Papillary thyroid carcinoma with atypical histiocytoid cells on fine-needle aspiration

Although papillary thyroid carcinoma (PTC) usually has classic cytological characteristics on fine-needle aspiration (FNA), it can present rarely with aberrant features resembling those of histiocytes in a cystic nodule. The aim of the current study was to describe PTC with atypical histiocytoid cells and distinguish it from benign histiocytes. A retrospective computerized search for FNAs with atypical features suggestive of PTC and cystic degeneration was performed, and if available, the corresponding resection specimens were compared. Four cases met the criteria for FNAs and three had surgical pathology follow-up, which showed PTC. One aspirate had some features typical of PTC, but the remaining FNAs had atypical histiocytoid cells, which had traits intermediate between those of PTC and histiocytes. Large cell size, pseudoinclusions, nuclear grooves, and multiple well-defined vacuoles in atypical histiocytoid cells favor PTC over benign histiocytes. Ancillary immunocytochemical studies can also be useful in confirming the diagnosis. Histiocytic cells are frequently present in thyroid aspirates, and occasionally, they have atypical features that represent an unusual presentation of PTC. Closer examination of these cells can provide diagnostic clues for preventing false-negative diagnosis of PTC.