Medullary thyroid carcinoma (MTC) occurs in sporadic and hereditary clinical settings and displays a variety of clinical behaviors ranging from moderately aggressive to extremely indolent. The discovery of the gene responsible for hereditary MTC has shed light on the biologic basis for this range of clinical presentations. MTC was described in 1959 by Hazard, Hawk, and Crile.1 MTC is a tumor of the thyroid C cells, also known as the parafollicular cells (Fig. 15-1). These cells are of neural crest derivation. Other neural crest-derived tumors include melanomas, pheochromocy-tomas, and neuroblastomas. C cells are dispersed throughout the thyroid gland, with the highest concentration in the upper poles. C cells secrete calcitonin, a hormone involved in calcium metabolism. Calcitonin is a sensitive and specific marker for the presence of MTC. It has been invaluable in the screening of individuals predisposed to the hereditary forms of the disease and in the follow-up of patients who have been treated. MTC cells have been noted to express and secrete a variety of substances in addition to calcitonin. Some of these are carcinoembryonic antigen (CEA), hista-minase, neuron-specific enolase, calcitonin gene-related peptide, somatostatin, thyroglobulin, thyrotropin-stimulating hormone, adrenocortical releasing hormone, gastrin-related peptide, serotonin, chromogranin, substance P, and proprio-melanocortin. MTC is often associated with a proliferative lesion, C-cell hyperplasia. It is likely that C-cell hyperplasia is a precursor of MTC.

MTC constitutes approximately 5% to 10% of all thyroid cancers. MTC occurs as unifocal or multifocal clonal populations of tumor cells. Regional lymphatic spread occurs to perithyroidal lymph nodes, paratracheal lymph nodes, nodes of the jugular chain, and upper mediastinal nodes.2 Nodal metastases are often found along the recurrent laryngeal nerves in the paratracheal regions and may be present where these nerves branch and insert into the larynx. Nodes may be in close association with the parathyroid glands. MTC may remain confined to the neck for long periods of time. In more advanced stages, metastases are found in liver, lungs, bone, and sometimes brain and subcutaneous tissues (Fig. 15-2). Histologic features of aggressiveness include vascular invasion, lymphatic invasion, invasion of the thyroid capsule, and extranodal spread of tumor in lymph node metastases.

The disease can be locally aggressive, and local structures may be invaded by the primary tumor or by tumor in nodal metastases. In the neck, the structures most commonly invaded include the trachea, recurrent laryngeal nerve, and strap muscles. Invasion of the trachea in the neck or mediastinum can cause death.

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