PAPILLARY CARCINOMA OF THE THYMUS - REPORT OF 4 CASES OF A NEW MICROSCOPIC TYPE OF THYMIC CARCINOMA

We describe four cases of papillary carcinoma arising in the thymus. T hree showed low-grade atypia and were associated with spindle cell-typ e thymoma. The fourth case showed high-grade atypia and had no evidenc e of an association with thymoma. Two of three low-grade cases invaded the lung and showed extensive lymphatic permeation. Cyst formation wa s seen in one case. Immunohistochemically, the tumor cells were positi ve for carcinoembryonic antigen (CEA), Leu-M1, Ber-EP4, calretinin (2 cases), and CD5 and were negative for thyroglobulin and pulmonary surf actant apoprotein. Ultrastructural findings for the one case studied b y electron microscopy gave evidence against a mesothelial origin of th e tumor. The histologic similarities and the intimate association with spindle cell thymoma indicate that papillary thymic carcinoma may ari se from the papillotubular formations sometimes seen in the former tum or. Papillary thymic carcinoma, a lesion hitherto undescribed, should be considered in the differential diagnosis of anterior mediastinal tu mors. Conceivably, it may be the source of some metastatic papillary c arcinomas with or without psammoma bodies in cervical lymph nodes, in which no tumor is found in the thyroid gland.