ABSTRACT
Modern radiotherapy is usually delivered using linear accelerators producing X-rays with energies of 4–25 MV which have generally superseded therapy with lower energy 60Co or 137Cs γ-rays.
X-rays and γ-rays are uncharged electromagnetic radiations, physically similar in nature to radio waves or visible light except that their wavelength is less than 10 picometers (10−12 m) so that the individual photons (‘packets’ of energy) are energetic enough to ionize molecules in tissues that they penetrate. This ionization results in the biological effects seen in radiotherapy. These X- and γ-rays all have roughly the same biological effect per unit dose, although there is a small dependence on the energy with lower energies being slightly more effective. The biological damage produced by high-energy photon beams is the result of ionizations by energetic electrons set in motion by photon interactions. Accordingly, the biological effects from beams of energetic electrons are similar to that from high-energy photon beams. While one could therefore refer to conventional radiotherapy as ‘particle therapy’, this terminology generally refers to another class of radiotherapy which is being increasingly adopted. The term particle therapy typically refers to radiotherapy using protons, neutrons, α-particles, fully stripped carbon ions or even heavier ions.
