Inclusion Criteria:  Pathologically-proven diagnosis of prostate adenocarcinoma  Clinical stage T3-4 or Gleason score 8-10 or PSA >20 ng/ml  °If radiographic T3-T4 is the only high-risk factor, it must be "consistent with" or >90% probability of having T3-T4 disease determined by the reading radiologist.  Decipher genomic score ≤0.6 Willing and able to provide written informed consent and Authorization for Use and Release of Health and Research Study Information (HIPAA authorization) Age ≥18 KPS ≥70 or ECOG 0-2 Estimated life expectancy >5 years Baseline prostate volume ≤75 cc Baseline IPSS ≤20 No contraindications to ADT, brachytherapy, or pelvic external beam radiation therapy as determined by the treating radiation oncologist per standard practice Patients who have already started ADT consisting of bicalutamide 50 mg PO daily with leuprolide or an equivalent GnRH analogue are eligible if given for ≤ 60 days prior to registration  Exclusion Criteria:  Regional lymph node or metastatic disease Prior pelvic radiation Prior prostate surgery (including TURP or cryosurgery) Prior history of inflammatory bowel disease Unable to undergo anesthesia or brachytherapy Active second malignancy or past history of malignancies diagnosed within the last 2 years that requires active therapy and/or in remission, with the exception of resected non-melanoma skin cancers, non-muscle invasive bladder cancer, stage I head and neck cancer, or stage I colorectal cancer