Where do gastrointestinal carcinoid tumors usually appear?

GI carcinoid tumors can occur throughout the GI tract. Most notably, primary tumors originated mostly to the small intestine specifically the ileum (45%), with the rectum following at 20%, colon 11%, and lastly the stomach 7%. There are three classifications of these tumors based upon their embryonic origen location which include forgut, midgut, hindgut, and extra intestinal. The foregut includes the stomach, duodenum, thymus and lungs, the midgut includes the jejunal and ileum portion of the small intestine, appendix, and ascending colon, the hindgut includes the transverse, sigmoid, descending colon, rectum, ovaries, and genitourinary. 

Most commonly, gastrointestinal carcinoid tumors grow in the small intestine, appendix, or rectum. This type of cancer can also develop in other gastrointestinal organ, including the stomach and colon. Later in the cancer's development, it can spread to the liver.