A new kind of radiation treatment using proton beams is now available. Protons are heavier than x-rays and tend not to scatter as much. Also, unlike x-rays, protons slow down as they travel through the body. It is possible that proton beam therapy delivers less unwanted energy to tissues around the tumor than IMRT, but studies have found mixed results, and some have shown that IMRT better spares certain tissues than proton treatment. But protons are very sensitive to the different densities of tissues they pass through, more so than x-rays. If healthy cells are spared, patients being treated for prostate cancer will have fewer side effects, such as bowel, bladder, or erection problems, from radiation treatment. Proton beam therapy is also more sensitive than IMRT to daily variations in a patient’s anatomy and positioning and theoretically can have more uncertainty in the dose delivered to the prostate.