Anaplastic carcinoma of the thyroid with osteoclast‐like giant cells

An unusual malignant thyroid neoplasm with a morphologic resemblance to giant cell tumor of the bone is reported in a patient who presented with a rapidly growing thyroid mass and a history of pre‐existing goiter. The light and electron microscopic studies disclosed areas of differentiated follicular carcinoma with gradual transition to undifferentiated carcinoma. Pleomorphic spindle‐shaped cells and giant cells were accompanied by numerous osteoclast‐like multinucleated giant cells. Desmosomes and interdigitating cell surfaces were apparent in the differentiated as well as undifferentiated areas on electron microscopy. These findings support an epithelial rather than a mesenchymal origin for this neoplasm. Cancer 52:2122‐2128, 1983.