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

Inclusion Criteria: 1. Patients with age between 18 and 75 2. Patient with histology-proven gastric adenocarcinoma. 3. Patients with Eastern Cooperative Oncology Group performance status (ECOG) of 0-1 4. Patients planned for at least 4 cycles of cytotoxic chemotherapy (either as part of curative therapy or as palliative therapy) 5. Patients with at least 6 months' life expectancy from date of recruitment 6. Patients with positive Hepatitis B Surface-antigen (HBsAg) 7. Patients with normal liver function tests including alanine aminotransferase (ALT), aspartate aminotransferase alkaline (AST), phosphatase (ALP), gamma glutamyl-transpeptidase (GGT), and bilirubin 8. Patients with negative HBV-DNA 9. Patients with no known history of radiological \&/or histological diagnosis of chronic active hepatitis or cirrhosis of any cause, or history of prior hepatitis B reactivation, or prior chronic therapy for HBV within 6 months 10. Patients with no evidence of autoimmune hepatitis, hepatitis C or D virus infection, HIV infection or radiological evidence of liver metastasis 11. adequate bone marrow, hepatic, and renal function within 14 days before recruitment 12. patients who sign the informed consent 13. Patients with good compliance during chemotherapy and follow-ups Exclusion Criteria: 1. Patients planned for radiation or radionuclide therapy 2. Pregnant female patients 3. Patients with a history of psychiatric drugs abuse and can't quit or with a mental disorder 4. Patients with immunodeficiency, other congenital or acquired immunodeficiency, or transplantation history 5. According to the investigators' judgment, patients with concomitant disease that seriously harms patients' safety or the completion of study.