It is an indolent (slow-growing) cancer that forms in cells that make hormones in the lining of the gastrointestinal tract (the stomach and intestines). It usually occurs in the appendix (a small fingerlike pouch of the large intestine), small intestine, or rectum. Having gastrointestinal carcinoid tumor increases the risk of forming other cancers of the digestive system.

Carcinoid tumors can be found wherever there are enterochromaffin cells, essentially, throughout the body. The majority (65%) of carcinoid tumors are found in the gastrointestinal tract. Approximately 25% of carcinoid tumors are found in the bronchial airways and the lung. The remaining 10% can be found almost anywhere. In some cases, doctors cannot locate the site of origin of the carcinoid tumors, though they know by the symptoms of the carcinoid syndrome that they are present.

Small intestinal carcinoid tumors

In general, small intestinal tumors (whether benign or cancerous) are rare, much more rare than colon or stomach cancers. Small intestinal carcinoid tumors typically produce no symptoms or produce only vague abdominal pain. Therefore, it is difficult to detect carcinoid tumors of the small intestine early, while they still can be completely removed and the patient cured. Approximately 10% of small intestinal carcinoids cause the carcinoid syndrome. Presence of the carcinoid syndrome usually means that the tumor is malignant and has spread to the liver.

Small intestinal carcinoid tumors often obstruct the small intestine when they reach a large size. Obstruction can be caused by two different mechanisms. The first mechanism is by enlargement and growth of the tumor into the lumen (channel) within the small intestine. The second mechanism is by kinking of the small intestine due to fibrosing mesenteritis, a condition caused by the tumor in which extensive scarring occurs in the tissue surrounding the small intestine. Fibrosing mesenteritis sometimes obstructs the arteries supplying blood to the intestines, resulting in death of a portion of the intestine (gangrene). The gangrenous intestine can rupture and be life-threatening.

Appendiceal carcinoid tumors

While tumors of the appendix are rare, carcinoid tumors are the most common tumor of the appendix, comprising approximately half of all appendiceal tumors. In fact, carcinoid tumors are found in 0.3% of resected (removed) appendices, but most of them are smaller than 1 cm and do not cause symptoms. They are found mostly in appendices removed for unrelated reasons. Most authorities believe that appendectomy is adequate treatment for these small appendiceal carcinoid tumors. The chances that a tumor would recur after appendectomy are very low. Carcinoid tumors limited to the appendix, even metastatic to local tissues, usually do no cause the carcinoid syndrome.

Rectal carcinoid tumors

Rectal carcinoid tumors are often discovered incidentally at the time of flexible sigmoidoscopy or colonoscopy. Carcinoid syndrome is rare with rectal carcinoid tumors. The probability of having metastases (malignant Carcinoid) correlates with the size of the tumor; those larger than 2 cm have a 60-80% chance of having metastases, and those smaller than 1 cm have less than a 2 % chance of having metastases. Therefore, small rectal carcinoid tumors usually can be successfully removed by simple excision, but the larger tumors (larger than 2 cm) need more extensive surgery that may involve removal of part of the rectum.

Gastric (stomach) carcinoid tumors

The origin of gastrointestinal carcinoid tumors is most commonly the small intestine, appendix, and rectum. Less common origins are the stomach and colon; and the least common origins are the pancreas, gallbladder, and liver (though carcinoid tumors in the liver usually are metastasis from elsewhere).