Aspiration cytopathology of metastatic mucinous papillary thyroid carcinoma

The light microscopic histopathology and cytopathology of papillary thyroid carcinoma (PTC) and its variants is amply depicted in reviews, books, and book chapters. One of the changes in PTC, however, that is infrequently discussed or illustrated in the literature is that of mucinous metaplasia. In the aspiration cytopathology literature, we are aware of only a rare report of PTC exhibiting extensive mucinous metaplasia. We present an example correlating the histopathology and fine needle aspiration cytopathology of a PTC that had metastasized on several separate occasions to cervical lymph nodes, and in the process demonstrated mucinous transformation. Without a prior history of PTC, the aspirate smears and tissue sections could have been mistaken easily for metastatic clear cell carcinoma of non-thyroidal origin. Mucinous metaplasia represents an extremely uncommon, but nonetheless potential, pitfall in the aspiration cytopathology and histopathology of metastatic PTC.