- Treatment: instead of chemotherapy through the veins, participants receive PIPAC. PIPAC is a laparoscopic operation under general anesthesia. The entire operation takes about 90 minutes. After the operation, patients stay in the hospital for at least one night. PIPAC is repeated every 6 weeks. The aim is to perform 3 PIPACs. If the peritoneal metastases respond well to PIPAC, participants may undergo more than 3 PIPACs. Participants may also undergo less than 3 PIPACs: for example if the peritoneal metastases do not respond PIPAC, if laparoscopy is unsafe for the patient, or if the surgeons cannot perform PIPAC due to adhesions in the abdomen. In case no further PIPACs can be performed, participants receive chemotherapy through the veins.