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Carcinoid Syndrome (cont.)

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Where do carcinoid tumors occur?

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. 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).

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, adenocarcinomas or carcinoids) are rare, and much more rare than colon or stomach cancers. Nevertheless, carcinoid tumors comprise one third of all small intestinal tumors, and are most commonly found in the ileum (the lower part of the small intestine close to the colon). 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. The few small carcinoid tumors that are found early usually are found incidentally when x-rays or procedures are performed for other purposes. Typically, small intestinal carcinoid tumors are diagnosed late, often years after the onset of symptoms and usually after local and distant metastases already are present.

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. Symptoms of small bowel obstruction include crampy abdominal pain, nausea and vomiting, and sometimes diarrhea. 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. Appendiceal carcinoid tumors larger than 2 cm at the time of diagnosis have approximately a 30% chance of being malignant and having local metastases. Thus, larger appendiceal carcinoid tumors need more extensive surgery such as removal of the right colon rather than simple appendectomy. Fortunately, large appendiceal carcinoid tumors are rare. Carcinoid tumors limited to the appendix, even metastatic to local tissues, usually do not 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

There are three types of gastric (stomach) carcinoid tumors; types I, II, and III.

Type I gastric carcinoid tumors, which account for 75% of gastric carcinoids, are typically smaller than 1 cm and usually are benign. There can be multiple tumors scattered throughout the body of the stomach. They typically develop in patients with pernicious anemia or chronic atrophic gastritis, conditions in which the stomach stops producing acid. The lack of acid causes the cells in the stomach that produce the hormone gastrin to secrete large amounts of gastrin into the blood. (Gastrin is a hormone normally produced by the body to stimulate stomach acid. Acid in the stomach shuts off the production of gastrin. In pernicious anemia or chronic atrophic gastritis, the lack of acid results in the production of increasing amounts of gastrin.) Gastrin, in addition to stimulating acid, also stimulates the growth of enterochromaffin cells in the stomach into benign carcinoid tumors. Treatment of type I carcinoid tumors include medications such as the somatostatin-type drugs which shut off production of gastrin or surgical removal of the gastrin producing part of the stomach.

Type II gastric carcinoid tumors are extremely rare and are very slow growing with a low probability of becoming malignant. They occur in patients with a rare genetic disorder called MEN (multiple endocrine neoplasia) type I. These patients have tumors in other endocrine glands such as the pituitary gland, parathyroid gland, and the pancreas.

Type III gastric carcinoid tumors tend to be larger than 3 cm and tend to be sporadic (occurring one or two at a time) in otherwise normal stomach (without the presence of pernicious anemia or chronic atrophic gastritis). Type III tumors usually are malignant and tend to invade deep into the stomach wall and metastasize. Type III tumors can cause local symptoms of abdominal pain and bleeding, as well as symptoms due to carcinoid syndrome. Type III gastric carcinoids usually require surgical removal of the stomach as well as the surrounding lymph nodes.

Colonic carcinoid tumors

Colonic carcinoid tumors typically occur in the right colon (ascending colon and right half of the transverse colon). Like small bowel carcinoid tumors, colonic carcinoid tumors are often discovered late. Thus, the average size of the tumors at the time of diagnosis is 5 cm, and metastases are present in two thirds of patients. Carcinoid syndrome is rare with colonic carcinoid tumors.

Medically Reviewed by a Doctor on 8/22/2016


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