[mohn]

There aren't a lot of clinics that can do it, but proton therapy has emerged as another method for treating tumors by irradiating them. Unlike a beam of high energy photons (used in conventional radiotherapy), a proton beam can be tuned so that most of its radiation dose will be deposited at a specific depth. By contrast, a photon beam will deposit more energy at depths shallower than the target, and it will also deposit some energy at depths beyond the target.

https://physics.stackexchange.com/questions/169665/dose-dept...