Medullary thyroid carcinoma is a tumor of C (Parafollicular) cells and comprises approximately 5-10% of thyroid carcinomas. About 80% of cases are sporadic and the remaining 20% are inherited. It is composed of polygonal or spindled tumor cells which may be arranged in solid sheets (shown here), trabecular, glandular, pseudopapillary, carcinoid-like, or paraganglioma-like patterns. The tumor cells have amphophilic granular cytoplasm, medium-sized nucleus with finely granular chromatin (salt & pepper look), and small punctate nucleoli. There is prominent delicate fibrovascular stroma which may contain focal amyloid deposits.