Common Histologic Type
Follicular thyroid cancer is an "epithelial carcinoma showing evidence of follicular cell differentiation but lacking the diagnostic features of papillary carcinoma" (11). It tends to be more common in regions with iodine deficiency and in patients older than 50 years. Ten-year survival rates are lower than in PTC ranging from 60% to 70%.
It usually presents as a solitary thyroid tumor. The diagnosis of malignancy depends on the presence of blood vessel and/or capsular invasion. Histologic diagnosis is divided into two categories according to the degree of invasiveness. This aspect confers prognostic significance.
Encapsulated or minimally invasive FTC is an encapsulated tumor whose growth pattern resembles that of a microcarcinoma. Blood vessel invasion is almost never seen. It has a better prognosis than the widely invasive variant.
Widely invasive FTC may be partially encapsulated but the tumor margins are infiltrative and vascular invasion is often extensive. It always has a follicular element but when follicular differentiation is poor or absent the tumor is classified as a poorly differentiated carcinoma.
It invades blood vessels, and metastasizes to lungs and bone. Brain and lymphatic metastases are rare. Metastases are frequent in the widely invasive variant and rare in the minimally invasive one.
Clear cell carcinoma: This is a rare follicular cancer variant with clinic characteristics similar to those of classic FTC. Tumor cells show glycogen accumulation or dilatation of the granular endoplasmic reticulum with a clear cell appearance.
Hurthle Cell Carcinoma (Oncocytic Carcinoma or Oxyphilic Variant Follicular Thyroid Cancer)
The precise classification of this tumor is controversial. The WHO considers this carcinoma as an oxyphilic variant of FTC (20). Other institutions consider that this tumor has different microscopic, behavioral, and etiopatho-genic features that set it apart from all others (21). Hurthle cell carcinoma is composed of greater than 75% of cells with oncocytic features. It is a usually solitary tumor with complete or partial encapsulation. Malignant behavior is predicted by vascular or capsular invasion (as in FTC) (22). It is more frequently associated with extrathyroid extension and distant and lymph-node metastases than the common FTC. Although Hurthle cell carcinomas usually produce Tg they mostly lack radioiodine uptake in comparison with standard DTC.
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