A rare type of tumor that can form in the pancreas or in other parts of the gastrointestinal tract, including the stomach, small intestine, colon, rectum, and appendix. Gastroenteropancreatic neuroendocrine tumors usually form in cells that secrete hormones. Some of these tumors make extra amounts of hormones and other substances that may cause signs and symptoms of disease, including a condition called carcinoid syndrome. Gastroenteropancreatic neuroendocrine tumors may be benign (not cancer) or malignant (cancer). They are sometimes called carcinoid tumors or islet cell tumors. Also called GEP-NET.