**Study Overview:** This study focuses on using a special test called **ctDNA deep sequencing** to find out if cancer is still present, even if it can't be seen with regular scans. **ctDNA** stands for "circulating tumor DNA," which are small pieces of DNA from tumors found in the blood. Researchers want to see if this test can help find cancer early, especially when it's hard to detect with other methods. **Key Points:** - **Time and Visits:** You will need to visit the study center multiple times for blood tests. - **Eligibility:** You should have esophageal, gastric, or pancreatic cancer in early stages (0-2) or be receiving certain cancer treatments. - **Benefits and Risks:** This test may help catch cancer early, but the study involves blood draws, which can be uncomfortable. This study aims to improve how doctors detect and treat cancer by using this new method. It is important for ensuring the right treatment is given at the right time. Participants will help researchers understand if this test works well for everyone.