This study will determine which of the two following treatment regimens provides greater benefit to patients with peritoneal carcinomatosis (gastrointestinal cancer that has spread throughout the abdomen):

• Surgical removal of tumors plus heated chemotherapy during surgery, followed by one dose of chemotherapy 7 to 12 days after surgery, followed 3 weeks later by 4 months of chemotherapy; or
• Surgical removal of tumors followed by 4 months of chemotherapy, starting 1 month after surgery.

Patients 18 years of age and older with peritoneal carcinomatosis may be eligible for this study. Candidates are screened with a medical history, physical examination, and blood tests; chest X-ray and CT scans; review of pathology slides; EKG, bone scan, and brain MRI, as needed; and laparotomy. Laparatomy is a surgical procedure in which an incision is made in the abdomen to look at the amount and location of tumors in the abdominal cavity. Patients whose surgery reveals that the tumors cannot be removed are taken off study. Those eligible for the study have their tumors removed during this screening laparotomy as part of the study procedure, as follows:

All participants undergo laparotomy and removal of as much tumor as possible, as described above. Patients are then randomly assigned to one of two treatment groups:

• Group 1 - During laparotomy, after tumor removal, patients receive continuous hyperthermic peritoneal perfusion (CHPP) with the anti-cancer drug cisplatin. For CHPP, the cisplatin solution is heated and delivered to the abdomen through a catheter (plastic tube), washed through the abdomen for 90 minutes, and then drained out of the body through another catheter. At the close of surgery, a small catheter, called a Tenckhoff catheter, is left in the abdomen and brought out through the skin. Between days 7 and 12 after surgery, one dose of fluorouracil and paclitaxel chemotherapy is delivered through this catheter. The catheter is removed following complete recovery from surgery and the patient is discharged from the hospital. Four to 6 weeks after the surgery, patients have CT scans of the chest, pelvis, and abdomen, and then begin intravenous (IV, through a vein) chemotherapy with oxaliplatin, 5-fluorouracil (5-FU), and leucovorin. The chemotherapy is given in 4-week courses as follows: oxaliplatin on day 1, infused over 2 hours through a vein in the arm or neck; leucovorin on days 1 and 2, infused over 2 hours, followed by 5-fluorouracil over 22 hours; leucovorin and 5-FU repeated two weeks later on days 15 and 16. This regimen is repeated two weeks later. Between each week of chemotherapy is a week break. A course of chemotherapy consists of 28 days (two weeks of chemotherapy and two 1-week breaks). Patients may receive up to four courses (total of 16 weeks) unless their disease progresses or they cannot tolerate further doses. Doses of the chemotherapy can be reduced if the side effects are too severe.
• Group 2 - Patients follow the same procedure as those in Group 1 for laparotomy, CT imaging, and IV chemotherapy with oxaliplatin, 5-fluorouracil, and leucovorin. They do not receive CHPP or chemotherapy into the abdomen.

All patients undergo repeat imaging tests six weeks after surgery and at the conclusion of the IV chemotherapy. They return for a physical examination and CT scans every three months for the first year, every four months for the next two years, and then every six months for up to five years after treatment. They are also asked to complete quality of life questionnaires before and after surgery, at the completion of chemotherapy, and at every follow-up visit.