Mixed Medullary-follicular-derived carcinomas of the thyroid gland.

Tumors of the thyroid are subclassified based on the cell of origin and commonly include follicular-derived tumors and C-cell-derived tumors. The most common follicular-derived tumors are papillary carcinoma and follicular carcinoma, whereas the malignant C-cell-derived tumor is medullary thyroid carcinoma. Rare cases in the literature describe patients who have follicular-derived and C-cell-derived tumors in the same thyroid gland. These can be synchronous but anatomically separate carcinomas, or they can show some mixing of the 2 components. The mixture may be at an interface, as in collision tumors, or can be throughout the entire lesion, as in true mixed medullary-follicular-derived carcinomas. The clinical, histologic, and molecular features of these mixed tumors and the classification guidelines are reviewed.