ABSTRACT
Radiation therapy started with kilovoltage X-ray beams in the 1910s, and with time the beam energies gradually increased to give better penetration, better skin sparing, and higher beam output. Proton beams and carbon ion beams have high LET, limited range, and ideal dose distribution for radiation therapy. They are produced in cyclotrons and guided through the beam lines to the treatment rooms. Radiation is classified as ionizing or non-ionizing [1,2]. This chapter illustrates the energy deposition mechanisms for ionizing radiation. In the atomic photoelectric effect (PEE), a photon disappears and an electron is ejected from an atom. When the photon energy is much larger compared to the shell binding energy (BE)s, the photons tend to see the electrons as free electrons since the BE becomes negligible compared to the photon energy. So, there is no Z dependence of electronic cross section in the Compton effect.
