Phase II Gemcitabine + Fractionated Stereotactic Radiotherapy for Unresectable Pancreatic Adenocarcinoma

Inclusion Criteria: 3.1.1 Histologically confirmed adenocarcinoma of the pancreas. 3.1.2 Unresectable disease as determined by a pancreatic cancer surgeon and assessment at a GI oncology tumor board (JHU - Johns Hopkins University, SU - Stanford University, or MSKCC - Memorial Sloan Kettering Cancer Center). 3.1.3 Up to 3 weeks of gemcitabine chemotherapy is allowed prior to SBRT. 3.1.4 Pancreatic tumors must be less than 7.5 cm in greatest axial dimension (or \<1000 cc in volume) at the time of treatment planning. 3.1.5 No prior upper abdominal or liver radiation therapy. 3.1.6 No chemotherapy within 2 weeks of radiotherapy, or chemotherapy within parameters set by Investigator for each institution. 3.1.7 Age \>=18 years. 3.1.8 No infections requiring systemic antibiotic treatment. 3.1.9 Karnofsky \>= 70% (see Appendix III). 3.1.10 Patients must have acceptable organ and marrow function as defined below (within 1 month prior to radiotherapy): * leukocytes: \>=3,000/microliter (uL) * absolute neutrophil count: \>=1,500uL * platelets: \>=100,000/uL * total bilirubin: within 1.5 times (1.5X) normal institutional limits * AST (aspartate aminotransferase)(SGOT -Serum glutamic oxaloacetic transaminase)/ALT(alanine aminotransferase)(SGPT-serum glutamic-pyruvic transaminase): \<=2.5 X institutional upper limit of normal * creatinine: within normal institutional limits OR \- creatinine clearance: \>=60 mL/min/1.73 m\^2 for patients with creatinine levels above institutional normal 3.1.11 The effects of radiation on the developing human fetus at recommended therapeutic doses can result in death of the fetus. If a woman is of child-bearing potential, a negative urine or serum pregnancy test must be demonstrated prior to treatment. Women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) for the duration of study participation and for up to 4 weeks following the study. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. 3.1.12 Ability to understand and the willingness to sign a written informed consent document. 3.1.13 Life expectancy \> 6 months Exclusion Criteria: 3.2.1 Patients who have had prior radiotherapy to the upper abdomen. 3.2.2 Patients receiving more than 1 cycle of gemcitabine chemotherapy or other therapy prior to SBRT. 3.2.3 Children are excluded because pancreatic tumors rarely occur in this age group. Furthermore, treatment requires a great deal of patient cooperation including the ability to lie still for several hours in an isolated room. 3.2.4 No laboratory personnel will be included. 3.2.5 Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements. 3.2.6 Any concurrent malignancy other than non-melanoma skin cancer, non-invasive bladder cancer, or carcinoma in situ of the cervix. Patients with a previous malignancy without evidence of disease for \> 5 years will be allowed to enter the trial. 3.2.7 Pregnant and breastfeeding women are excluded. Women of child-bearing potential who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy for the entire study period and for up to 4 weeks after the study are excluded. This applies to any woman who has experienced menarche and who has not undergone successful surgical sterilization or is not postmenopausal (defined as amenorrhea for at least 12 consecutive months, or women on hormone replacement therapy with serum FSH (follicle stimulating hormone) levels greater than 35 IU/mL (international units/milliliter). A negative urine or serum pregnancy test must be obtained within 72 hours prior to the start of study medication in all women of childbearing potential. Male subjects must also agree to use effective contraception for the same period as above.