Follicular Thyroid Carcinoma

Epidemiology

Follicular carcinoma is a relatively uncommon thyroid malignancy that accounts for 2 to 4% of thyroid cancers. The incidence is increased in regions where iodine deficiency is endemic. When the diet is supplemented with iodine, the incidence of papillary carcinoma increases. It is believed that the majority of follicular carcinomas develop from a preexisting adenoma. Follicular carcinoma often coexists with papillary thyroid carcinoma. This tumor is more common in elderly females. Children are rarely affected.

Clinical Findings

These tumors usually present as fullness or swelling in the region of the thyroid gland. Clinical examination reveals a palpable solitary nodule. This may be associated with dysphagia or dyspnea due to pressure on the esophagus or trachea, respectively. The tumor spreads hematogeneously and metastases may occur in the bone, lungs, brain, and liver. Cervical nodal metastases disease is relatively uncommon (2–10%).

Pathology

A pure follicular carcinoma is very rare. Follicular carcinomas frequently coexist with the papillary forms. According to the Armed Forces Institute of Pathology (AFIP), a tumor is classified as a follicular carcinoma when more than 50% of the thyroid mass is composed of follicles. Capsular and vascular invasion are the features that distinguish a follicular carcinoma from its benign counterpart, a follicular adenoma. There are three histological types: encapsulated, minimally invasive, and widely invasive. The encapsulated tumor shows minimal capsular invasion without capsular penetration or vascular invasion. The minimally invasive tumor shows capsular invasion and penetration without vascular invasion. The widely invasive form shows obvious vascular invasion.