Pembro With Radiation With or Without Olaparib

Eligibility Criteria

• * Male participants with histologically confirmed adenocarcinoma of the prostate
• * High-risk / very high-risk status per NCCN guidelines
• * ECOG performance status 0 to 1
• * No pelvic nodes \>2 cm in long axis as established by CT imaging
• * Agree to use contraception during the treatment period and for at least 120 days after the last dose of study intervention and refrain from donating sperm during this period.
• * Ability to understand and the willingness to sign a written informed consent document.
• * Adequate organ and marrow function
• * Prior pharmacologic androgen ablation for prostate cancer is allowed only if the onset of androgen ablation is ≤90 days prior to the date of registration
• * Prior 5-alpha reductase inhibitor (for example, finasteride) for prostatic hypertrophy is allowed if discontinued at least 60 days prior to registration.
• * 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.
• * PSA \> 150ng/ml
• * Prior hormonal therapy with LHRH agonists (e.g., Lupron) and LHRH antagonists (e.g., Degarelix)for prostate cancer continuously for more than 90-days prior to study enrollment.
• * Prior radiation to the prostate. Previous pelvic RT or major surgery (colorectal anastomosis, total cystectomy, radical prostatectomy, TURP, etc.). History of Ulcerative proctitis.
• * Concurrent active, additional malignancy in the last 2 years.
• * Prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to randomization.
• * Patients with distant metastases