This research study is for people who have pancreas cancer for which surgery is not recommended. Potential patients must have already received several months of chemotherapy before they are eligible for this study and there will not have been any detectable spread of their tumor on imaging studies following this chemotherapy course.

In this study the investigators want to find out more about the efficacy of giving higher doses of radiation with concurrent chemotherapy in controlling unresectable pancreas cancers than are used in either the pre-operative or post-operative setting. The investigators will assess acute and late side effects (problems and symptoms) of radiation therapy given at these higher doses of radiation (dose escalated) following full dose chemotherapy given before the radiation and with concurrent chemotherapy for pancreas cancer. Radiation therapy is given in higher doses that are limited by the proximity of normal organs to the radiation dose distribution to improve the likelihood of controlling the tumor in the pancreas while minimizing the risk of radiation injury to these organs. There are two chemotherapy drugs, Capecitabine is an oral drug taken twice per day on the same day that radiation therapy is given and Gemcitabine is an intravenous drug given once per week, during radiation therapy. Everyone in this study will have already received chemotherapy alone first. Everyone in this study will receive radiation therapy and concurrent chemotherapy.

Inclusion Criteria:

• Pathologically confirmed (histologic or cytologic), locally advanced, adenocarcinoma of the pancreas; patients must have unresectable disease based on institutional standardized criteria of unresectability or medical inoperability.
• Participants with and without regional adenopathy are eligible.
• No distant metastases, based upon the following minimum diagnostic workup:
• History/physical examination, including collection of weight and vital signs, within 28 days prior to study entry;
• Abdominal/pelvic CT scan with IV contrast within 21 days prior to study entry;
• Chest CT scan, or X-ray within 21 days prior to study entry.
• Abdominal/pelvic MR prior to radiation with perfusion and diffusion- weighted sequences and MR and CT sim for radiation planning Pet scan within 21 days prior to study entry, Functional renal study.
• Zubrod performance status 0-1 within 1 week of study entry.
• Age ≥ 18.
• Hematology and cancer antigen (CA) 19-9 / carcinoembryonic antigen (CEA) within 14 days prior to study entry, as follows.
• Absolute neutrophil count (ANC) ≥ 1,500 cells/mm^3;
• Platelets ≥ 100,000 cells/mm^3;
• Hemoglobin ≥ 8.0 g/dl (Note: The use of transfusion or other intervention to achieve Hgb ≥ 8.0 g/dl is acceptable.);
• Serum creatinine ≤ 1.5 mg/dl;
• Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) < 3 x upper limit of normal (ULN);
• Total bilirubin < 3.0 mg/dL;
• Alkaline phosphatase < 3 x ULN;
• Fasting blood glucose < 160 mg/dl.
• Negative serum pregnancy test (if applicable) within 14 days prior to study entry.
• Ability to swallow oral medications.
• Participants must have had at least 4 months of prior systemic chemotherapy.
• Participants must provide study specific informed consent prior to study entry.
• Women of childbearing potential and male participants who are sexually active must practice adequate contraception.

Exclusion criteria:

• Distant metastatic disease, second malignancy or peritoneal seeding;
• Prior invasive malignancy (except non-melanomatous skin cancer) unless disease free for a minimum of 3 years (For example, carcinoma in situ of the breast, oral cavity, or cervix are all permissible).
• Prior radiotherapy to the region of the study cancer that would result in overlap of radiation therapy fields.
• Any major surgery within 28 days prior to study entry
• Severe, active co-morbidity, defined as follows:
• Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months;
• Transmural myocardial infarction within 3 months prior to study entry;
• Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration;
• Chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy within 30 days before registration;
• Uncontrolled malabsorption syndrome significantly affecting gastrointestinal function;
• Any unresolved bowel or bile duct obstruction;
• Major resection of the stomach or small bowel that could affect the absorption of capecitabine
• Acquired immune deficiency syndrome (AIDS) based upon current Center for Disease Control (CDC) definition;
• Pregnancy or women of childbearing potential and men who are sexually active and not willing/able to use medically acceptable forms of contraception during the course of the study and for women, for 3 months after the last study drug administration.
• Women who are lactating at the time of registration and who plan to be lactating through 3 months after the last study drug administration.
• Prior allergic reaction to capecitabine or gemcitabine
• Inability to undergo an MR of the abdomen/pelvis
• Participation in another clinical treatment trial while on study.

Up to two years and seven weeks.