Prediction of Peritoneal Dissemination of Digestive Tumors Through the Study of Circulating Tumor DNA

After screening, verification of inclusion and exclusion criteria, information and consent, patients will be enrolled in the study. Several samples will be collected at different time points during patient management, in addition to samples already performed as part of routine care. No intervention or sampling will occur at a time point specifically required for research purposes. Blood and peritoneal fluid samples will always be collected concomitantly. They will then be transported to the molecular biology laboratory for initial DNA extraction. After extraction, samples will be stored at -80 °C until analysis. The treatments offered to the patient follow standard-of-care recommendations and are not modified by participation in the study. Three independent cohorts will be evaluated: gastric cancer, colon cancer, and pancreatic cancer.Depending on the type of cancer and the therapeutic strategy, different samples will be collected. Systematic sampling: Plasma: at diagnosis, at the time of surgery, in the early postoperative period, and 1 month after surgery (during the postoperative visit). Peritoneal fluid: at the time of surgery and in the early postoperative period. Specific sampling: Peritoneal fluid: during pre-therapeutic exploratory laparoscopy (gastric cancer) or during diverting colostomy (obstructive colon cancer). In total: Gastric cancer: 3 peritoneal fluid samples and 4 blood samples. Colon cancer: 2 peritoneal fluid samples and 3 blood samples systematically, plus 1 optional blood and peritoneal fluid sample (only in case of diverting colostomy). Pancreatic cancer: 2 peritoneal fluid samples and 3 systematic blood samples, plus 1 optional blood sample (in case of neoadjuvant treatment). Blood sampling: 2 EDTA tubes (molecular biology) Peritoneal fluid sampling: 2 dry tubes (molecular biology, pathology) For intraoperative sampling, a peritoneal washing is performed with 200 mL of 0.9% NaCl instilled into the abdominal cavity; then 50 mL are collected from the pouch of Douglas. For the postoperative period, the sample will be collected between postoperative day 2 and day 5 directly from the drainage fluid (collected from the drain reservoir left in place at the end of surgery). ctDNA analysis: NGS (next-generation sequencing), or high-throughput sequencing, is a molecular biology method that enables the rapid sequencing of millions of DNA molecules simultaneously and is used to identify point mutations as well as insertions, duplications, and deletions. It will be used for both plasma analysis and peritoneal fluid analysis.