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 …