Atypical Histiocytoid Cells in Metastatic Papillary Thyroid Carcinoma An Underrecognized Cytologic Pattern

Objectives: 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.