Mucoepidermoid carcinomas of the thymus. A clinicopathologic study of six cases.

Six cases of mucoepidermoid carcinoma of the thymus are presented. The patients were two men and four women aged 17 to 66 years (median age, 34.5). Clinically, three patients had symptoms of chest discomfort and three were asymptomatic. Grossly, the tumors in three patients were described as cystic structures varying in size from 7 to 8 cm in greatest dimension, with focal areas of induration within the walls of the cyst that averaged from 1.5 to 3 cm. The other three cases had grossly and radiographically well-circumscribed, homogeneous tumor masses. Histologically, the lesions showed a spectrum of features that ranged from those of well-differentiated, to moderately well-differentiated, to poorly differentiated mucoepidermoid carcinoma, with sheets and solid islands of squamoid cells admixed with mucin-secreting epithelium lining gland-like spaces. In four cases, the tumor was histologically seen in continuity with the epithelial lining of multilocular cystic structures; the nonneoplastic components of the cysts contained abundant inflammation and showed the features of otherwise conventional acquired multilocular thymic cysts. Clinical follow-up showed that the two patients with intermediate and high-grade tumors died within 2 and 7 months after initial diagnosis. One of these patients showed at autopsy residual tumor limited to the mediastinum, whereas the other patient died with metastases to pericardium and myocardium despite postoperative radiation therapy. Two patients with low-grade tumors were alive and well with no residual disease 2 and 3 years after surgery, and the other two were lost to follow-up. Mucoepidermoid carcinoma of the thymus should be included in the differential diagnosis of cystic neoplasms of the thymus. As with their counterparts at other sites, the biologic behavior of these tumors closely correlates with their degree of differentiation and amount of cytologic atypia.