Carcinoid Syndrome (cont.)

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How common are carcinoid tumors and carcinoid syndrome?

The prevalence of carcinoid tumors is difficult to determine since many carcinoid tumors are not detected because they produce no symptoms. In one autopsy series, carcinoid tumors were estimated to have a prevalence of 8/100,000 people annually, but 90% of the carcinoids found at autopsy were not the cause of death.

The carcinoid syndrome is very rare. This is because many carcinoid tumors do not produce hormones. Another reason that the syndrome is rare is that hormones released by carcinoid tumors, particularly from tumors within the abdomen, often are destroyed by the liver before they reach the general circulation to cause symptoms. For example, hormones produced by small intestinal carcinoid tumors are released into the portal vein. The portal vein blood passes through the liver before reaching the heart and the general circulation. As the portal vein blood passes through the liver, the hormones are destroyed by the liver.

Only those carcinoid tumors that can release hormones directly into the general circulation (and not into the portal veins which pass through the liver) can produce the carcinoid syndrome. Thus, the most common cause of carcinoid syndrome is small intestinal carcinoid tumors that have metastasized to the liver. The metastases in the liver can release hormones directly into the circulation. Another rare example is carcinoid tumors of the bronchial airways. Carcinoid tumors in the bronchial airways can release hormones directly into the general circulation via the pulmonary veins without passing through the liver.

What is the prognosis and natural history of carcinoid tumors?

Carcinoid tumors can be benign (non-cancerous) or malignant (cancerous). Benign carcinoid tumors are typically small (less than 1 cm). They usually can be removed completely and, in most cases, they do not come back. Cells from benign carcinoid tumors do not spread to other parts of the body. Benign carcinoid tumors typically produce no symptoms and are commonly found by chance during flexible sigmoidoscopy or upper gastrointestinal endoscopy.

Cancerous carcinoid tumors are typically large (larger than 2 cm) at the time of diagnosis. Cells from these malignant tumors can invade and damage tissues and organs near the tumor. Moreover, malignant cells can break away and enter the bloodstream or lymphatic system and spread to form new tumors in other parts of the body. (The distant tumors are called metastases.) Common sites for carcinoid metastases include the lymph nodes, liver, lung, bone, and skin.

Carcinoid tumors between 1.0 and 2.0 cm in size have approximately a 10% chance of being cancerous at the time of diagnosis.

Carcinoid tumors are typically slow growing. They grow much more slowly than other cancers such as colon, pancreas, liver, and lung cancer. Many small carcinoid tumors produce no symptoms and are not fatal; they are found incidentally at autopsy. Even patients with larger, malignant carcinoid tumors (with or without metastasis) can survive years or decades with a good quality of life. This is especially true with modern treatments to control the carcinoid syndrome and to control the growth of the carcinoid tumors. However, there is a rare form of carcinoid tumor called adenocarcinoids that are more aggressive than the typical malignant carcinoid tumor, and they have a poor prognosis. An experienced pathologist can identify adenocarcinoid tumors by examining the tissue from the tumor under a microscope.

Medically Reviewed by a Doctor on 1/7/2014

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