Carcinoid tumors develop from neuroendocrine cells in the GI tract (90%—see Tumors of the GI Tract: Small-Bowel Tumors), pancreas, and pulmonary bronchi (see Tumors of the Lungs: Bronchial Carcinoid). More than 95% of all GI carcinoids originate in only 3 sites: the appendix, ileum, and rectum. Although carcinoids are often benign or only locally invasive, those affecting the ileum and bronchus are frequently malignant.
Carcinoids can be endocrinologically inert or produce various hormones. The most common endocrinologic syndrome is carcinoid syndrome; however, most patients with carcinoids do not develop carcinoid syndrome. The likelihood that a tumor will be endocrinologically active varies with its site of origin, being highest for tumors originating in the ileum and proximal colon (40 to 50%). The likelihood is lower with bronchial carcinoids, lower still with appendiceal carcinoids, and essentially zero with rectal carcinoids.
Endocrinologically inert carcinoids are suspected because of their symptoms and signs (eg, pain, luminal bleeding, GI obstruction). They can be detected by angiography, CT, or MRI. Small-bowel carcinoids may exhibit filling defects or other abnormalities on barium x-rays. Definitive diagnosis is made histologically after biopsy or resection.
Endocrinologically active carcinoids are diagnosed and treated as described below.
Last full review/revision August 2012 by B. Mark Evers