J Am Coll Radiol. 2015 Nov;12(11):1204-6. doi: 10.1016/j.jacr.2015.08.002. Epub 2015 Sep 26.
Langen K1, Mehta M2.
External radiation therapy is typically administered using photons. The depth dose profile for a photon beam builds up from a low surface dose to a maximum at a depth of a few centimeters and attenuates exponentially thereafter. Protons, on the other hand, deposit modest dose initially, a dose peak just before they stop, and no dose beyond their range. The range of protons depends on their initial energy and can be easily adjusted to treat tumors at various depths. Figure 1 compares dose versus depth for a photon and a 165-MeV proton beam.