The digestive system processes food for energy and rids the body of solid waste. After food is chewed and swallowed, it enters the esophagus, 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.

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. When neuroendocrine tumors develop in the pancreas, they are known as pancreatic islet cell tumors. Because of the size of the digestive system, it has the most neuroendocrine cells of any organ. 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, and they may help control the growth of other types of digestive system cells.

Carcinoid (Neuroendocrine) Tumors and Cancers

Like most cells of the body, GI tract neuroendocrine cells sometimes go through certain changes that cause them to grow too much and form tumors. These are known as neuroendocrine tumors and neuroendocrine cancers.

In 2000, the World Health Organization (WHO) revised its classification of carcinoids. Before that time, most abnormal growths of neuroendocrine cells were called carcinoids. The WHO now divides these growths into neuroendocrine tumors (growths that are benign or have uncertain potential to spread to other parts of the body, and which may also be called carcinoids) and neuroendocrine cancers (which spread to other parts of the body). The WHO also subdivides neuroendocrine cancers (also known as neuroendocrine carcinomas) into well differentiated (tending to be less aggressive) and poorly differentiated (tending to be more aggressive) groups. Some doctors do not use this new terminology and use the term carcinoid to refer to either neuroendocrine tumors or well differentiated neuroendocrine cancers.

Neuroendocrine tumors and cancers act like the cells they come from. They often release certain hormone-like substances into the bloodstream. In about 1 out of 10 people with carcinoid tumors, they spread and grow very large and release high amounts of those hormones. This can cause symptoms such as facial flushing (redness and warm feeling), wheezing, diarrhea, and a fast heartbeat. These symptoms are grouped together and called the carcinoid syndrome. While most types of cancer cause symptoms only in the organs they start in or spread to, neuroendocrine tumors and neuroendocrine cancers can cause symptoms throughout the body.

Most tumors in the GI tract start from different glandular cells (the kind of gland cell that produce mucus rather than hormones) of the inner lining of the digestive system. These tumors can be either adenomas (benign) or adenocarcinomas (malignant). They are quite different from carcinoid tumors in their symptoms, their prognosis (course of the disease and outlook for survival), and their treatment. For these reasons, it is important for doctors to find out whether a patient has a neuroendocrine tumor or 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 there is a wide range among these tumors 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.