FTC is a malignant tumor of follicular cell origin of the thyroid gland that lacks the nuclear features of papillary carcinoma, usually with an envelope and an infiltrative growth pattern. The incidence is 6% to 10%. The subtypes include: (1) follicular carcinoma, microinfiltrative (envelope invasion only); (2) follicular carcinoma, vascular infiltration within the envelope; (3) follicular carcinoma, extensive infiltration. The mutations of BRAF and RET fusions are uncommon.
Hürthle cell tumors are a group of follicular tumors with more than 75% eosinophils. They usually have an envelope and are also of follicular cell origin and may be classified as FTC or as a separate type, which is less common. The diagnostic criteria for benign malignancy are the same as for FTC. The incidence of BRAF mutations, RET fusions and RAS mutations is low in eosinophilic carcinomas. They can be classified as Hürthle cell adenoma (eosinophilic adenoma) and Hürthle cell carcinoma (eosinophilic carcinoma).