https://phys.org/news/2018-11-high-contrast-imaging-cancer-therapy-protons.html > Atomic nuclei as weapons against cancer > An alternative is radiation therapy with charged atomic nuclei such as protons. The penetration depth of these particles depends on their initial energy. They release their maximum dose at the end of their trajectory. No dose will be deposited beyond this so-called "Bragg peak." The challenge for physicians administering this kind of therapy is to control the proton beam exactly to match the shape of the tumor tissue and thus spare as much of the surrounding normal tissue as possible. Before the treatment, they conduct an X-ray-based computed tomography (CT) scan to select their target volume.