Background: Metastases to the thyroid gland, although rare, are important entities to consider when evaluating malignant cells on a thyroid fine-needle aspiration (TFNA) specimen. Cellular TFNA specimens with small round blue cells should prompt a broad differential: florid lymphocytic thyroiditis, lymphoma, metastases, as well as primary thyroid malignancies with similar morphologies such as poorly differentiated (insular) and medullary carcinomas. Age, clinical presentation and prior history must be considered in every case. Case Report: We report, to the best of our knowledge, the first case of metastatic alveolar rhabdomyosarcoma (ARMS) to the thyroid gland, definitively diagnosed by TFNA. A 21-year-old female patient presented with a large mass in the right lobe of the thyroid. Her past history was significant for ARMS diagnosed 24 months earlier, currently in remission after successfully completing 40 weeks of chemoradiation therapy. The diagnosis of metastatic ARMS in the TFNA prompted a more thorough examination revealing previously unknown additional sites of metastases. Conclusion: Metastases to the thyroid gland are uncommon but should be considered in cases where atypical morphology is encountered. Small round blue cell tumors can metastasize to the thyroid gland, and clinical presentation, morphology, immunohistochemistry and molecular studies are helpful in differentiating between them.

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