Isolated carcinoid tumor metastatic to the thyroid gland: report of a case initially diagnosed by fine needle aspiration cytology

BACKGROUND: Neuroendocrine tumor metastatic to the thyroid gland is rare and may be difficult to differentiate from primary thyroid neuroendocrine tumors, such as medullary thyroid carcinoma (M/ITC). This report describes an unusual case of bronchial carcinoid metastatic to the thyroid diagnosed by fine needle aspiration cytology (FNAC). CASE: A 42-year-old woman with an undiagnosed bronchial carcinoid tumor presented to our clinic with a solitary nodule in the thyroid gland. FNAC of the nodule showed loosely cohesive groups of cuboidal tumor cells with scant, slightly granular cytoplasm; centrally located nuclei with a coarsely granular, salt-and-pepper chromatin pattern and inconspicuous nucleoli. Immunocytochemically the tumor cells were positive for neuron-specific enolase, chromogranin and synaptophysin and negative for thyroglobulin, calcitonin and carcinoembryonic antigen. The cytologic diagnosis of a metastatic neuroendocrine carcinoma was confirmed histologically. CONCLUSION: Metastasis to the thyroid gland may pose a diagnostic problem, particularly with tumors of neuroendocrine origin, as these have similar cytologic features in various organs. The correct preoperative cytologic diagnosis of metastatic carcinoid tumor in patients without a prior history of cancer and differential diagnosis with MTC are crucial because prognosis, workup and treatment are different in each.