Carcinoid tumors.

Carcinoid tumors most commonly occur in the gastrointestinal tract and are known best for the bizarre manifestations of the carcinoid syndrome caused by the release of tumor products. However, many carcinoid tumors do not present with or cause the carcinoid syndrome, and this has led to the development of new diagnostic markers for these tumors. New treatments have emerged recently and are being developed, because although these tumors are relatively indolent, they can metastasize. The biology of carcinoid tumors of the gastrointestinal tract can be classified in large part by the embryologic regions of the gut in which they occur. More carcinoid tumors will be discovered as the indications and use of gastrointestinal endoscopies increase, so the diagnosis and initial management of carcinoid tumors will fall largely on the gastroenterologist.