Undifferentiated (anaplastic) thyroid carcinoma: Practical immunohistochemistry and cytologic look-alikes

Aspirate smears of undifferentiated (anaplastic) thyroid carcinoma (ATC) are, in most instances, readily recognized as malignant. Nonetheless, pitfalls exist with this neoplasm in part due to the absence of epithelial markers, overlapping features with other malignancies that may metastasize to or arise within the thyroid, and potential confusion with non-neoplastic conditions that simulate malignancy. We highlight the salient morphologic features of ATC and its variants, useful discriminatory ancillary immunostains to recognize it, and ATC mimickers that have the potential to confuse the cytopathologist.