Sensitive cytologic criteria for the identification of follicular variant of papillary thyroid carcinoma in fine-needle aspiration biopsy

The cytologic diagnosis of follicular variant of papillary thyroid carcinoma (FVPTC) can be extremely challenging and may be associated with false negative diagnoses. The purpose of this study was to determine the minimal cytologic criteria needed to identify FVPTC. We examined sixty-nine fine-needle aspiration (FNA) cases, processed with Diff-Quik and Papanicolaou stains, that were either diagnostic or suspicious of FVPTC. All cases had histologic confirmation. These cases included 29 FVPTC, 18 classic papillary thyroid carcinoma (PTC), 17 follicular neoplasm (6 adenomas, 10 carcinomas, 1 neoplasm NOS), 2 lymphocytic thyroiditis and 3 nodular goiter. Seven of the most commonly cited cytomorphologic features, including flat syncytial sheets, nuclear enlargement, fine chromatin, nuclear grooves, nuclear pseudoinclusions, and amount of colloid and cytoplasm, were evaluated. A diffuse distribution of fine chromatin, nuclear grooves, and colloid was seen more often in FVPTC than in follicular neoplasm (p<0.01). The combination of flat/syncytial sheets, nuclear enlargement, and fine chromatin was observed in all our cases of FVPTC, and is therefore considered a sensitive marker in detecting FVPTC. Logistic regression analysis revealed colloid to be the only positive predictor in favor of FVPTC over classic PTC.