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 (MTC). 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 became prognosis, workup and treatment are different in each.