Thyroid carcinosarcoma.

A thyroid carcinosarcoma composed of follicular carcinoma intermixed with osteosarcoma and chondrosarcoma was studied. Results of immunoperoxidase staining for thyroglobulin were positive in the areas of follicular carcinoma and negative in the sarcomatous component. Likewise, the carcinomatous areas showed thyroid epithelial characteristics by ultrastructure, while the sarcomatous areas showed typical mesenchymal differentiation. This is an extremely rare neoplasm, with only 16 cases reported in the literature. It occurs predominantly in elderly women, is thought to arise through a process of neoplastic metaplasia, and has a uniformly dismal prognosis. Immunoperoxidase and ultrastructural studies, the first reported in such lesions, proved helpful in ascertaining the sarcomatous nature of the anaplastic, mesenchymal component of this unusual and aggressive neoplasm.