Pembro With Radiation With or Without Olaparib

Inclusion 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.

Exclusion Criteria:

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