Carcinoid and other neuroendocrine tumors of the colon and rectum.

Neuroendocrine tumors (NETs) are found throughout the intestinal tract and arise from the Kulchitsky cells located in the crypts of Lieberkühn. They are classified by site of origin and by degree of differentiation, with well-differentiated lesions representing those tumors formerly referred to as carcinoid tumors. The focus of this review is NETs of the appendix, colon, and rectum. The clinical presentation of these tumors is dependent on the primary site and many are discovered incidentally, either during screening or during the investigation of nonspecific abdominal complaints. Treatment is primarily via surgical removal as the response to chemotherapy has been traditionally poor. A noted exception to this has been with treatment of the carcinoid syndrome, which occurs almost exclusively in patients with liver metastases and is due to the release of bioactive peptides and amines directly into the systemic circulation. The use of somatostatin congeners to block the release of these substances has greatly ameliorated the devastating symptoms of this condition. Postresection follow-up is advocated, but specific recommendations are lacking an evidentiary basis. NETS, particularly those of the small bowel, colon, and appendix, are seen in association with other synchronous or metachronous malignancies, often of the gastrointestinal tract. However, the utility of subsequent screening and surveillance is unproven.