Randomized Trial of Image -Guided Stereotactic Radiation Therapy (IG-SRT) in Prostate Cancer

This is a randomized, two arm study for patients with low-risk or favorable intermediate-risk prostate cancer as defined by 1.2016 NCCN criteria. Patients will be randomized to either rectal spacer placement or endorectal balloon placement, daily prior to each radiation treatment. 1. Endorectal balloon (ERB): Immobilization device manually placed into the rectum prior to radiation treatment planning CT and daily treatment delivery, to immobilize the prostate and reduce prostate motion. 2. Rectal spacer (RS): Biodegradable gel that is transperineally injected between the rectum and prostate under transrectal ultrasound guidance, to increase physical distance and thereby reduce radiation dose to the anterior rectal wall. The spacer begins to biodegrade in 2-3 months, and is fully absorbed within 6 months. This study plans to enroll a total of 40 patients with an accrual period of 4 years.