Fine-needle aspiration of the macrofollicular and microfollicular subtypes of the follicular variant of papillary carcinoma of the thyroid

Abstract

Background: The follicular variant of papillary carcinoma of the thyroid (FVPCT) is being increasingly diagnosed on excised thyroid nodules. However, the fine-needle aspiration (FNA) and intraoperative diagnosis is often that of a follicular neoplasm, especially in papillary carcinomas with a pure or predominantly follicular pattern. The authors undertook this study in an attempt to refine the cytologic criteria for the diagnosis of FVPCT.

Methods: The authors reviewed 16 cases with cytologic diagnoses of FVPCT (9 cases), suspicious for FVPCT (6 cases), or cellular adenomatoid nodule (1 case) based on aspirates stained with Papanicolaou stain or a Giemsa-type stain (Diff-Quik). All cases had been confirmed histologically as pure or predominantly (>80%) FVPCT in 13 cases and as follicular adenoma in 3 cases. Cytologic features evaluated included cellularity, cell arrangement, nuclear features, cytoplasm, and colloid.

Results: Twelve of 13 cases of FVPCT were correctly diagnosed cytologically. Features that proved useful in the diagnosis of FVPCT were the concomitant and conspicuous presence of ovoid or pear-shaped nuclei with hypochromasia and nuclear grooves. Soft features included nuclear overlap and eccentric, small nucleoli. Cytoplasmic features were not useful in making this diagnosis. Based on cell arrangement and colloid, it was possible to predict microfollicular and macrofollicular variants. The microfollicular subtype showed rosettes or microfollicles and scant, thick colloid in casts and blobs. The macrofollicular subtype had predominantly sheets or spherules with abundant, thick background colloid. Nuclear pseudoinclusions and psammoma bodies were absent and multinucleated giant cells rarely found. Pitfalls leading to a "false-positive" FVPCT diagnosis included oncocytic adenoma (in 2 cases) and atypical adenoma (in 1 case). A cytologic diagnosis of cellular adenomatoid nodule was made in one case of macrofollicular FVPCT.

Conclusions: The authors present improved cytologic criteria for the diagnosis of pure FVPCT on smears stained with Papanicolaou stain or Diff-Quik, and they elaborate on the clues and pitfalls associated with this diagnosis. The cytologic features of the macrofollicular and microfollicular subtypes of FVPCT are also described.