Regular radiation, regardless of the source, be it a linear accelerator, an implanted radioactive source or one of the new computer guided robotic machines typically deliver x-rays. X-rays, when pointed at the human body, will almost immediately begin to deposit energy. As they move deeper they give up more and more of their energy hitting a peak then gradually tailing off until they exit the opposite side of the body. Radiation may damage healthy tissue throughout this long path. Protons are an entirely different form of radiation from x-rays and they have a very different effect within the human body. Protons pass through the initial layers of tissue depositing little of their energy. But, at a programmed depth protons will very quickly give up all of their energy and stop. There is no exit dose and much less normal tissue is affected. For almost any cancer in any part of the body, there will be a tissue-sparing advantage with protons compared to x-rays.
Learn More about how proton therapy works.