Inclusion Criteria:  Biopsy-proven NCCN low- or intermediate-risk prostate cancer, which includes patients with: Gleason score ≤7 disease PSA ≤20 ng/ml  Clinical T1-T2c disease  Note: Patients who only have radiographic evidence of possible or probable T3 disease (i.e., extracapsular extension or seminal vesical invasion) will not be excluded. KPS ≥ 80% Prostate size as determined on MRI to be < 100 cc. Prostate size can be determined on CT scan if MRI is not available. Male 18 years of age or older IPSS ≤ 20 Patient must be a candidate for and agree to placement of intraprostatic fiducial markers and a hydrogel rectal spacer Patient must be available for at least 2 years of follow-up  Exclusion Criteria:  Prior prostate surgery (including cryosurgery)  Note: Patients who underwent TURP or greenlight PVP are eligible if it was > 12 weeks prior to the anticipated start date of SBRT Prior history of chronic prostatitis or urethral stricture Currently active cancer(s) other than non-melanoma skin cancers. Patients are not considered to have currently active cancers if they have completed therapy and are considered by their physicians to be at <5% risk of relapse within 2 years. Life expectancy of < 2 years