ctDNA Screening in Advanced HER2 Positive Gastric Cancer

Inclusion Criteria: 1. Male/female patients aged over 18 years. 2. Histologically confirmed gastric adenocarcinoma. Gastric tumors should be treatment naïve unresectable or metastatic disease, or recurrence over 6 months after finish of adjuvant chemotherapy. 3. HER2 status is confirmed by IHC/FISH. HER2 positive: IHC 3+ or IHC 2+ plus FISH positive, HER2 negative: IHC 0/1+ or IHC 2+ plus FISH negative. 4. At least one measurable lesion should be confirmed by imaging examination. 5. Eligible peripheral blood samples 6. Patients with enough organ function and performance status (ECOG 0-2) can tolerant chemotherapy. 7. For HER2 positive patients, trastuzumab should be used as first-line treatment. The regimen of chemotherapy should be platinum plus oral fluorouracil. 8. For HER2 negative patients, clinicopathological characteristics should be matched to HER2 positive patients. 9. Willing to provide clinicopathological information and imaging information. Exclusion Criteria: 1. Patients received systemic treatment before enrolled or finished adjuvant chemotherapy less than 6 months. 2. With second primary malignant diseases. 3. Other situations assessed by investigator can disturb quality control of the investigation.