Gastroenteropancreatic neuroendocrine tumor metastases to the thyroid gland: differential diagnosis with medullary thyroid carcinoma

Neuroendocrine tumors (NET) of the thyroid gland are rare. Apart from medul lary thyroid carcinoma (MTC), metastases of gastroenteropancreatic (GEP) NE T may also occur. Features of six patients (five men, one female; age range , 39-67 years) with thyroid metastases from a GEP-NET are described. Thyroi d metastases were bilateral in all patients and were associated with enlarg ed neck lymph nodes in five. In four cases, the thyroid tumor was either th e first sign of the disease (n=2) or was an isolated site of recurrence (n= 2). The tumors were well (n=3) or poorly differentiated (n=3). Five tumors for which the primary site could be determined corresponded to foregut-deri ved tumors (3 lungs, 1 thymus and 1 pancreatic NET), One tumor demonstrated calcitonin (CT) production as shown by immunohistochemistry and elevated p lasma CT levels. However, the disease history and the clinical features str ongly favored a metastasizing GEP-NET No tumoral RET proto-oncogene mutatio n was found in this patient. The differential diagnosis between metastatic GEP-NET and MTC is crucial because prognosis, work-up, and treatment differ greatly.