High or Standard Intensity Radiation Therapy After Gemcitabine Hydrochloride and Nab-paclitaxel in Treating Patients With Pancreatic Cancer That Cannot Be Removed by Surgery

PRIMARY OBJECTIVES: * I. To determine if intensified radiochemotherapy following gemcitabine and nab-paclitaxel in patients with unresectable pancreatic cancer will show a signal for improved 2-year overall survival (OS) from 10% to 22.5% as compared to chemotherapy with gemcitabine and nab-paclitaxel alone. * II. To determine if standard radiochemotherapy, following gemcitabine and nab-paclitaxel, in patients with unresectable pancreatic cancer will show a signal for improved 2-year OS from 10% to 22.5% as compared to chemotherapy with gemcitabine and nab-paclitaxel alone. SECONDARY OBJECTIVES: * I. To evaluate patterns of failure (local and systemic progression) by SMAD family member 4 (SMAD4) status and intensity of radiation therapy. * II. To evaluate the impact of radiochemotherapy on OS for the subset of SMAD4 intact patients. * III. To evaluate adverse events associated with the treatments. * IV. To evaluate correlation between SMAD4 status determined by immunohistochemistry (IHC) and genetic SMAD4 status. Patients are randomized to 1 of 3 treatment arms. After completion of study treatment, patients are followed up at 1 month and then every 3 months.