Radiation Physics

The word “atom” derives from the Greek word “atomos” which means indivisible, and atom was the smallest indivisible component of matter according to some philosophers in Ancient Greece. Nevertheless, it is now known that atoms are composed of subatomic particles: neutrons and protons in the nucleus of the atom, and electrons orbiting that nucleus. Radiation is termed as propagation of energy from a radioactive source to another medium. This energy transmission can be in the form of particulate or electromagnetic radiation. High-energy ionizing radiation can remove electrons from atoms. In the clinical radiation oncology practice, electromagnetic photons and particulate electron, proton, or neutron radiation are used in the treatment of malignancies and some benign conditions. Radiation is scattered and absorbed when passing through the tissue in five types of interactions, three of which, namely photoelectric, Compton effects, and pair production, determine the absorption coefficient. Basic concepts that are crucial to understanding radiation physics are reviewed in depth in this chapter.