Protons and their Properties
The proton range depends on the capture cross-section and the material with which the protons interact as they are slowed down. Photons deposit radiation dose as they penetrate through the body. Thus, healthy tissues in the path of the beam are also irradiated. In the case of proton beams, the protons decelerate continuously between the surface of the body and the point at which the beam is stopped. Therefore, tissues in the beam path absorb only a small dose relative to photon beams. At the end of the range, the protons are stopped and transfer a large amount of energy to the tissue. This gives rise to a peak in the dose that is termed the Bragg Peak. Beyond this depth the dose deposition falls to zero within millimetres.
In this way, the proton beams can be generated to deposit the highest radiation dose in the tumour, in a small volume or a spot at the end of the beam range. Thus, they irradiate less healthy tissues in the path of the beam than in treatments with photon beams.
In the figure on the right-hand side the radiation path is demonstrated for a single narrow “pencil beam” of protons. The bottom part of the figure also shows that tissues beyond the target volume are irradiated in the photon beam, while in the proton beam case no dose is deposited beyond the target volume.