A small number of GISTs start outside the GI tract in nearby areas such as the omentum (an apron-like layer of fatty tissue that hangs over the organs in the abdomen) or the peritoneum (the thin lining over the organs and walls inside the abdomen). 
A small number of GISTs start outside the GI tract.  In nearby areas such as the omentum (an apron-like layer of fatty tissue that hangs over the organs in the abdomen) or the peritoneum (the thin lining over the organs and walls inside the abdomen). 