Follicular carcinoma (FC) is the second most common malignancy of the thyroid after papillary thyroid carcinoma (PTC), representing approximately 15% of all thyroid carcinomas. Most FCs are minimally invasive and are categorized as well-differentiated tumors that have an excellent prognosis. A small subset of FCs, however, are widely invasive carcinomas (i.e., grossly recognizable as carcinomas) with a much more aggressive clinical course. In addition, the classic subtype of poorly differentiated thyroid carcinoma is insular carcinoma, a rare, aggressive follicular-derived tumor.
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