Digestive system cancer is a term used to describe any type of cancer that affects the gastrointestinal tract. This includes the stomach, small, large, rectum, and anus. Cancer of the digestive system is relatively rare in dogs, but it can be deadly. The most common type of digestive system cancer in dogs is lymphoma, which starts in the lymph nodes and can spread to other organs. Other types of digestive system cancer include gastrointestinal stromal tumors (GISTs), adenocarcinomas, and leiomyosarcomas.