Benign thyroid nodules with papillary hyperplasia (BTN-PH) are sometimes misinterpreted cytologically as papillary thyroid carcinoma (PTC). We evaluated a fine-needle aspiration biopsy (FNAB) series of BTN-PH to identify the causes of diagnostic error and to better define its cytologic criteria.
A total of 48 FNAB cases of histologically confirmed BTN-PHs from 44 patients were identified from our archives. The available cytological and histological slides were reviewed and scored for 10 cytomorphological features. Fifteen FNAB cases of classical PTC were also reviewed for comparison.
The FNAB diagnoses for the 48 BTN-PHs were: benign, 18; suspicious for malignancy (S-PTC), 12); atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS), 10; follicular neoplasm, 5; malignant (PTC), 2; and nondiagnostic, 1. The extent of PH in the corresponding histology was highest in FNAB cases diagnosed as S-PTC/PTC versus all other categories (54% vs 27%; P = .0084). Papillary cytoarchitectural features were present in 53% of FNAB cases overall and in 89% of cases diagnosed as S-PTC/PTC (P = .0093). Nuclei were smaller in FNABs of BTN-PH compared with PTC (mean diameter/range, 8.2/6-12 vs 14.2/7-26 μm, respectively; P = .0001). Twenty-six percent of BTN-PHs contained cytoplasmic pigment. All FNAB cases diagnosed as S-PTC/PTC had focal nuclear atypia including grooves (87.5%), enlargement and crowding (75%), and chromatin pallor and pseudoinclusions (25%); however, the extent of nuclear atypia was significantly less than in the control group of PTCs (P < .0001).
BTN-PH represents a significant pitfall in thyroid FNABs in which it can be misinterpreted as S-PTC/PTC. Cytologic clues to this pitfall include limited and variable nuclear atypia, small nuclear size, and cytoplasmic pigment.