Thymic epithelial neoplasms with sebaceous differentiation: a clinicopathological and immunohistochemical study of 8 cases.

Eight cases of primary thymic epithelial neoplasms corresponding to 7 thymomas and 1 thymic carcinoma with sebaceous differentiation are presented. The patients are 5 men and 3 women between the ages of 45 and 63 years (average, 54 years) who presented with nonspecific symptoms related to their mediastinal mass. All patients underwent complete surgical resection of the mediastinal mass. Grossly, all the tumors were described as round to oval measuring from 3.5 to 6.0 cm in greatest diameter. In 4 cases, the tumors were described as with infiltrative borders. Histologically, 1 tumor corresponded to thymic carcinoma characterized by irregular islands of tumors cells showing cellular atypia and mitotic activity. Six of the 7 thymomas showed mixed histologies corresponding to spindle cell, lymphocyte predominant, and mixed lymphocyte/epithelial types (World Health Organization types A, B1, and B2). One thymoma was mixed lymphocyte/epithelial (World Health Organization type B2). The areas of sebaceous differentiation characterized by clusters or strands of epithelial cells with ample clear cytoplasm were present within the lymphocytic component, whereas in the thymic carcinoma, the areas of sebaceous differentiation were identified within the epithelial component of the tumor. Follow-up information was obtained in 5 patients, showing that the patient with thymic carcinoma died 20 months after initial diagnosis, whereas the patients with thymoma whether encapsulated or minimally invasive remained alive without recurrence 12 to 24 months after initial diagnosis. The current cases represent an unusual feature that occasionally may be seen in thymomas and thymic carcinomas.