ON THIS PAGE: You will find some basic information about this disease and the parts of the body it may affect. This is the first page of Cancer.Net’s Guide to Carcinoid Tumors. To see other pages, use the menu on the side of your screen. Think of that menu as a roadmap to this full guide.
About the endocrine system
The body’s endocrine system is made up of cells that produce hormones. Hormones are chemical substances that are carried through the bloodstream to have a specific effect on the activity of other organs or cells in the body.
About endocrine and neuroendocrine tumors
A tumor begins when healthy cells change and grow uncontrollably, forming a mass. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body if it is not found early and treated. A benign tumor means the tumor can grow but will not spread. A benign tumor usually can be removed without it causing much harm.
An endocrine tumor  is a mass that begins in the parts of the body that produce and release hormones. Because an endocrine tumor develops from cells that produce hormones, the tumor can also produce hormones, causing serious illness.
A neuroendocrine tumor  begins in the hormone-producing cells of the body’s neuroendocrine system, which is made up of cells that are a combination of hormone-producing endocrine cells and nerve cells. Neuroendocrine cells are found throughout the body in organs such as the lungs and gastrointestinal (GI) tract, including the stomach and intestines. Neuroendocrine cells perform specific functions, such as regulating air and blood flow through the lungs and controlling the speed at which food moves through the GI tract.
About carcinoid tumors
A carcinoid tumor is a specific type of neuroendocrine tumor. Carcinoid tumors most often develop in the GI tract, in organs such as the stomach or intestines, or in the lungs. However, a carcinoid tumor can also develop in the pancreas, a man’s testicles, or a woman’s ovaries. More than one carcinoid tumor can develop in the same organ.
Here is a general overview of where carcinoid tumors begin:
39% occur in the small intestine.
15% occur in the rectum.
10% occur in the bronchial system of the lungs.
7% occur in the appendix.
5% to 7% occur in the colon.
2% to 4% occur in the stomach.
2% to 3% occur in the pancreas.
About 1% occurs in the liver.
Carcinoid tumors rarely occur in the ovaries, testicles, and other organs.
Because carcinoid tumors develop from neuroendocrine cells, they can make high levels of neuropeptides and amines, which are hormone-like substances. However, these substances may not be released in large enough amounts to cause symptoms, or the substances may be defective and not cause symptoms. As a result, a carcinoid tumor can grow slowly for many years without causing symptoms. Although a carcinoid tumor is cancerous, it is often described as "cancer in slow motion."
Types of lung carcinoid tumors
There are two types of lung carcinoid tumors: typical and atypical. The difference is based on how a tumor processes and makes serotonin (5-HT). Serotonin is a neurotransmitter involved in behavior and depression.
Typical. A typical lung carcinoid tumor produces high levels of serotonin and chromogranin-A that can be measured in the blood. Typical lung carcinoid tumors also produce high levels of 5-HIAA, a product of serotonin breakdown, that can be measured in the urine.
Atypical. People with an atypical lung carcinoid tumor have normal levels of serotonin and chromogranin-A in their blood and normal levels of 5-HIAA in their urine. However, high levels of serotonin and the amino acid 5-HTP can be found in the urine, as atypical lung carcinoid tumors can make 5-HTP.
The next section in this guide is Statistics , and it helps explain how many people are diagnosed with this type of tumor and general survival rates. Or, use the menu on the side of your screen to choose another section to continue reading this guide.