Atypical histiocytoid cells in fine‐needle aspiration cytology of metastatic papillary thyroid carcinoma: A potential diagnostic pitfall

Abstract

Background Fine needle aspiration (FNA) cytology of cervical lymph nodes is an effective diagnostic tool to detect metastatic papillary thyroid carcinoma (PTC) when typical cytomorphologic features of PTC are observed. However, the presence of atypical histiocytoid cells (AHCs) due to cystic degeneration sometimes poses a diagnostic challenge. The purpose of this study was to evaluate the presence and cytomorphology of AHCs in metastatic PTC. Methods We analyzed a total of 76 FNA cytological samples of cervical lymph nodes from 66 patients with metastatic PTC diagnosed during approximately 10‐year period, from January 2010 to April 2020. Samples were either liquid based preparation (n = 53) or conventional smear (n = 23). Results AHCs were present in 38 (50%) of the 76 FNA cases and the remaining 38 cases showed classic PTC features. Among the 38 cases, eight displayed pure AHCs that constituted 10.5% of all the metastatic PTC in the lymph nodes. Pure AHCs were more commonly detected in the liquid based preparation (7/53, 13.2%) than the conventional smear (1/23, 4.3%). The remaining 30 cases had AHCs mixed with the characteristic PTC components. The presence of AHCs was found to be statistically associated with cystic background (p < .002). Conclusion Metastatic PTC frequently exhibits cystic degeneration and the FNA cytology may not yield classic cytological features of PTC. Pure AHCs composed 10.5% of all cases and might be a potential pitfall for liquid based preparation in diagnosing metastatic PTC. The finding of AHCs within the cystic background should raise the concern of metastatic PTC.