Postoperative histopathology of the resected tumor. (a) The thyroid tumor featured large zones of necrosis (right) (hematoxylin and eosin, ×100). (b) The tumor was composed of highly pleomorphic cells with large, bizarre nuclei containing prominent nucleoli and mitotic figures. Much of the tumor was infiltrated by neutrophils, a common feature of anaplastic thyroid carcinoma (hematoxylin and eosin, ×400). While the predominant pattern of the tumor was pleomorphic/giant cell, there were areas that were spindled (c) hematoxylin and eosin, ×200) or epidermoid (d) hematoxylin and eosin, ×200). (e) Focally admixed with the undifferentiated tumor was a well-differentiated oncocytic component (hematoxylin and eosin, ×200). (f) The well-differentiated component transitioned directly into the predominant anaplastic component. The anaplastic carcinoma was positive for cytokeratins AE1/AE3 (g) (AE1/AE3 immunohistochemistry, ×200) and PAX8 (h) (PAX8 immunohistochemistry, ×200). (i) The well-differentiated component of the tumor (right) expressed thyroid transcription factor-1, while the anaplastic component (left) did not (thyroid transcription factor-1 immunohistochemistry, ×200).