Follicular variant of papillary carcinoma. Cytologic and histologic correlation.

The cytologic diagnosis of follicular variant of papillary thyroid carcinoma (FVPTC) can be difficult owing to the paucity of nuclear changes of papillary carcinoma and overlapping features with other follicular-derived lesions. We report our fine-needle aspiration (FNA) experience with 17 histologically proved cases of FVPTC with a preceding FNA. All cases showed increased cellularity and a background of moderate to abundant thin watery colloid with variable amounts of thick eosinophilic colloid. The cells were arranged mainly in monolayer sheets and syncytial fragments; occasional microfollicle formation with abundant fine eosinophilic cytoplasm was seen. Nuclear enlargement was a consistent finding in all cases. Only 5 cases showed prominent nuclear features of papillary carcinoma. Histologic examination showed encapsulated follicular-patterned nodules with multifocal random distribution of nuclear features of papillary carcinoma that were more pronounced in the subcapsular locations of the lesions. This morphologic heterogeneity explains the variability seen in FNA specimens of FVPTC and may result in false-negative diagnoses. We believe that these findings should be considered when interpreting follicular lesions; suspicion of an FVPTC should be conveyed in the cytopathology report, which may prompt intraoperative assessment to avoid a second surgical intervention for completion thyroidectomy.