Follicular-patterned lesions of the thyroid form the largest and most heterogeneous group of lesions ranging from benign, nonneoplastic, follicular hyperplasias to follicle-forming, infiltrating carcinomas. These lesions have overlapping cytologic features, and fine-needle aspiration (FNA) is not always a reliable tool in their differential. Follicular neoplasia (FN) on cytology, sometimes categorized as “indeterminate,” has been a major cause of debate for thyroid pathologists and clinicians alike.
While the histological lesions in this category carry a wide spectrum of biological behavior, morphologic patterns of these lesions in aspirate smears have common characteristics. The stereotypical example of a follicular neoplasm on cytology contains abundant cellularity, uniform population of follicles, predominantly microfollicles with architectural crowding, bland follicular epithelial cells that lack the characteristic nuclear features of papillary thyroid carcinoma, and paucity or complete lack of background colloid.
Oncocytic lesions of the thyroid are also grouped under this category, even if the architectural follicular characteristics might not be as apparent in cytology. Follicular variant of papillary thyroid carcinoma (FVPTC), when the nuclear features are not diagnostic, is commonly associated with the FNA diagnosis of FN.
While the overall risk of malignancy is 15% to 30% according to the Bethesda terminology, malignancy risk with the FNA diagnosis of FN also shows a heterogeneous pattern in those practices where FN diagnosis is divided into subgroups.
Morphologic details of follicle forming lesions are discussed for FNA specimens with pertinent features particularly of those that might be associated with a different risk of malignancy.