Carcinoma of the thymus with clear-cell features. Report of eight cases and review of the literature.

We report eight cases of thymic carcinoma with clear-cell features and review the five previously reported cases. The tumor was composed of cells with clear cytoplasm and rather bland nuclear features, and showed a lobular or occasionally sheet-like growth pattern. Cytoplasmic glycogen was demonstrable in 92% of cases, whereas mucin was absent in all cases tested. The tumor cells were uniformly immunoreactive for low- and high-molecular-weight keratins and were reactive for epithelial membrane antigen in 22% of cases. The differential diagnosis includes mediastinal seminoma, parathyroid carcinoma, and metastatic clear-cell carcinoma. The diagnosis of primary thymic carcinoma depends on the exclusion of other primary sites clinically, particularly lung and kidney, and the demonstration of postivity for keratins and negativity for placental alkaline phosphatase. Thymic carcinoma with clear-cell features behaves as a high-grade thymic carcinoma: including our cases and the previously reported cases, nine of 13 patients (69%) are either dead of disease or have persistent disease at the latest follow-up. The importance of this entity is recognition of its aggressive clinical behavior and its distinction from other primary and metastatic clear-cell neoplasms of the mediastinum.