FINE-NEEDLE ASPIRATION OF THE MACROFOLLICULAR AND MICROFOLLICULAR SUBTYPES OF THE FOLLICULAR VARIANT OF PAPILLARY CARCINOMA OF THE THYROID

BACKGROUND. The follicular variant of papillary carcinoma of the thyro id (FVPCT) is being increasingly diagnosed on excised thyroid nodules. However, the fine-needle aspiration (FNA) and intraoperative diagnosi s is often that of a follicular neoplasm, especially in papillary carc inomas with a pure or predominantly follicular pattern. The authors un dertook this study in an attempt to refine the cytologic criteria for the diagnosis of FVPCT. METHODS, The authors reviewed 16 cases with cy tologic diagnoses of FVPCT (9 cases), suspicious for FVPCT (6 cases), or cellular adenomatoid nodule (1 case) based on aspirates stained wit h Papanicolaou stain or a Giemsa-type stain (Diff-Quik). All cases had been confirmed histologically as pure or predominantly (>80%) FVPCT i n 13 cases and as follicular adenoma in 3 cases. Cytologic features ev aluated included cellularity, cell arrangement, nuclear features, cyto plasm, and colloid. RESULTS. Twelve of 13 cases of FVPCT were correctl y diagnosed cytologically. Features that proved useful in the diagnosi s of FVPCT were the concomitant and conspicuous presence of ovoid or p ear-shaped nuclei with hypochromasia and nuclear grooves. Soft feature s included nuclear overlap and eccentric, small nucleoli. Cytoplasmic features were not useful in making this diagnosis. Based on cell arran gement and colloid, it was possible to predict microfollicular and mac rofollicular variants. The microfollicular subtype showed rosettes or microfollicles and scant, thick colloid in casts and blobs. The macrof ollicular subtype had predominantly sheets or spherules with abundant, thick background colloid. Nuclear pseudoinclusions and psammoma bodie s were absent and multinucleated giant cells rarely found. Pitfalls le ading to a ''false-positive'' FVPCT diagnosis included oncocytic adeno ma (in 2 cases) and atypical adenoma (in 1 case). A cytologic diagnosi s of cellular adenomatoid nodule was made in one case of macrofollicul ar FVPCT. CONCLUSIONS. The authors present improved cytologic criteria for the diagnosis of pure FVPCT on smears stained with Papanicolaou s tain or Diff-Quik, and they elaborate on the clues and pitfalls associ ated with this diagnosis. The cytologic features of the macrofollicula r and microfollicular subtypes of FVPCT are also described. Cancer (Ca ncer Cytopathol) 1998;84:235-44. (C) 1998 American Cancer Society.