Leiomyosarcoma of the thyroid gland. A case report with a comparative study of five cases of anaplastic carcinoma.

We report on a case of leiomyosarcoma of the thyroid with ultrastructural features and expression of intermediate filament proteins different from that of anaplastic carcinomas. Histologic examination of multiple sections showed that they were leiomyosarcomatous, and neither epithelial features nor giant cell configurations were found. Electron microscopic examination showed microfilament bundles with dense patches and discontinuous basal lamina. Desmin, vimentin, and actin were positive for tumor cells, and cytokeratin, thyroglobulin, and calcitonin were negative. In contrast, the anaplastic carcinoma cases did not contain microfilament bundles with dense patches in their tumor cells. There were no desmin-positive or cytokeratin-negative tumor cells. We concluded that this case was a primary leiomyosarcoma, presumably derived from the smooth muscle cell of the vascular wall.