STOMACH CANCER | Gastrointestinal Stromal Tumor with ulcer ENDOSCOPY (GISTs) Gastrointestinal stromal tumors (GISTs) account for less than 1% of gastrointestinal tumors, but they are the most common mesenchymal neoplasms of the gastrointestinal tract. GISTs are usually found in the stomach or small intestine but can occur anywhere along the GI tract and rarely have extra-GI involvement