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).
CONCLUSIONS: 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.

OBJECTIVE: Fine-needle aspiration (FNA) of head and neck lymph nodes (LNs) is useful in diagnosing metastatic papillary thyroid carcinoma (PTC) and most commonly shows classic cytologic features of PTC. Metastatic PTC, however, may occasionally present with a pattern unfamiliar to most pathologists: atypical histiocytoid cells (AHCs).
METHODS: All PTC thyroidectomy specimens with associated FNA of LNs were retrieved from our files for 2007 to 2013. We aimed to assess cytologic features of metastatic PTC, as well as the presence of AHCs and their morphology.
RESULTS: Fifty-six FNAs from LNs with metastatic PTC were reviewed. AHCs were identified in 38 (68%) cases, while only PTC with classic cytologic features was seen in 18 (32%) cases. AHCs did not show diagnostic nuclear features of PTC and presented as large cells with abundant cytoplasm either vacuolated or dense. Nuclei varied from vesicular with prominent nucleoli to dark and smudgy. Thirty-one cases showed mixed AHCs and classic PTC, but seven cases (13% of all metastatic PTCs in LNs) consisted only of AHCs.
CONCLUSIONS: AHCs are an often unrecognized metastatic morphologic pattern of cystic PTC, as it does not show diagnostic classic nuclear features of PTC. AHCs are the predominant cytologic finding in approximately 13% of metastatic PTCs to neck LNs.