There has been no report on whether the patients with gastric cancer who are also inactive Hepatitis B carriers should receive prophylactic use or preemptive Use of an Anti-viral Drug Entecavir. This open, randomized controlled clinical trial aims to compare the impact of the prophylactic use or preemptive use of an anti-viral drug Entecavir on the outcomes of patients with gastric cancer who are also inactive hepatitis B carriers during chemotherapy and the subsequent follow-ups.

Patients with gastric cancer who are also inactive hepatitis B carriers are enrolled and randomized into two groups as following. Patients in experimental group are treated with entecavir prophylactically in the dose of 0.5mg p.o. every day from the initiation of chemotherapy till 6 months after the end of chemotherapy.Patients in active comparator group are only treated with entecavir in the dose of 0.5mg p.o. every day from the time that the DNA copies of hepatitis B virus are more than 100 IU/ml till 6 months after the end of chemotherapy.

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.