Proton therapy is a type of external beam radiotherapy using ionizing radiation. During treatment, a particle accelerator is used to target the tumor with a beam of protons. These charged particles damage the DNA of cells, ultimately causing their death or interfering with their ability to reproduce. Cancerous cells, because of their high rate of division and their reduced ability to repair damaged DNA, are particularly vulnerable to attack on their DNA.
Due to their relatively large mass, protons have little lateral side scatter in the tissue; the beam does not broaden much, stays focused on the tumor shape and delivers small dose side-effects to surrounding tissue. All protons of a given energy have a certain range; very few protons penetrate beyond that distance. Furthermore, the dose delivered to tissue is maximum just over the last few millimeters of the particle.