The authors report a case of a 70-year-old woman with an anaplastic giant cell thyroid carcinoma, along with immunohistochemical and electron microscopic findings. Histologically, the tumor is characterized by mononucleated and multinucleated giant cells, lack of architectural cohesion, atypical mitoses, and extensive areas of coagulative necrosis. Tumor cells showed AE1/AE3 positivity as well as nuclear overexpression of p53 and ki-67. Semithin sections revealed multiple nuclei with

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... us size ranging from micronuclei to large-size (giant) nuclei. Micronuclei were confirmed by electron microscopy that disclosed also the presence of nuclear blebs, strings, and pockets. Morphological findings of these abnormal nuclear structures in conjunction with p53 and Ki-67 nuclear overexpression suggested a faulty mitotic checkpoint/mitotic catastrophe in the progression of anaplastic giant cell thyroid carcinoma.