Tall cell variant of papillary thyroid carcinoma: Its characteristic features demonstrated by fine-needle aspiration cytology and immunohistochemical study

Tall cell variant (TCV) of papillary thyroid carcinoma is a rare tumor, which is usually associated with poor outcome, and pathologists often face the dilemma of proper diagnosis of TCV, not only by cytology but also histology. To allow surgeons to determine aggressiveness of the tumor before operation, it is important for pathologists to detect tall cell features correctly by fine-needle aspiration cytology (FNAC). However, the current criteria and definition of TCV are still controversial and confounded by another problem, the differential diagnosis from poorly differentiated thyroid carcinoma (PDC). In this report, we describe two cases of TCV and present characteristic FNAC findings. The tumor cells had a peculiar shape, which included tall, columnar, and oxyphilic cytoplasm with "eccentric," basally located nuclei. We propose new terms for these cells, such as "tail-like cells" or "tadpole cells." In the surgically-resected specimens, both cases exhibited remarkable extrathyroidal invasion accompanying prominent vascular invasions. They showed high Ki-67 (MIB-1) labeling index by immuno-histochemistry, which indicated a higher proliferation activity of TCV than conventional form of papillary thyroid carcinoma. Furthermore, we discuss in this report the problematic issue of differential diagnosis of TCV from PDC and oxyphilic papillary thyroid carcinoma.