Erectile dysfunction (ED) as a result of prostate cancer radiation treatment can be a common late side effect affecting quality of life. Radiation dose to the penile bulb which lies at the apex of the prostate is thought to be directly related to the risk of erectile dysfunction. Recent prostate cancer trials have shown that spacers (hydrogel that expands and solidifies) significantly reduce dose to nearby organs. The spacer is stable during radiation treatment and subsequently absorbed by the body. In this preliminary feasibility study, we evaluate the dose to penile bulb based on treatment planning scans and impact on clinical erectile function as observed by patients using the Validated International Index for Erectile Function (IIEF) in8patients who will undergo curative radiation treatments for prostate cancer with hydrogel spacer injected in the perirectal region.