HER2 Positive CTC in Advanced Gastric Cancer

Inclusion Criteria: * Having signed informed consent * Age≥ 18 years old * HER2 overexpression confirmed by IHC or ISH (IHC 3+,or IHC2+/ISH+) * Histologically confirmed gastric adenocarcinoma * Unresectable recurrent or metastatic disease * Previous neo-adjuvant or adjuvant treatment for gastric cancer, if applicable, more than 6 months * Measurable disease according to the RECIST criteria * Karnofsky performance status ≥70 * Life expectancy of ≥3 month * No prior radiotherapy except radiotherapy at non-target lesion of the study more than 4 weeks * ALT and AST\<2.5 times ULN (≤5 times ULN in patients with liver metastases) * Serum albumin level ≥3.0g/dL * Serum AKP \< 2.5 times ULN * Serum creatinine \<ULN, and CCr \< 60ml/min * Bilirubin level \< 1.5 ULN * WBC\>3,000/mm3, absolute neutrophil count ≥2000/mm3, platelet\>100,000/mm3, Hb\>9g/dl Exclusion Criteria: * Previous systemic therapy for metastatic gastric cancer * Surgery (excluding diagnostic biopsy) within 4 weeks prior to study entry Contraindications of nuclear magnetic resonance image such as fitment of cardiac pacemaker , nerve stimulator, or aneurysm clip, and metallic foreign body in eye ball and so on. * Allergic constitution or allergic history to protium biologic product or any investigating agents. * Severe heart disease or such history as recorded congestive heart failure, uncontrolled cardiac arrhythmia, angina pectoris needing medication, cardiac valve disease, severe abnormal ECG findings, cardiac infarction , or retractable hypertension. * Pregnancy or lactation period * Other previous malignancy within 5 year, except non-melanoma skin cancer * Legal incapacity