Unlike most thyroid carcinomas, undifferentiated carcinoma (anaplastic carcinoma) is an extremely aggressive malignancy with a poor prognosis. It generally occurs in elderly patients where it presents as a large, firm mass that infiltrates extrathy-roid tissues. For most undifferentiated carcinomas, surgical resection is not an effective treatment and only palliative therapies are used. Consequently, the pathologist may be called upon to establish the diagnosis of undifferentiated carcinoma by fine needle aspiration (FNA) to guide the clinical management (Figure 11.1).
One of the key entities in the differential diagnosis of undifferentiated carcinoma is a metastasis. Metastatic disease involving the thyroid gland can present as diffuse thyroid enlargement, as multiple nodules, or as a solitary nodule, but it is quite uncommon, being detected in less than 0.1% of all thyroid FNAs. The most frequent metastatic tumors to the thyroid include kidney, colorectal, lung, breast, melanoma, lymphoma, and head and neck squamous cell carcinoma. A majority of patients with thyroid metastases have a prior history of cancer, and FNA is an accurate and reliable method for its detection.
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