MSH: A tumor that secretes VASOACTIVE INTESTINAL PEPTIDE, a neuropeptide that causes VASODILATION; relaxation of smooth muscles; watery DIARRHEA; HYPOKALEMIA; and HYPOCHLORHYDRIA. Vipomas, derived from the pancreatic ISLET CELLS, generally are malignant and can secrete other hormones. In most cases, Vipomas are located in the PANCREAS but can be found in extrapancreatic sites.,NCI: An often clinically aggressive neuroendocrine tumor located in the pancreas or small intestine. It is composed of cells containing vasoactive intestinal peptide. It may cause intractable diarrhea and metabolic disturbances.