RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage tumor cells. Combining radiation therapy with chemotherapy may kill more tumor cells.

PURPOSE: Phase I trial to study the effectiveness of radiation therapy plus chemotherapy in treating patients who have cancer of the pancreas or stomach.

OBJECTIVES: I. Determine the maximum tolerated dose of gemcitabine and cisplatin that can be administered during a standard course of radiation therapy for patients with unresectable or locally recurrent pancreatic and gastric cancer. II. Describe the tolerance of gemcitabine, cisplatin, and radiation therapy in this patient population. III. Seek preliminary evidence of the therapeutic activity of this regimen in this patient population as measured by survival.

OUTLINE: This is a dose escalation study. Patients undergo radiotherapy to the tumor and lymph nodes, followed by a decrease in radiotherapy to the tumor alone. Radiation therapy is administered for a total of 5.5 weeks. Patients receive intravenous gemcitabine twice weekly on Tuesday and Friday over the first 3 weeks of radiotherapy. Cisplatin is administered intravenously twice weekly following gemcitabine therapy. Three patients are treated at each dose level. Dose escalation does not occur until all patients at a given dose level have completed radiotherapy and returned for a 4 week follow up. The dose limiting toxicity (DLT) is defined as the dose at which at least 2 of 6 patients experience unacceptable toxic effects. The maximum tolerated dose is defined as the dose at which no more than 1 of 6 patients experiences DLT. Patients exhibiting stable disease remain on therapy until disease progression or intolerable toxic effects. Patients experiencing toxic effects and no disease progression are retreated at a lower dose. Patients are followed every 3 months for the first 2 years then every 6 months for the next year.

DISEASE CHARACTERISTICS: Histologically proven unresectable (including incomplete resections) or locally recurrent pancreatic or gastric cancer No evidence of metastases outside of the planned radiation field No cystadenocarcinoma of the pancreas or pancreatic tumors of neuroendocrine origin

PATIENT CHARACTERISTICS: Age: 18 and over Performance Status: ECOG 0-1 Life Expectancy: Not specified Hematopoietic: Absolute neutrophil count at least 2,000/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin within normal limits Alkaline phosphatase no greater than 3.0 times upper limit of normal (ULN) AST no greater than 3.0 times ULN Renal: Creatinine no greater than 1.3 times ULN Other: Not pregnant or nursing Fertile patients must use effective contraception No significant infection or medical illness No significant nausea or vomiting At least 1200 calories per day of oral nutrition

PRIOR CONCURRENT THERAPY: Biologic therapy: No prior or concurrent biologic therapy Chemotherapy: No prior or concurrent chemotherapy Endocrine therapy: Not specified Radiotherapy: No prior radiotherapy (except focal fields for skin cancer that do not overlap with planned radiotherapy fields) Surgery: At least 21 days since laparotomy surgery