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.