Gemcitabine, Ascorbate, Radiation Therapy for Pancreatic Cancer, Phase I

This phase 1 study will test the safety of adding high dose ascorbate (vitamin C) to standard chemoradiation. The ascorbate is infused during external beam radiation therapy treatment. For patients eligible for this trial, standard treatment for their cancer includes radiation therapy combined with weekly gemcitabine (a chemotherapy). Participants will: * receive high doses of intravenous (IV) ascorbate during their daily radiation therapy treatments. Radiation treatments are given once a day, Monday through Friday. * have routine doctor's visits and be asked about any side effects they are experiencing. This is a phase 1 study that will evaluate the side effects of adding ascorbate to standard therapy. The dose given to a participant will be determined by how well other participants have tolerated ascorbate.