ABSTRACT
Ionizing radiation (IR) consisting of electromagnetic radiation, or photons, is the type of radiation most commonly used for the treatment of patients with radiotherapy. Typical energies of the photons produced by 4-25 MV linear accelerators found in radiotherapy departments range from less than 100 keV to several MeV (the maximum energy of the machine being used). From its name, the principal damaging effects of this type of radiation arise from its ability to ionize, or eject electrons, from molecules within cells. Almost all the photons produced by linear accelerators have sufficient energy to cause such ionizations. Most biological damage, however, is done by the ejected electrons themselves, which go on to cause further ionizations in molecules they collide with, progressively slowing down as they go. At the end of electron tracks, interactions with other molecules become more frequent, giving rise to clusters of ionizations (Goodhead, 2006). The pattern and density of ionizations and their relationship with the size of the DNA double helix is shown in Fig. 2.1. The clusters
are such that many ionizations can occur within a few base pairs of the DNA. These clusters are a unique characteristic of IR, in contrast to other forms of radiation such as UV or DNA-damaging drugs such as topoisomerase inhibitors. Only a few per cent of the damage is clustered, but when these clusters occur in DNA, the cell has particular difficulty coping with the damage.
