Inclusion Criteria

          -  Age >18 years.

          -  Karnofsky Performance Status >70%

          -  Histologically confirmed adenocarcinoma of the pancreas or ampulla of Vater; at least
             the majority of the histopathologic specimen must be identified as adenocarcinoma as
             opposed to another histologic subtype. In patients with a diagnosis of recurrent
             disease (based on radiographic progression and/or rising CA19-9 levels) and a history
             of a biopsy-proven adenocarcinoma of the pancreas or the ampulla of Vater, repeat
             biopsy of the recurrence site is not required for participation of the trial.

          -  Pancreatic or periampullary tumors must be less than 8.0 cm in greatest axial
             dimension at the time of treatment planning.

          -  Patients who have been treated with any combination of surgical resection and
             neoadjuvant/adjuvant conventional chemoradiation therapy for resectable disease or
             conventional chemoradiation as definitive treatment for unresectable or borderline
             resectable disease are eligible for the study, provided that at least 180 days have
             elapsed since completing any previous radiation treatment. Patients who have been
             receiving continued chemotherapy following their initial radiation treatment are
             eligible regardless of when the most recent chemotherapy was received. Those patients
             who have received prior radiation therapy will constitute Cohort A and will receive
             stereotactic body radiation (SBRT) as 5 gray (Gy) x 5.

          -  Patients who have not previously undergone radiation therapy can have a history of
             treatment with either chemotherapy (for unresectable/borderline resectable disease) or
             any combination of surgery and chemotherapy (for resectable disease). Patients with no
             history of prior radiation treatment will constitute Cohort B and will receive SBRT as
             6.6 Gy x 5. Please note that patients must have received at least two cycles of
             chemotherapy (with selection of drugs at the discretion of the treating oncologist)
             before SBRT treatment on protocol.

          -  Acceptable organ and marrow function as defined below (within 2 weeks prior to
             radiotherapy):

               -  Leukocytes >3,000/μL

               -  Absolute neutrophil count >1,500μL

               -  Platelets >100,000/μL

               -  Total Bilirubin ≤1.5x institutional upper limit of normal

               -  Aspartate transaminase (AST(SGOT))/Alanine transaminase (ALT(SGPT)) <2.5x
                  institutional upper limit of normal

               -  Creatinine ≤ institutional upper limit of normal OR creatinine clearance >60
                  mL/min/1.73 m2 for patients with creatinine levels above institutional normal

          -  Ability to understand and the willingness to sign a written informed consent document.

          -  Life expectancy > 3 months.

          -  Radio-opaque markers must be present within the tumor bed. In patients who have
             undergone surgical resection, radio-opaque surgical clips within the tumor bed can be
             used as fiducials. Patients without surgical clips in the tumor bed must be able to
             have fiducials placed endoscopically, laparoscopically, or through a CT- or
             ultrasound-guided technique. If not, the tumor must be posterior and adjacent to the
             aorta, and treatment will only be permitted at the discretion of the Principal
             Investigator.

        Exclusion Criteria

          -  Age < 18 years.

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection (or infections requiring systemic antibiotic treatment), active upper GI
             ulceration or hemorrhage, symptomatic congestive heart failure, unstable angina
             pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would
             limit compliance with study requirements.

          -  Any concurrent malignancy other than non-melanoma skin cancer, non-invasive bladder
             cancer, early stage prostate cancer, or carcinoma in situ of the cervix. Patients with
             a previous non-pancreatic, non-periampullary malignancy without evidence of disease
             for > 5 years will be allowed to enter the trial.

          -  Pregnant and breastfeeding women are excluded as are women of child-bearing potential
             who are unwilling or unable to use an acceptable method of birth control (hormonal or
             barrier method of birth control; abstinence) to avoid pregnancy for the duration of
             the study. Should a woman become pregnant or suspect she is pregnant while
             participating in this study, she should inform her treating physician immediately.

          -  Women who are not post-menopausal (as defined in Appendix III) and have a positive
             urine or serum pregnancy test or refuse to take a pregnancy test.

          -  Patients with a life expectancy of < 3 months.

          -  Patients with metastatic disease.

          -  Patients with evidence of gross tumor invasion into the lumen of the stomach or small
             bowel are not eligible; if imaging suggests luminal invasion of tumor, this must be
             ruled out endoscopically before the patient can be enrolled on study.