A proton beam is made up of charged particles that have a well-defined range of penetration into tissues. How deep it can penetrate is decided by both the beam's energy and the density of the tissue through which it passes. As the proton beam penetrates the body, the particles slow down, and the beam deposits its dose sharply near the end of its range. This is a phenomenon known as the Bragg peak. By adjusting the Bragg peak, the doctor can deliver a full, localized, uniform dose of energy to the treatment site while sparing the surrounding normal tissues. The proton beam is ideal for treatments where organ preservation is very important, such as lung cancer.