Gastrointestinal Stromal Tumors (GISTs) are a rare form of cancer located within the gastrointestinal (GI) tract. More than half of GISTs start in the stomach. Most of the others start in the small intestine but they can start anywhere in the GI tract. Rarely, they can occur outside the GI tract. Some GISTs are more likely to grow or spread to other areas of the body than others. Doctors will look at specific features of the GIST to help predict whether a GIST is likely to grow and spread quickly, such as the size of the tumor, where it is located in the GI tract, and how fast the tumor cells are dividing (mitotic rate).