This is a research study for pancreatic cancer. One way of improving the results of current standard treatments is to try new approaches. This study will examine the use of a more advanced radiation therapy technique, called intensity modulated radiation therapy (IMRT), with chemotherapy. All subjects on this study will be treated with gemcitabine. This chemotherapy agent has been used for patients with pancreatic cancer. The researchers have already done studies using radiation therapy and gemcitabine. They want to build on the information they have from this previous research. The researchers want to find the best dose of IMRT that can be given at the same time that patients are receiving gemcitabine. To do this, they will vary the total dose of radiation received by patients on this study based on the information they have available from previously treated patients. The goal of the research is to identify the highest dose of IMRT that can be given at the same time as the chemotherapy without causing severe side effects.