This study investigates the prognostic role of liquid biopsy in patients with locally advanced gastric cancer.

Liquid biopsy for the detection of circulating tumor DNA will be performed:

• In the peritoneal lavage, during staging laparoscopy (if indicated) and during curative gastrectomy
• In plasma, before staging laparoscopy (if indicated), before curative gastrectomy, at hospital discharge, three months after surgery/at the end of adjuvant therapy, and in case of disease recurrence.

The aim of this study is to determine the predictive power of liquid biopsy on overall survival and disease free survival.

The purpose of this prospective observational study is to determine the prognostic role of liquid biopsy for the detection of circulating tumor DNA (ctDNA) in patients with locally advanced gastric cancer.

Patients with cT3-4 and or N+ gastric cancer will proceed to either staging laparoscopy, if preoperative chemotherapy is indicated, or upfront radical surgery within 2 weeks from the last imaging assessment.

Liquid biopsy will be performed with peritoneal lavage and blood samples. Peritoneal lavage is performed with 500 ml of saline, starting from the retrogastric space (100 ml) and following clockwise from right upper quadrant to right lower using 100 ml for each quadrant. At the end of peritoneal lavage, 250 ml of saline solution is collected over the liver, after having positioned the patient in Trendelenburg position and on the right side. Of the collected fluid, 50 mL will be sent for cytological examination and 50 mL will be stocked for ctDNA analyses.

In case of upfront radical surgery, liquid biopsy will be performed at the following time points:

• Through blood samples performed before skin incision and 7-10 days after radical surgery;
• Through peritoneal lavage, immediately after laparotomy/laparoscopy.

In case of staging laparoscopy, liquid biopsy will be performed with blood samples, just before skin incision, and with peritoneal lavage. Patients with no evidence of peritoneal dissemination after staging laparoscopy will undergo induction chemotherapy according to the FLOT scheme or per investigators' judgment, and restaged after 4 cycles. Patients still fulfilling the inclusion criteria will undergo radical surgery within 3 weeks from the last chemotherapy cycle. Patients will then undergo liquid biopsy with the same scheme as for upfront radical surgery.

Patients who are clinically fit, and for whom there is clinical indication, will undergo adjuvant chemotherapy. Blood samples for ctDNA will be collected at the end of the last cycle of adjuvant chemotherapy, or at the end of the third postoperative month for those patients not undergoing postoperative chemotherapy. Then, patients will be followed up according to standard protocols by means of CT scan, CEA assessment and clinical visit every six months for the first three years after surgery and then yearly up to five years after local treatments. Blood samples for ctDNA will be collected at detection of cancer recurrence.

Inclusion Criteria:

• Histologically proven gastric or gastro-esophageal junction (GEJ -Siewert type II -III) adenocarcinoma
• More than cT2 and/or N + (UICC 8th edition)
• Written informed consent

Exclusion Criteria:

• Presence of ascites or distant metastases
• Previous chemotherapy for gastric adenocarcinoma
• Previous gastric surgery
• Absolute contraindications to surgery
• Concomitant malignancies or history of other malignancies in the previous 5 years (with exception of in situ cervical carcinoma, basal cell carcinoma, superficial bladder tumors are allowed if curatively treated).

Patients with histologically proven gastric or GEJ Siewert type II and III adenocarcinoma with indication to either staging laparoscopy followed by perioperative chemotherapy and surgery with curative intent, or surgery with curative intent without perioperative treatment, will be included.