Unusual findings in thyroid cytology and histology

Introduction: Although cytology and histopathology of thyroid lesions generally fall into common, well-defined categories, there are uncommon cases with unusual fine needle aspiration (FNA) findings or histology. Herein, we review the prevalence and characteristics of rare thyroid cytology and histopathology findings at a tertiary hospital. Methods: Institutional data from >31,000 patients with a thyroid pathology from 1995 to 2013 were queried. Both cytology and histology were available in 6,693 patients. After exclusion of the common cytological categories detailed by The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) and common histopathology categories, 90 patients with either an unusual FNA, histopathology, or both were identified. Results: A total of 90 cases were included (19: only unusual FNA; 25: only unusual histology; 46: both unusual cytology and histopathology). The positive predictive value of an unusual FNA for discovering an unusual lesion was 71% (95% CI: 58%–81%). The majority (66%) were females and median age was 59 years. On histopathology, 80 (88%) cases were malignant, 72 (90%) of which were initially diagnosed as malignant on FNA. Of the 10 benign lesions, 8 (80%) also had a benign FNA. Patients with unusual malignant lesions were significantly older than those with unusual benign lesions (62 vs. 44 years; P: 0.004). Conclusion: Unusual cytopathological and histopathological findings in thyroid comprise a varied group of tumors that are individually rare but collectively common. A preoperative FNA with an unusual cytopathology is likely to lead to an unusual histopathological diagnosis; however, its diagnostic accuracy in differentiating benign from malignant is lower than the accuracy of cytopathology of conventional TBSRTC. Diagn. Cytopathol. 2017;45:185–190.