Inclusion Criteria:  Age > 18 y Histologically confirmed prostate adenocarcinoma Low risk: cT1c-T2a, Gleason score 6, PSA < 10ng/mL Favorable intermediate risk: 1 intermediate risk factor, Gleason 3+4 or less, < 50% positive biopsy cores) Unfavorable intermediate risk: > 1 intermediate risk factor, Gleason 4+3, > 50% positive biopsy cores) Limited high risk: cT3a with PSA < 40ng/mL or cT2a-c with a Gleason score > 7 and/or a PSA > 20ng/mL but < 40ng/mL World Health Organization performance score 0-2 Written informed consent  Intermediate risk factors: T2b-T2c, Gleason 7, PSA 10-20 ng/mL  Exclusion Criteria:  Transurethral resection (TUR) < 3months before SBRT International Prostate Symptom Score (IPSS) > 19 Prostate volume > 100cc on transrectal ultrasound (TRUS) Stage cT3b-T4 N1 disease (clinically or pathologically) M1 disease (clinically or pathologically) PSA > 40ng/mL inflammatory bowel disease immunosuppressive medications prior pelvic RT contra-indications for MRI