**Prospective Single-center Study About the Use of ctDNA in the Management of Patients With Tumors of the Gastrointestinal Tract** This study looks at how a special test, called ctDNA (circulating tumor DNA), can help doctors manage patients with tumors in their digestive system. Tumors are lumps that can grow in places like the stomach or intestines. **ctDNA** is a kind of DNA that can be found in blood and gives clues about the tumor. This study collects blood and poop samples to check for these clues, which can help doctors make better treatment decisions. - **Key Point 1:** This study does not involve new or experimental drugs. - **Key Point 2:** Participants will provide blood and poop samples at specific times during the study. - **Key Point 3:** Must be over 18, have certain types of digestive tumors, and provide consent to join. To join, you should be 18 or older and have a confirmed diagnosis of certain types of digestive cancers. You shouldn't join if you don't have these types of cancer or don't want to sign a consent form. All personal information is kept private.