly low-grade tumors, and may be associated with multilocular thymic cysts. Poor prognosis is related to high-grade histology and high-stage disease.