Prophylactic or Preemptive Entecavir in Patients With Gastrointestinal Cancer Who Are Inactive Hepatitis B Carriers

Inclusion Criteria: 1. Patients with age between 18 and 75 2. Patient with histology-proven locally advanced unresectable or metastatic gastrointestinal cancers (colorectal cancer, gastric cancer, esophageal cancer, hepatocellular carcinoma, pancreatic cancer, and cholangiocarcinoma) 3. Planned to receive first-, second-, or third-line anti-tumor therapy (chemotherapy or PD-1/PD-L1 monoclonal antibody immunotherapy) 4. Patients with Eastern Cooperative Oncology Group performance status (ECOG) of 0-2 5. Patients planned for at least 4 cycles of chemotherapy or immunotherapy 6. Patients with at least 6 months' life expectancy from date of recruitment 7. Patients with chronic or past HBV infection (HBsAg-positive or HBcAb-positive), and hepatitis B is inactive 8. Patients with normal liver function tests including alanine aminotransferase (ALT), aspartate aminotransferase alkaline (AST), and bilirubin 9. Patients with negative HBV-DNA 10. Adequate major organ function (laboratory tests 14 days before randomization meeting requirements for anti-tumor therapy) 11. Patients who sign the informed consent 12. Patients with good compliance during chemotherapy and follow-ups. Exclusion Criteria: 1. History of liver cirrhosis 2. Prior HBV reactivation 3. Received anti-HBV therapy for chronic hepatitis B within 6 months before enrollment 4. Active co-infection with other hepatitis viruses 5. HIV infection 6. Autoimmune hepatitis 7. History of hepatic radiotherapy 8. Scheduled hepatic radiotherapy or radioisotope therapy 9. Pregnant or lactating women 10. Patients with a history of psychiatric drugs abuse and can't quit or with a mental disorder 11. Patients with immunodeficiency, other congenital or acquired immunodeficiency, or transplantation history 12. According to the investigators' judgment, patients with concomitant disease that seriously harms patients' safety or the completion of study.