Medullary cell carcinoma (MTC) is a neuroendocrine thyroid malignancy derived from calcitonin producing C cells. MTC can be sporadic or familial, associated with MEN2a, MEN2b or familial medullary carcinoma syndrome. Autosomal dominant germline activating mutation in RET oncogene plays role in pathogenesis of familial cases, while up to 2/3 of sporadic MTC have somatic RET mutation. Familial cases develop in 3rd decade, about two decades earlier than sporadic.

Patients usually present with thyroid mass and cervical lymph node enlargement. In familial cases tumors may be multiple and bilateral, and the history and symptoms associated with other syndromic lesions may be present. Serum calcitonin and CEA are increased.

The tumor is usually well-circumscribed, composed of cells arranged in sheets, nests and cords in fibrovascular stroma. The cells can be round, plasmacytoid or spindled. Nuclei are round, with salt and pepper stippled chromatin. Amyloid (calcitonin-derived) is present in up to 80% of cases. Entrapped normal follicles may be seen within the tumor.

C cell hyperplasia is present in the thyroid gland in familial cases.

Tumor cells are positive for pCEA, calcitonin and neuroendocrine markers. TTF1 immunohistochemistry is positive, but thyroglobulin is negative.

Extrathyroidal extension, lymphovascular invasion and metastasis are important prognostic factors.

Differential diagnosis is from other thyroid carcinomas, paraganglioma and metastasis.