Anaplastic thyroid carcinoma: an ultrastructural study of 10 cases.

Anaplastic carcinoma of the thyroid is an aggressive, rapidly fatal neoplasm that is generally believed to arise from the epithelium of the thyroid follicle. When differentiated carcinoma is not present, the diagnosis can be difficult and confusion with a sarcoma is frequently a problem. Ten anaplastic thyroid carcinomas have been examined by light and electron microscopy and compared with two biologically aggressive, solid follicular carcinomas. Ultrastructural study revealed evidence of epithelial differentiation in all the anaplastic carcinomas, confirming their origin from thyroid follicular cells. The study illustrates the value of electron microscopy in establishing the diagnosis of anaplastic carcinoma and in differentiating it from sarcoma.