ABSTRACT
Let us estimate the sort of energy and beam current necessary to achieve a desirable penetration depth and dose rate. We first consider the beam energy. The deepest target in the human body is in the pelvis. Let us suppose that we need to reach a depth of 30 cm with no more than 50% beam attenuation. In this case, we need photons with energy such that the half-value layer thickness equals 30 cm in water. This corresponds to photons with an energy of approximately 5 MeV. The average energy in a bremsstrahlung spectrum is about one-third of the maximum energy. Therefore, the electron beam energy needs to be about 15 MeV. We would not want to use beam energies much larger than this because this would waste photons that would pass right through the patient and because we want the beam to be attenuated beyond the target to reduce the dose to normal tissues.
