Papillary carcinoma of the thymus: report of four cases of a new microscopic type of thymic carcinoma.

We describe four cases of papillary carcinoma arising in the thymus. Three showed low-grade atypia and were associated with spindle cell-type thymoma. The fourth case showed high-grade atypia and had no evidence of an association with thymoma. Two of three low-grade cases invaded the lung and showed extensive lymphatic permeation. Cyst formation was seen in one case. Immunohistochemically, the tumor cells were positive for carcinoembryonic antigen (CEA), Leu-M1, Ber-EP4, calretinin (2 cases), and CD5 and were negative for thyroglobulin and pulmonary surfactant apoprotein. Ultrastructural findings for the one case studied by electron microscopy gave evidence against a mesothelial origin of the tumor. The histologic similarities and the intimate association with spindle cell thymoma indicate that papillary thymic carcinoma may arise from the papillotubular formations sometimes seen in the former tumor. Papillary thymic carcinoma, a lesion hitherto undescribed, should be considered in the differential diagnosis of anterior mediastinal tumors. Conceivably, it may be the source of some metastatic papillary carcinomas with or without psammoma bodies in cervical lymph nodes, in which no tumor is found in the thyroid gland.