Combined Systemic and Intraperitoneal Chemotherapy for Synchronous Gastric and/or Gastroesophageal Peritoneal Carcinomatosis

Inclusion Criteria: * Histologically proven, synchronous gastric and/or gastroesophageal junction (Siewert 3) adenocarcinoma with either positive peritoneal cytology (micPC) or macroscopic carcinomatosis (macPC) (stage IV), diagnosed by imaging or laparoscopy or laparotomy. * Patients must be 18 years of age or older at time of informed consent. * ECOG performance status of ≤ 2\* \*Note: ECOG PS of 2 is allowed only if debilitation is caused by the gastric cancer and not by other comorbidities. * Must have normal organ and marrow function following completion of 4-6 months systemic therapy and prior to enrollment as assessed on basic laboratory tests: Hemoglobin ≥ 8.0 g/dL Leukocytes ≥ 3,000/mcL Absolute neutrophil count ≥ 1,500/mcL Platelet count ≥ 100,000/mcL Total bilirubin \< 2mg/dl AST (SGOT) ≤ 2.5 X institutional upper limit of normal (ULN) ALT (SGPT) ≤ 2.5 X institutional ULN Serum creatinine within normal institutional limits * Patient should have a life expectancy of \>6 months per the discretion of the treating surgical oncologist and/or medical oncologist in relation to comorbidities and volume of disease. * Patients must have received at least 4 months (maximum 6 months) of 1st or 2nd line systemic therapy at the discretion of the treating medical oncologist and/or surgical oncologist and exhibit no evidence of extraperitoneal disease progression prior to enrollment. * Patients with stable oligometastatic liver only metastasis (≤ 3 lesions, each \< 3 cm in m…