Ultrastructure of a primary fibrosarcoma of the human thyroid gland.

A primary fibrosarcoma of the thyroid occurring in a patient with a nodular goiter is described. Light microscopy showed interwoven bundles of spindle cells admixed with plump ovoid cells, and foci of multinucleated giant cells. Remnants of distorted thyroid follicles were found only at the periphery of the tumor. "Transitional" epithelial elements were not discernible but the tumor resembled an anaplastic giant cell carcinoma. Multiple blocks studied by electron microscopy, however, revealed that the tumor cells, including the giant cells, have the ultrastructure features of a fibroblast.