A Study of Salvage Radiotherapy With or Without Enzalutamide in Recurrent Prostate Cancer Following Surgery

Inclusion Criteria: * Pathologically (histologically) proven adenocarcinoma confirmed by prostatectomy performed within 10 years prior to registration and any type of radical prostatectomy is permitted, including retropubic, perineal, laparoscopic or robotically assisted. * Prostate-specific antigen (PSA) level (≥ 0.2 ng/mL) within 120 days prior to registration. Patients must have a PSA ≥ 0.2 ng/mL prior to starting ADT. For patients being followed by an ultrasensitive PSA assay, a serum PSA concentration of ≥ 0.10 ng/mL will be considered eligible. * GnRH analog may be started no more than 42 days prior study entry. * Hemoglobin ≥ 9.0 g/dL, independent of transfusion and/or growth factors within 90 days prior to registration. * Platelet count ≥ 75,000 x 10\^9/µL independent of transfusion and/or growth factors within 90 days prior to registration. * At least 1 of the following aggressive features: * Gleason score of 8-10 (note any Gleason score is eligible) * Seminal vesicle invasion (SVI) (note any pT stage American Joint Committee on Cancer (AJCC) v8.0 is eligible but a pT stage ≥ pT3b is considered aggressive) * Locoregional node involvement at radical prostatectomy (RP) (pN1) * Persistently elevated PSA post-RP nadir (PEPP) defined as PSA \> 0.1 ng/mL after radical prostatectomy * PSA ≥ 0.7 ng/mL * Serum albumin ≥ 3.0 g/dL within 90 days prior to registration * Glomerular filtration rate (GFR) ≥35 mL/min estimated by Cockcroft-Gault or measured directly…