ABSTRACT

Key points 76 Bibliography 77 Further reading 77

Modern radiotherapy is usually given by linear accelerators producing X-rays with high-energy of 4-25 MV which have generally superseded therapy with lower energy 60Co or 137Cs γ-rays.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 picometres (1012 m) so that the individual photons (‘packets’ of energy) are energetic enough to ionize molecules in tissues that they penetrate. It is this ionization that 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. Electron beams are quantum-mechanically similar to X-rays and produce similar biological effects. Two other classes of radiations that are being increasingly adopted in radiotherapy are often referred to as:

● Light particles – e.g. protons, neutrons and α-particles.