A tumor that forms in islet cells (hormone-making cells) of the pancreas. Pancreatic neuroendocrine tumors may be benign (not cancer) or malignant (cancer). Islet cells make several different hormones that affect body functions, including controlling the amount of glucose (sugar) in the blood and helping digest food in the stomach. Functional pancreatic neuroendocrine tumors make extra amounts of these hormones, which can cause symptoms. Nonfunctional pancreatic neuroendocrine tumors do not make extra amounts of hormones, but they may cause symptoms as they grow and spread. Also called islet cell tumor and pancreatic endocrine tumor.