Gastrointestinal Cancer can occur anywhere in the GI tract − from the esophagus to the anus. Most cancers that start in the GI tract, including most esophagus cancers, stomach cancers, and colon and rectum cancers, start in the gland cells that line almost all of the GI tract. If you are experiencing any symptoms of bleeding, pain or vomiting, it is important you consult a physician right away.