Concomitant Intraperitoneal and Systemic Chemotherapy in Patients With Extensive Peritoneal Carcinomatosis of Gastric Origin

Inclusion Criteria: * Patients with a histologically confirmed diagnosis of HER2-negative gastric cancer. * A histologically confirmed diagnosis of peritoneal carcinomatosis. * Age ≥ 18 years old. * Written informed consent according to the ICH-GCP and national/local regulations. * A peritoneal cancer index (PCI) ≥7 evaluated by laparoscopy or laparotomy before inclusion in this trial. * Patients must be ambulatory: World Health Organisation (WHO) performance status 0 or 1. * Life expectancy of at least 3 months. * Ability to return to the Erasmus MC for adequate follow-up as required by this protocol. * Patients must have normal organ function and adequate bone marrow reserve as assessed by the following laboratory requirements: * absolute neutrophil count \>1.5 \* 10\^9/l; * platelet count \>100\*10\^9/l; * Hb\>6.0mmol/l; * Bilirubin \< 1.5x upper limit of normal (ULN); * Serum aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) \< 2.5 x ULN; * Glomerular Filtration Rate (GFR) \>45 and Creatinine clearance \<2 x ULN. Exclusion Criteria: * Medical or psychological impediment to probable compliance with the protocol. * Serious concomitant disease or active infections. * Distant metastasis other than peritoneal metastasis or metastatic lymph nodes. * No sufficient oral food intake. * Polyneuropathy grade 2 or worse according to CTCAE version 5.0. * History of auto-immune disease or organ allografts, or with active or chronic infection, including HIV and viral hepatitis. * Serious intercurrent chronic or acute illness such as pulmonary (COPD or asthma) or cardiac (NYHA class III or IV) or hepatic disease or other illness considered by the study coordinator to constitute an unwarranted high risk for participation in this study. * Homozygous UGT1A1\*28 genotype. * Homozygous dihydropyrimidine dehydrogenase (DPYD) genotype (tested for \*2A, \*13, 2846A\>T, and 1236G\>A). * Current use of strong CYP3A4-inhibitors or inducers. If patients use this CYP3A4-modulating medication, it is allowed to stop it within 14 days of start of treatment. * Pregnant or lactating women. * Concomitant participation in a competing clinical study. * Absence of assurance of compliance with the protocol. * An organic brain syndrome or other significant psychiatric abnormality which would comprise the ability to give informed consent, and preclude participation in the full protocol and follow-up.