
For background information and definitions, this is a short www.cancer.gov paper &&url

Proton radiation is a form of external-beam radiation treatment. Radiation oncologists (physicians who specialize in radiation treatments) can treat in two major ways: by implanting radioactive materials in the body [brachytherapy] or by using machines to generate beams that penetrate the body from outside [external beam]. Proton radiation is delivered by the second method. Other forms of external-beam radiation therapy include x-ray therapy and Cobalt-60 gamma-ray therapy.  External beam radiation does not remain in the body. Your body does not become radioactive, and there is no risk of radiation exposure to others. 

Proton beam therapy is a radiation treatment that delivers high dose radiation to a very localized site. Protons, being particles instead of x-rays, slow down faster than photons. They deposit more energy as they slow down, culminating in a peak (called a Bragg peak). This allows the majority of radiation to be delivered to the target site with less scattering of radiation around and beyond to the adjacent normal tissues.  

Proton beams can be conformed [shaped in three dimensions] to fit the target area.  The beam can be carefully shaped to the dimensions of the tumor, and so deliver most of the radiation to the targeted tumor mass, not to the surrounding normal tissue. This tissue-sparing is unique to proton beam therapy.  

Proton beam radiotherapy contrasts with conventional Xray or Gamma ray radiation therapy [often called PHOTON beam] due to the unique properties of minimal scatter as the proton beams pass through the tissue, and deposit  the ionizing energy at a precise depth (the Bragg peak). Thus radiation exposure to surrounding normal tissues is minimized, permitting higher radiation doses to the target area and improved local control, with less damage and side effects.

Protons, which are positively charged subatomic particles, deposit energy differently than x-ray beams do. Compared to an x-ray beam, a proton beam that is delivered with sufficient energy (or "modulated") has a low "entrance dose" (the dose in front of the tumor), a high-dose "Bragg peak" region, which is designed to cover the entire tumor, and no "exit dose" beyond the tumor. In contrast, X-ray beams may deposit most of their dose in tissues in front of the tumor, and continue to sleet through the body after passing through the target area. 
         
As of November 1, 2002, more than 32,000 patients around the world had been treated with proton beam radiotherapy.  The effectiveness of proton beams for treating cancer show in almost every tumor site examined. The higher tumor doses and lower normal-tissue exposure have improved local control and reduced acute and late complications as compared with x-ray therapy. [from Loma Linda website information]
