The gastrointestinal system

The gastrointestinal (GI) system processes food for energy and rids the body of solid waste. It is also known as the digestive system. After food is chewed and swallowed, it enters the esophagus. This is a tube that carries food through the neck and chest to the stomach. The esophagus joins the stomach just beneath the diaphragm (the breathing muscle under the lungs). The stomach is a sac-like organ that holds food and begins the digestive process by secreting gastric juice. The food and gastric juices are mixed into a thick fluid, which is then emptied into the small intestine.

The small intestine continues breaking down the food and absorbs most of the nutrients. It is the longest section of the gastrointestinal (GI) tract, measuring more than 20 feet. The small intestine then joins the colon (large intestine). This is a wider, muscular tube about 5 feet long. The appendix is found near the junction of small intestine and colon. The colon absorbs water and mineral nutrients from the food matter and serves as a storage place for waste. The waste left after this process goes into the rectum. From there it passes out of the body through the anus as stool (feces).

The diffuse neuroendocrine system

Carcinoid tumors start from cells of the diffuse neuroendocrine system. This system consists of cells that are like nerve cells in certain ways and like hormone-making endocrine cells in other ways. These cells do not form an actual organ like the adrenal or thyroid glands. Instead, they are scattered throughout other organs like the esophagus, stomach, pancreas, intestines, and lungs. The digestive system is large and contains more neuroendocrine cells than any other part of the body. This may explain why carcinoid tumors most often start in the digestive system.

Neuroendocrine cells help control the release of digestive juices and the speed at which food moves in the GI tract. They may also help control the growth of other types of digestive system cells.

Neuroendocrine tumors

Like most cells in the body, GI tract neuroendocrine cells sometimes go through certain changes that cause them to grow too much and form tumors. These tumors are known as neuroendocrine tumors (NET) and neuroendocrine cancers. In the past, most abnormal growths of neuroendocrine cells were called carcinoids. But in 2000, the World Health Organization (WHO) revised its classification of carcinoids. The WHO now divides these growths into neuroendocrine tumors and neuroendocrine cancers. Neuroendocrine tumors are growths that look benign but that might possibly be able to spread to other parts of the body. Neuroendocrine cancers are abnormal growths of neuroendocrine cells which can spread to other parts of the body. Neuroendocrine cancers (also known as neuroendocrine carcinomas) can then be divided into groups based on the way the cells look under the microscope. A cancer with cells that do not look very abnormal is called well differentiated. These tumors tend to be less aggressive -- they grow and spread slowly. Well differentiated neuroendocrine cancers can look identical to benign neuroendocrine tumors when examined under the microscope. Sometimes the only way to know for certain that a mass is a neuroendocrine cancer (and not a benign tumor) is when it spreads to other organs or tissues. If the cells of a cancer look very abnormal, it is called poorly differentiated. These cancers tend to be more aggressive meaning that they grow and spread quickly.

Neuroendocrine tumors of the pancreas

Neuroendocrine tumors that arise in the pancreas are known as islet cell carcinomas or pancreatic neuroendocrine tumors. These are not the same as carcinoid tumors. Islet cell tumors have a different outlook and respond differently to treatment than carcinoids do. These tumors are not discussed further in this document. For more information about pancreatic neuroendocrine tumors, please see our document, Pancreatic Cancer.

Carcinoid tumors

Carcinoid is the term used to describe a well to moderately differentiated neuroendocrine tumor in the stomach, intestine, appendix, rectum, and lung. For more information about carcinoid tumors that start in the lung, please see our document, Lung Carcinoid Tumors).

Neuroendocrine tumors and cancers act like the cells they come from, often releasing certain hormone-like substances into the bloodstream. In most people with carcinoid tumors, the levels of these hormones are not high enough to cause symptoms. But in about 1 person out of 10 with carcinoid tumors, the tumor spreads and grows enough to release high amounts of these hormones. This can cause a set of symptoms known as the carcinoid syndrome. Some symptoms of the carcinoid syndrome include flushing (redness of the skin with a feeling of warmth), wheezing, diarrhea, and a fast heartbeat.

Other gastrointestinal tumors

Carcinoids and other neuroendocrine tumors are different from the more common tumors of the GI tract. Most GI tract tumors start from the glandular cells that produce mucus and make up the inner lining of the digestive system. When these tumors are benign, they are called adenomas. When these cells develop into cancer, the tumors are known as adenocarcinomas. These tumors are quite different from carcinoid tumors in their symptoms, their prognosis (course of the disease and outlook), and their treatment. For these reasons, it is important to know what type of tumor you have: a neuroendocrine tumor, a neuroendocrine cancer, an adenoma, an adenocarcinoma, some other type of tumor, or a non-cancerous condition. And it is important for patients to understand that neuroendocrine tumors and neuroendocrine cancers are not the same as other, more common types of GI tract tumors.

In general, neuroendocrine tumors and neuroendocrine cancers grow more slowly than other cancers in the GI tract. But these tumors vary widely -- in how they grow and whether or not they spread to other areas. This depends to some extent on which part of the body the tumor starts in.

Last Medical Review: 06/19/2009
Last Revised: 06/19/2009