A METASTASIZING COMPOSITE CARCINOMA OF THE THYROID WITH DISTINCT MEDULLARY AND PAPILLARY COMPONENTS

Thyroid tumors with differentiation of both parafollicular and follicu lar epithelial cells are rare. The majority of those reported have bee n composed of a single cell population with combined features of the t wo cell types. We describe a 48-year-old man with a thyroid tumor comp osed of two discrete cell populations: thyroglobulin-positive papillar y carcinoma intermixed with calcitonin-containing medullary carcinoma. The tumor metastasized to regional lymph nodes and maintained this co mposite differentiation. We propose that such tumors with two discrete intermingled cell populations be called composite thyroid carcinomas, as opposed to mixed thyroid tumors, which describes tumors with a sin gle cell population that exhibits features of two cell types. Dual dif ferentiation in thyroid neoplasms has been interpreted as indicative o f a common stem cell origin. In this case, the presence of two mature, highly differentiated elements may suggest a proliferative response o f two distinct cell populations to a common tumorigenic stimulus.