Inclusion Criteria:

          -  Pathologically (histologically) proven diagnosis of prostate adenocarcinoma. Any type
             of radical prostatectomy is permitted, including retropubic, perineal, laparoscopic or
             robotically assisted

          -  Any T-stage is eligible

          -  Appropriate stage for study entry based on fluciclovine F-18 positron emission
             tomography (PET) scan (FACBC, Axumin) within 90 days prior to registration that is
             negative for distant metastatic (M1a, M1b, M1c) disease; (Note that though every
             effort should be made to obtain a fluciclovine F-18 PET [FACBC, Axumin] scan, if the
             patient has already had a recent gallium Ga 68-labeled PSMA-11 [Ga-68 PSMA] PET scan
             or C-11 or F-18 choline PET scan within 90 days prior to registration [to include scan
             report] then repeat molecular imaging with a fluciclovine F-18 PET [FACBC, Axumin]
             scan will not be required.)

          -  Pathologically node positive disease with nodal involvement only in the pelvis in the
             prostatectomy specimen (including external iliacs, internal iliacs, and/or obturator
             nodes)

          -  History/physical examination within 90 days prior to registration

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 within 90 days
             prior to registration

          -  Detectable PSA after radical prostatectomy. Detectable PSA is defined as serum PSA > 0
             ng/mL at least 30 days after prostatectomy and within 90 days of registration and
             before start of GnRH agonist/antagonist

          -  Patients who have already started on post-prostatectomy GnRH agonist/antagonist for =<
             45 days prior to registration are eligible (Note: patients who started on an oral
             antiandrogen are eligible if started =< 45 days and stopped prior to registration)

          -  Hemoglobin >= 9.0 g/dL, independent of transfusion and/or growth factors (within 90
             days prior to registration)

          -  Platelet count >= 100,000 x 10^9/uL independent of transfusion and/or growth factors
             (within 90 days prior to registration)

          -  Serum potassium >= 3.5 mmol/L within 90 days prior to registration

          -  Creatinine clearance (CrCl) >= 30 mL/min estimated by Cockcroft-Gault (please use
             actual weight for calculation unless greater than 30% above ideal body weight then use
             the adjusted body weight) (within 90 days prior to registration)

          -  Total bilirubin =< 1.5 x institutional upper limit of normal (ULN) (Note: In subjects
             with Gilbert?s syndrome, if total bilirubin is > 1.5 x ULN, measure direct and
             indirect bilirubin and if direct bilirubin is =< 1.5 x ULN, subject is eligible)
             (within 90 days prior to registration)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase [SGOT]) or
             alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT]) =< 2.5 x
             institutional ULN (within 90 days prior to registration)

          -  Serum albumin >= 3.0 g/dL (within 90 days prior to registration)

          -  Discontinue or substitute concomitant medications known to lower the seizure threshold
             at least 30 days prior to registration

          -  The patient must agree to use a condom (even men with vasectomies) and another
             effective method of birth control if he is having sex with a woman of childbearing
             potential or agree to use a condom if he is having sex with a woman who is pregnant
             while on study drug and for 3 months following the last dose of study drug

          -  Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral
             therapy (didanosine [DDI] is not permitted) with undetectable viral load within 6
             months are eligible for this trial and have a CD4 count >= 200 cells/microliter within
             30 days prior to registration. Note: HIV testing is not required for eligibility for
             this protocol

          -  For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral
             load must be undetectable on suppressive therapy within 30 days prior to registration,
             if indicated. Note: HBV viral testing is not required for eligibility for this
             protocol

          -  Patients with a history of hepatitis C virus (HCV) infection must have been treated
             and cured. For patients with HCV infection who are currently on treatment, they are
             eligible if they have an undetectable HCV viral load within 30 days prior to
             registration

          -  Patients with a prior or concurrent malignancy whose natural history or treatment does
             not have the potential to interfere with the safety or efficacy assessment of the
             investigational regimen are eligible for this trial. Note: Any patient with a cancer
             (other than keratinocyte carcinoma or carcinoma in situ) who has no evidence of
             disease for < 3 years must contact the principal investigator, Ron Chen, Doctor of
             Medicine (MD)

          -  The patient or a legally authorized representative must provide study-specific
             informed consent prior to study entry

        Exclusion Criteria:

          -  Definitive radiologic evidence of metastatic disease (M1a, M1b or M1c) on molecular
             imaging (e.g. fluciclovine F-18 PET, PSMA, F-18 choline 11)

          -  Prior systemic chemotherapy for the study cancer; note that prior chemotherapy for a
             different cancer is allowed (completed > 3 years prior to registration)

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

          -  Current use of 5-alpha reductase inhibitor. NOTE: if the alpha reductase inhibitor is
             stopped prior to randomization the patient is eligible

          -  Didanosine (DDI) antiretroviral therapy is not permitted

          -  History of any of the following:

               -  Seizure or known condition that may pre-dispose to seizure (e.g. prior stroke
                  within 1 year prior to registration, brain arteriovenous malformation,
                  Schwannoma, meningioma, or other benign central nervous system [CNS] or meningeal
                  disease which may require treatment with surgery or radiation therapy)

               -  Severe or unstable angina, myocardial infarction, arterial or venous
                  thromboembolic events (e.g., pulmonary embolism, cerebrovascular accident
                  including transient ischemic attacks), or clinically significant ventricular
                  arrhythmias within 6 months prior to registration

               -  New York Heart Association functional classification III/IV (Note: Patients with
                  known history or current symptoms of cardiac disease, or history of treatment
                  with cardiotoxic agents, should have a clinical risk assessment of cardiac
                  function using the New York Heart Association functional classification.)

               -  History of any condition that in the opinion of the investigator, would preclude
                  participation in this study

          -  Current evidence of any of the following:

               -  Known gastrointestinal disorder affecting absorption of oral medications

               -  Active uncontrolled infection

               -  Presence of uncontrolled hypertension (persistent systolic blood pressure [BP] >=
                  160 mmHg or diastolic BP >= 100 mmHg). Subjects with a history of hypertension
                  are allowed, provided that BP is controlled to within these limits by
                  anti-hypertensive treatment

               -  Any chronic medical condition requiring a higher dose of corticosteroid than 10
                  mg prednisone/prednisolone once daily

               -  Baseline moderate and severe hepatic impairment (Child-Pugh Class B & C)

               -  Inability to swallow oral pills

               -  Any current condition that in the opinion of the investigator, would preclude
                  participation in this study

          -  Patients must not plan to participate in any other therapeutic clinical trials while
             receiving treatment on this study