Follicular Thyroid Carcinoma

Thyroid carcinoma with follicular differentiation but no papillary nuclear features. Although the majority of follicular cell-derived thyroid carcinomas are sporadic, approaching 10% are hereditary.

The most common somatic mutations in FTC are RAS point mutations being reported in up to half of FTC, with an overall rate of approximately 30% in published studies.

 

Clinical features 

FTC occurs predominantly in adults (with a mean age of 45-50 years at diagnosis) and is rare in children. Most patients present with a painless thyroid mass that is detected on self-examination, routine physical examination, or is incidentally detected on imaging. Up to 25-35% of FTC are over 40 mm; however, only a subset of these tumors produces dysphagia, hoarseness, or stridor. Regional lymph node enlargement at presentation essentially does not occur with FTC.

 

 

 

Macroscopic features 

The vast majority of FTC are grossly encapsulated, representing either minimally invasive FTC or encapsulated angioinvasive FTC. Most FTC are over 20 mm; however, tumor size cannot be used to differentiate FTC from Follicular Adenoma. FTC are usually solid and tan/gray on cut surface.

Microscopic features

The diagnosis of FTC requires capsular and/or vascular invasion. FTC is divided into three subtypes:

minimally invasive (capsular invasion only),

encapsulated angioinvasive (venous invasion present with or without capsular penetration),

widely invasive.

Although distinction between FTC and FA is seemingly straight-forward, it is recognized that there is debate over what constitutes capsular invasion and vascular invasion, as well as issues with inter-observer reproducibility.

Vascular invasion in an encapsulated angioinvasive follicular thyroid carcinoma. Note the endothelial wrapping that is evident.

 

Clear cell FTC, the most frequent FTC variant, is defined as having >50% clear cells. Clear-cell change can occur in a range of benign and malignant thyroid tumors and occurs secondary to an accumulation of glycogen, lipids, thyroglobulin, or vesicles of apparent mitochondrial derivation.

Signet-ring-cell variant of FTC is characterized by cells with cytoplasmic vacuoles with eccentrically displaced nuclei. The cytoplasmic vacuoles are similar to follicular lumens and have an accumulation of thyroglobulin. The nuclei are generally uniform and bland. These tumors must be distinguished from metastatic adenocarcinomas with a signet-ring-cell morphology. (3)

The tumors have a follicular architecture and are composed of cells with round normochromatic nuclei. 

Bibliography

3.        D’Avanzo A, Treseler P, Ituarte PHG, Wong M, Streja L, Greenspan FS, et al. Follicular thyroid carcinoma: Histology and prognosis. Cancer. 2004 Mar 15;100(6):1123–9.