• Histologically confirmed, medically operable, resectable diffuse or mixed type (according to Lauren's classification) adenocarcinoma of the gastroesophageal junction (AEG II-III) or the stomach (uT3, uT4a, any N category, M0), or any T N+ M0 patient

• Patient has received 3 to 6 cycles of neoadjuvant FLOT (de-escalation or dose modification allowed)

• No preceding cytotoxic or targeted therapy other than neoadjuvant FLOT (including de-escalated or dose reduced schema) therapy

• No prior partial or complete tumor resection

• Female and male patient ≥ 18 and ≤ 75 years. Female patient with childbearing potential needs to have a negative pregnancy test within 7 days prior to study start. Males and females of reproductive potential must agree to practice highly effective contraceptive measures\* during the study. Male patients must also agree to refrain from father a child during treatment and additionally to use a condom during treatment period. Their female partner of childbearing potential must also agree to use an adequate contraceptive measure.

• \*highly effective (i.e. failure rate of \<1% per year when used consistently and correctly) methods: intravaginal and transdermal combined (estrogen and progestogen containing) hormonal contraception; injectable and implantable progestogen-only hormonal contraception; intrauterine device (IUD); intrauterine hormone-releasing system (IUS); bilateral tubal occlusion; vasectomised partner; sexual abstinence (complete abstinence is defined as refraining from heterosexual intercourse during the entire period of risk associated with the study treatments).

• ECOG ≤ 1

• Exclusion of distant metastases by CT or MRI of abdomen, pelvis, and thorax, bone scan or MRI (if bone metastases are suspected due to clinical signs). Exclusion of the infiltration of any adjacent organs or structures by CT or MRI

• Laparoscopic exclusion of peritoneal carcinomatosis at initial staging, before start of FLOT chemotherapy

• Hematological, hepatic and renal function parameters adequate to allow surgical procedure and HIPEC at investigator´s discretion

⁃ Patient able and willing to provide written informed consent and to comply with the study protocol and with the planned surgical procedures