Depth dose curve for an unmodulated proton beam, 10 MeV photon radiation, and spread-out Bragg Peak (SOBP) proton radiation

The dose distribution of an unmodulated proton beam consists of a slowly rising dose, a rapid rise to a maximum (also known as the Bragg Peak), followed by a fall to near zero. As a result, a significant portion of the beam energy is deposited in a small volume. In contrast, the energy deposition of photon irradiation is dictated by the laws of exponential decay and is distributed over a larger volume. Because of the sharpness of the Bragg Peak, a single monoenergetic proton beam will only irradiate an area approximately the size of a pituitary gland. Irradiation of larger targets is achieved by superimposing proton beams of different energies. The end result of this superposition is known as the spread-out Bragg Peak (SOBP).