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The small intestine keeps breaking down food and absorbs most of the nutrients. It is the longest section of the gastrointestinal (GI) tract, measuring more than 20 feet (6 meters). The small intestine then joins the colon. This is a wider, muscular tube about 5 feet (1.5 meters) long. The appendix is near the junction of small intestine and colon. The colon absorbs water, minerals, and nutrients from food and serves as a storage place for waste. The waste that is left after this process goes into the rectum. From there it leaves the body through the anus as stool (feces).

The neuroendocrine system

The neuroendocrine system has cells that act like nerve cells in certain ways and like hormone-making endocrine cells in others. These cells don’t form an actual organ like the adrenal or thyroid glands. Instead, they are scattered throughout organs like the esophagus, stomach, pancreas, intestines, appendix, and lungs. The digestive system has more neuroendocrine cells than any other part of the body. This might be why carcinoid tumors most often start there.

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

Neuroendocrine (carcinoid) tumors

Neuroendocrine tumors (NETs) are mostly slow growing, but some are not and can possibly spread to other parts of the body

They are classified by tumor grade which describes how quickly the cancer is likely to grow and spread..

• Grade 1 (low grade) NETs have cells that look more like normal cells and are not multiplying quickly.
• Grade 2 (intermediate grade) NETs have features in between those of low- and high-grade tumors.
• Grade 3 (high grade) NETs have cells that look very abnormal and are multiplying faster.

Cancers that are grade 1 or grade 2 are called GI neuroendocrine tumors. These cancers tend to grow slowly and can possibly spread to other parts of the body.

Cancers that are grade 3 are called GI neuroendocrine carcinomas (NECs).  These cancers tend to grow and spread quickly and can spread to other parts of the body.

The term “carcinoid” is often used to describe grade 1 and grade 2 GI NETs. The term carcinoid will be used here unless referring to NECs specifically. Carcinoid tumors that start in the lungs are not covered here, but you can find more information in Lung Carcinoid Tumor.

Other gastrointestinal tumors

Neuroendocrine (carcinoid) 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.

Pancreatic neuroendocrine 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 (see Pancreatic Neuroendocrine Tumors).

These tumors differ quite a lot in their symptoms, their outlook, and their treatment. For these reasons, it is important to know what type of tumor you have. Information about other kinds of tumors of the GI tract can be found in Esophagus Cancer, Stomach Cancer, Small Intestine Cancer, and Colorectal Cancer.