Metastatic Tumors to the Thyroid

Metastatic tumors to the thyroid are relatively uncommon, constituting less than 1% of all thyroid malignancies. Fine-needle aspiration of thyroid nodules is a reliable technique to diagnose primary thyroid lesions with excellent sensitivity and specificity; however, cytologic diagnosis of tumors metastatic to the thyroid can be challenging, particularly if prior clinical history is not available. Clinical presentation is very much similar to that of primary thyroid tumors. Although renal cell carcinoma appears to be the most common secondary tumor of the thyroid, any tumor can metastasize to the thyroid, particularly breast, lung, colon, stomach, and head and neck primary tumors. Carcinomas are most common, including adenocarcinoma and squamous cell carcinoma, but melanoma and sarcoma, although less frequent, are also encountered. Metastatic tumors may show low-grade cytologic features similar to well differentiated primary thyroid tumors or high-grade morphologic features as seen in anaplastic thyroid carcinoma in fine-needle aspirations. Cytologic diagnosis of a secondary thyroid tumor depends on the pathologist’s knowledge of the patient’s prior clinical history, morphologic features, and use of ancillary techniques for confirmation. Management and prognosis of these tumors depend on the stage of the primary tumor. Patients with isolated metastasis to the thyroid generally do well, whereas the prognosis of patients with widespread malignant disease is grim.