Patients with gastric or colon cancer with peritoneal carcinomatosis will receive a biopsy of the tumor during their primary curative surgery. The operation is performed according to standard and includes resection of the primary tumor and any metastases and followed by HIPEC (Intraperitoneal hyperthermic chemoperfusion) according to the respective hospital standard. Organoid cultures from the biopsies are established in the research laboratory.

Various chemotherapeutic agents are tested on these tumor organoids in the laboratory and the tumor organoids are analyzed in detail with regard to genetic alterations in order to find alterations that can be addressed, if necessary, by means of targeted drugs against peritoneal carcinomatosis.

Inclusion Criteria:

• (Suspected) synchronous or metachronous peritoneal metastasis of adenocarcinoma of the stomach / gastroesophageal junction (GEJ) or of the colon or rectum
• intraoperative histological confirmation of synchronous or metachronous peritoneal carcinomatosis in gastric carcinoma (incl. GEJ) or colon carcinoma (incl. rectal carcinoma)
• Intraoperative peritoneal cancer index (PCI) ≤ 15 for gastric carcinoma and ≤ 20 for colon carcinoma.
• Possibility of surgical resection of peritoneal carcinomatosis (cytoreductive surgery) in curative intention with achievement of a Completeness of Cytoreduction Score (CCS) of 0-1
• No contraindication to surgery
• No contraindication against the performance of HIPEC
• Expected survival of 6 months at least
• ECOG ≤ 2
• Female and male patients ≥ 18 years of age
• Patient is able and willing to give written informed consent and comply with the study protocol

Exclusion Criteria:

• Presence of non-resectable distant metastases
• Patients with extensive metastasis (e.g., multiple bilobular liver metastases, hepatic and pulmonary metastases, multiple retroperitoneal lymph node metastases; oligometastasis is allowed)
• Patients with recurrence of peritoneal carcinomatosis (e.g., previous peritonectomy in the course of primary tumor resection)
• Patients after previous palliative chemotherapy or radiation of the tumor (exception: neoadjuvant and/or adjuvant therapies)
• Hypersensitivity/allergy to components of the planned intraperitoneal chemotherapy
• Patients not eligible for surgery/HIPEC (e.g., heart failure NYHA ≥III, myocardial infarction within the last 3 months before surgery, high-risk cardiac arrhythmias)
• Secondary malignant disease that occurred <5 years ago (exception: early stage of a localized tumor with in-sano resection, for example in situ carcinoma of the cervix, Adequately treated basal cell carcinoma of the skin)
• Patients who are housed in a closed facility
• Pregnant or breastfeeding patients, or patients who plan to become pregnant within 7 Months after the end of treatment to become pregnant

Eligible patients are recruited consecutively during of the initial surgical treatment planning