METASTATIC NEUROENDOCRINE TUMORS TO THE THYROID-GLAND MIMICKING MEDULLARY CARCINOMA - A PATHOLOGICAL AND IMMUNOHISTOCHEMICAL STUDY OF 6 CASES

The clinical, pathological, and immunohistochemical features of six ca ses of metastatic neuroendocrine and carcinoid tumors to the thyroid s imulating medullary thyroid carcinoma (MTC) are described. The patient s were women between the ages of 24 and 70 years who, without symptoms or significant past medical histories, presented with either a single mass or multiple thyroid nodules. The primary source of the tumor was only discovered on follow-up. Two of the neoplasms were classical car cinoid tumors, one was a carcinoid predominantly composed of large cel ls, another showed a prominent oval to spindle cell component, and the two remaining cases were atypical carcinoid/high-grade neuroendocrine carcinomas. The immunohistochemical profile was inconsistent with MTC in that all tumors were negative for calcitonin and only two were foc ally positive for carcinoembryonic antigen (CEA). A variable pat tern of staining for other neuroendocrine and epithelial markers was obtain ed in each case. Despite the morphologic and immunohistochemical simil arities with MTC, the diagnosis of a metastatic neuroendocrine tumor t o the thyroid should be favored in the presence of a predominantly int erstitial pattern of spread; occurrence of multiple tumor foci; follic ulotropism; rosette formations with lumen and cuticular borders: and l ack of immunoreactivity for calcitonin and CEA. The differential diagn osis between MTC and metastatic neuroendocrine carcinoma to the thyroi d is of importance because of the vast differences in treatment and pr ognosis.