What Is a Gastrointestinal Carcinoid Tumor?

Gastrointestinal carcinoid tumors are a type of cancer that forms in the lining of the gastrointestinal (GI) tract. Cancer starts when cells begin to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about what cancer is and how it can grow and spread, see What Is Cancer?

To understand gastrointestinal carcinoid tumors, it helps to know about the gastrointestinal system, as well as the diffuse neuroendocrine system.

The gastrointestinal system

The gastrointestinal (GI) system, also known as the digestive system, processes food for energy and rids the body of solid waste. After food is chewed and swallowed, it enters the esophagus. This tube 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 then empties into the small intestine.

color illustration of the digestive system which shows the location of the esophagus, stomach, pancreas, transverse colon, descending colon, sigmoid colon, anus, rectum, appendix, cecum, ascending colon, small intestine, gallbladder and liver

The small intestine continues breaking down 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. 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 food 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 don’t 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 has more neuroendocrine cells than any other part of the body. This might be why carcinoid tumors most often start in the digestive system.

Neuroendocrine cells help control the release of digestive juices and how fast 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 (NETs) and neuroendocrine cancers. In the past, most abnormal growths of neuroendocrine cells were called carcinoids. But in 2000, the World Health Organization (WHO) reclassified carcinoids as neuroendocrine tumors and neuroendocrine cancers.

Neuroendocrine tumors are growths that look benign but can possibly 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) are divided into groups based on the way the cells look under a microscope:

  • Well-differentiated neuroendocrine cancers have cells that do not look very abnormal and are not multiplying rapidly. These tumors tend to grow and spread slowly. Well-differentiated neuroendocrine cancers can look identical to benign neuroendocrine tumors when seen 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.
  • Poorly-differentiated cancers have cells that look very abnormal and are multiplying more rapidly. Poorly-differentiated cancers tend to grow and spread quickly.
  • Moderately-differentiated cancers have features in between those of well-differentiated and poorly-differentiated cancers.

Neuroendocrine tumors of the pancreas

Neuroendocrine tumors in the pancreas are known as islet cell carcinomas or pancreatic neuroendocrine tumors. Islet cell tumors are not the same as carcinoid tumors. They have a different prognosis (course of disease and outlook) and respond differently to treatment. Neuroendocrine tumors of the pancreas are not covered here, but you can find more information in Pancreatic Cancer.

Carcinoid tumors

Carcinoid is the term used to describe well to moderately-differentiated neuroendocrine tumors in the stomach, intestine, appendix, rectum, and lung. Carcinoid tumors that start in the lungs are not covered here, but you can find more information in Lung Carcinoid Tumor.

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 a carcinoid tumor, 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 differ quite a lot from carcinoid tumors in their symptoms, their 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, or some other type of tumor. Information about adenocarcinomas of the GI tract can be found in Esophagus Cancer, Stomach Cancer, Small Intestine Cancer, and Colorectal Cancer.

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

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: February 26, 2015 Last Revised: February 8, 2016

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