Mixed medullary-follicular carcinoma of the thyroid gland: a clinicopathologic variant of medullary thyroid carcinoma.

A rare case of mixed medullary-follicular carcinoma of the thyroid gland, which occurred in a 44-year-old man, is reported. The thyroid tumor was composed of solid nests of polygonal cells, with an admixture of many evenly distributed thyroid follicles that contained colloid. The lymph node metastases were the same composition as the primary, with follicle formations that contained colloid. Immunohistochemically, in both the primary and metastatic lesions, calcitonin and carcinoembryonic antigen were present in the predominant solid areas of medullary carcinoma, whereas thyroglobulin was demonstrated in the follicular structures. At the ultrastructural level, most of the tumor cells contained numerous neurosecretory granules, but some showed follicular cell differentiation. These findings fulfilled the criteria of mixed medullary-follicular carcinoma of the thyroid according to the World Health Organization classification and also suggested dual neuroendocrine and follicular differentiation of this type of thyroid carcinoma. We reviewed the literature on mixed medullary-follicular carcinoma of the thyroid and concluded that it might constitute another clinicopathologic entity different from conventional medullary thyroid carcinoma; it occurs predominantly in younger males and is associated with a more favorable clinical course than the usual medullary thyroid carcinoma.