Gemcitabine With or Without Radiation Therapy in Treating Patients With Pancreatic Cancer

Inclusion Criteria: * Histologically or cytologically confirmed adenocarcinoma of the pancreas * Locally advanced or regional (encompassable within the same radiotherapy portals) * Adenosquamous cancers are allowed * Unresectable disease * Measurable and/or non-measurable disease as determined by computed tomography (CT) scan or magnetic resonance imaging (MRI), which must be performed within 4 weeks prior to randomization. * Age\>=18 * ECOG Performance status of 0-1 * Life expectancy \>= 12 weeks * Adequate bone marrow reserve,liver and renal function within 2 weeks of randomization: * Absolute granulocyte count at least 2,000/mm\^3 * Platelet count at least 100,000/mm\^3 * Bilirubin less than 3 mg/dL (unless secondary to biliary obstruction or cholangitis) * Serum glutamic-oxaloacetic (AST) less than 5 times upper limit of normal (ULN) * Albumin greater than 2.5 g/dL * Creatinine no greater than 1.5 times ULN * Fertile patients must use effective contraception * Willing and able to attend follow-up visits * Concurrent enrollment on protocol ECOG-E1Y03 allowed * More than 4 weeks since prior investigational agents Exclusion Criteria: * Candidate for surgical excision based on local extent of disease (e.g., T3, N1, M0) * Stage M1 disease * Small cell, mucinous cystadenocarcinoma, islet cell or papillary cystic histology * Pregnant or nursing * Active infection within within 4 weeks of randomization * Malignancy within the past 5 years except nonmelanoma s…