Inclusion Criteria:

          -  Histologic proof of primary head of pancreas invasive adenocarcinoma managed with a
             potentially curative resection (i.e., removal of all gross tumor) involving a classic
             pancreaticoduodenectomy (Whipple) or a pylorus preserving pancreaticoduodenectomy;
             patients with invasive adenocarcinoma that also contains a component of intraductal
             papillary mucinous neoplasm (IPMN) are eligible

               -  The operating surgeon must document in the operative note that a complete gross
                  excision of the primary tumor was achieved; the pathology report must include
                  documentation of the margin status and the size of the tumor; the pathology
                  report must also include the status of the three major margins-bile duct,
                  pancreatic parenchyma, and retroperitoneal (uncinate)

          -  For patients who have not started their chemotherapy prior to registration, the
             interval between definitive tumor-related surgery and 1st step registration must be
             between 21-70 days; for patients entering on the study who have already received up to
             3 months of adjuvant chemotherapy as per the treating institution, the interval
             between definitive tumor-related surgery and day one of adjuvant chemotherapy must be
             between 21-77 days

          -  Patients will be staged according to the 6th edition American Joint Committee on
             Cancer (AJCC) staging system with pathologic stage T1-3, N0-1, M-0 being eligible

          -  Zubrod performance status 0 or 1

          -  Complete history and physical examination including weight and Zubrod status within 31
             days of study entry (or within 31 days prior to day 1 of chemotherapy post-surgery for
             those patients having started chemotherapy prior to first step registration)

          -  Before starting therapy the patient should be able to maintain adequate oral nutrition
             of >= 1500 calories estimated caloric intake per day and be free of significant nausea
             and vomiting

          -  Complete blood count (CBC)/differential obtained within 21 days of registration on
             study (or within 21 days prior to day 1 of chemotherapy post-surgery for those
             patients having started chemotherapy prior to first step registration)

          -  Absolute neutrophil count (ANC) >= 1,500/mm^3

          -  Platelets >= 100,000/mm^3

          -  Hemoglobin (Hgb) >= 8.0 g/dL (transfusion or other intervention to achieve Hgb >= 8.0
             g/dl is acceptable)

          -  Post resection serum cancer antigen (CA)19-9 =< 180 units/mL AND prior to any systemic
             treatment

          -  Serum total bilirubin =< twice the institutional upper limit of normal (ULN) within 21
             days of registration on study (or within 21 days prior to day 1 of chemotherapy
             post-surgery for those patients having started chemotherapy prior to first step
             registration)

          -  Creatinine levels =< twice the institutional upper limit of normal within 21 days of
             registration on study (or within 21 days prior to day 1 of chemotherapy post-surgery
             for those patients having started chemotherapy prior to first step registration)

          -  Serum glutamic oxaloacetic transaminase (SGOT) must be =< 2.5 x institutional ULN
             within 21 days of registration on study (or within 21 days prior to day 1 of
             chemotherapy post-surgery for those patients having started chemotherapy prior to
             first step registration)

          -  Negative serum pregnancy test for women of childbearing potential within 14 days of
             study registration

          -  Abdominal/pelvic computed tomography (CT) scan with contrast is preferred; abdominal
             CT alone is acceptable only if insurance restrictions are experienced; chest CT/x-ray
             (CT of chest preferred) within 31 days of registration on study (or within 31 days
             prior to day 1 of chemo post-surgery for those patients having started chemotherapy
             prior to first step registration); patients allergic to intravenous (IV) contrast can
             have magnetic resonance imaging (MRI) of the abdomen/pelvis instead

          -  Signed study-specific informed consent

          -  Consultation, agreement, and documentation in the patient's chart by a radiation
             oncologist that patient is suitable to receive radiotherapy per this protocol

          -  Women of childbearing potential and male participants must practice adequate
             contraception

          -  Patients with active human immunodeficiency virus (HIV) infection are eligible if
             their cluster of differentiation (CD)4 count is > 499/cu mm and their viral load is <
             50 copies/ml; use of highly active antiretroviral treatment (HAART) is allowed

        Exclusion Criteria:

          -  Patients with non-adenocarcinomas, adenosquamous carcinomas, islet cell
             (neuroendocrine) tumors, cystadenomas, cystadenocarcinomas, carcinoid tumors, duodenal
             carcinomas, distal bile duct, and ampullary carcinomas; patients with tumors that are
             largely IPMN with a minimal or minor component of invasive carcinoma are not eligible;
             patients with acinar carcinomas are not eligible; patients with IPMN's that contain
             some secondary (minor) foci of adenocarcinoma are also not eligible

          -  Patients managed with a total pancreatectomy, a distal pancreatectomy, or central
             pancreatectomy

          -  Patients entering on the study after pancreaticoduodenectomy, who have not already
             started chemotherapy must not have had prior systemic chemotherapy for pancreas
             cancer; note that prior chemotherapy for a different cancer is allowable; for patients
             entering on the study who have already received up to 3 months of adjuvant
             chemotherapy as per the treating institution, patients must not have received adjuvant
             chemotherapy with agents other than gemcitabine, nab-paclitaxel, oxaliplatin,
             fluoropyrimidine, or irinotecan for the current pancreatic cancer; prior chemotherapy
             for a different cancer is allowable

          -  Prior radiotherapy to the region of the study cancer that would result in overlap of
             radiation therapy fields

          -  Previous history of invasive malignancy (except non-melanoma skin cancer) unless the
             patient has been disease free for at least 2 years prior to study entry (or first day
             of chemotherapy for patients having started chemotherapy prior to first step
             registration); patients with a previous history of carcinoma in situ are eligible

          -  Severe, active co-morbidity, defined as follows per time points indicated below (or
             per time points indicated below prior to the first day of chemotherapy for patients
             having started chemotherapy prior to first step registration):

               -  Unstable angina and/or congestive heart failure requiring hospitalization within
                  the last 6 months

               -  Transmural myocardial infarction within the 3 months of study registration

               -  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 at the time of registration

          -  Pregnant or lactating women

          -  Women of childbearing potential and men who are sexually active and not willing/able
             to use medically acceptable forms of contraception

          -  If surgical margin status cannot be determined after consultation with the operating
             surgeon and the institutional pathologist, the patient will be ineligible