Anaplastic variants of medullary thyroid carcinoma: a light-microscopic and immunohistochemical study.

Three unusual cases of anaplastic medullary thyroid crcinoma, two having small cell characteristics and one having the features of an anaplastic giant-cell carcinoma, were encountered during a review of 110 cases of familial and sporadic medullary carcinoma. Immunoperoxidase demonstration of calcitonin within tumor cells in these three cases enabled differentiation of the two small cell variants from lymphoma or other small cell carcinomas, and differentiation of the anaplastic giant-cell variant from other giant cell carcinomas of the thyroid. Although the existence of small cell carcinomas of the thyroid gland has been questioned, this study shows that small cell carcinomas of the thyroid gland do exist and that at least some are variants of medullary carcinoma. Whereas previous immunohistochemical studies have been successfully employed in defining the spectrum of C-cell proliferative disorders and in understanding the evolution of medullary thyroid carcinoma, this study demonstrates the value of these techniques in diagnostic pathology. Immunoperoxidase staining can be successfully used in differentiating unusual variants of medullary carcinoma from other thyroid tumors.