Carcinoid tumors are a rare form of cancer that originate in the gastrointestinal tract. Occasionally, these tumors also develop in organs outside the digestive tract, such as the lungs. Carcinoid tumors develop slowly and are highly treatable when caught in the early stages of development.

Cancer can occur in any part of the gastrointestinal tract from the mouth to the rectum. Most cancers in the digestive tract occur in glandular tissues that produce mucus. Such cancers include stomach cancer and colorectal cancer. The tumors that develop in these areas are usually of the type called adenomas or adenocarcinomas.

Carcinoid tumors originate in the neuroendocrine cells, which are found in the digestive system and lungs. Neuroendocrine cells produce hormones that regulate the digestive juices and muscles that move food through the stomach and intestines. When these cells undergo certain changes, they may grow uncontrollably and form carcinoid tumors.  The majority of neuroendocrine cells are in the digestive system, so most carcinoid tumors develop there.

Within the digestive system, carcinoids occur most commonly in the small intestine, rectum and the appendix. They are the most common cancerous condition that affects the small intestine.  Gastrointestinal carcinoid tumors tend to grow slowly and may occur in multiple sites, especially when they are located within the small intestine.

The location and size of the tumor play a large role in determining whether or not a carcinoid tumor will spread. Very small tumors – 1 centimeter (about 1/2 inch) or less – usually do not spread (metastasize), although larger tumors frequently spread beyond their area of origin.

When detected early, gastrointestinal carcinoid tumors are usually treatable. However, they typically do not produce symptoms during their early stages of development and these early stage tumors may only be discovered as incidental findings, during abdominal surgery performed for other conditions. However, even when these tumors are not treated or removed, many patients may live for a long time because of the tumors’ slow growth. Gastrointestinal carcinoid tumors produce the same hormones as neuroendocrine cells. The hormones produced by tumors are usually destroyed by enzymes in the blood and liver. As long as the liver successfully breaks down these hormones, a patient usually experiences no symptoms related to the tumors. Many carcinoid tumors go undiagnosed because they progress slowly and may not cause readily identifiable symptoms during an individual’s lifetime.

However, in some cases, the tumors secrete excess hormones and produce varying symptoms. One collection of symptoms is called carcinoid syndrome and may affect breathing, skin and the circulatory system.

About 11,000 to 12,000 carcinoid tumors are diagnosed in the United States each year, according to the American Cancer Society. About half of these tumors are gastrointestinal carcinoids, and another third occur in the lungs. One-half of all gastrointestinal carcinoid tumors are found at an early stage of development.