Gastrointestinal carcinoid tumors are a type of cancer you can get in the GI tract. They're most common in the appendix, small intestine, and rectum, which is part of the large intestine. They're the most common type of neuroendocrine tumor (NET). 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 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.