In resectable gastric cancer participants who received curative surgery, to early and more accurately detect peritoneal carcinomatosis or occult metastasis is important. Also, investigators will look at CTC numbers in different timings after operation, to investigate the possibility of early detection for peritoneal carcinomatosis or occult metastasis. Also, this study will correlate the relationship of CTC and participants' survival.

Circulating tumor cells (CTCs) are an emerging "liquid biopsy" that provide prognostic value for various types of solid cancer on early recurrence and survival. The evaluation of CTCs might be a useful strategy to predict tumor progression and prognosis in Gastric adenocarcinoma (GC). Previous study has shown that the frequency of CTC detection was higher in advanced GC than early GC, in poorly differentiated GC than well/moderately differentiated GC, and in GC with lymphatic metastasis than that without lymphatic metastasis. However, the impact of CTCs in the detection of PM in GC is still under debate. Peritoneal metastasis (PM) is highly related to recurrence and metastasis in GC; therefore, it was significantly related to disease free and overall survival of participants.

Consequently, several important questions and goals will be answered by this study:

To elucidate the clinical relationship between CTCs and PM in GCs before the operation; therefore, it could be an indicator of prophylactic during operation, which may possibly prolong the disease free and overall survival.

To establish a good model to follow-up a specific surface marker on CTCs, which could be possibly utilized as a more sensitive marker, comparing with CEA or image study, to more accurately detect the early recurrence or metastasis in GC.

To verify that dynamically monitoring CTCs status and changes during long-term follow-up and anti-cancer treatment are feasible and clinically meaningful to survival or treatment responses.

Inclusion Criteria:

• Enrolling 150 cases
• Stage I or more advanced gastric cancer, pathology proved
• Diagnosed at age ≥ 20 years
• Enrolled before surgery at the General Surgery Department, both inpatient and outpatient services.

Exclusion Criteria:

• Patient's refusal
• Poor compliance, unable to cooperate for blood sampling for CTC isolation as time schedule or clinical treatment or follow-up
• Difficult blood sampling
• No more needs for CTCs evaluation, decided by clinicians.

In resectable gastric cancer patients, who undergo radical intent gastrectomy surgery.