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.

Inclusion Criteria:

• Is the patient 18 years of age or older.
• Does the patient have histologically or cytologically proven carcinoma of the pancreas
• Is the tumor unresectable or medically inoperable
• Does the patient have a Zubrod performance status of ≤ 2 (appendix I).
• Does the patient have an absolute neutrophil count of ≥ 1500/mm3, and platelets ≥ 100,000/mm3
• Does the patient have adequate renal function (creatinine < 2 mg/dl) and hepatic function (bilirubin < 3 mg/dl), with relief of biliary obstruction if present
• Is the patient free of significant co-morbid conditions that would preclude safe administration or completion of protocol therapy
• If the patient is of reproductive potential, has he or she agreed to use an effective method of contraception during treatment on this trial and for 6 months after treatment
• Is the patient aware of the investigational nature of the therapy such that they can provide written informed consent

Exclusion Criteria:

• Does the patient have a neuroendocrine tumor of the pancreas
• Does the patient have metastatic disease
• Does the patient have a history of abdominal radiation therapy
• Is there history of more than 1 month of therapy with single agent gemcitabine
• Has the patient used any investigational agent in the month before enrollment into the study