ABSTRACT
Ionizing radiation consisting of electromagnetic radiation, or photons, is the type of radiation most commonly used for the treatment of patients with radiotherapy. Typical photon energies 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). The principal damaging effects of this type of radiation arise from its ability to eject electrons from (ionize) 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 become more frequent, giving rise to clusters of ionizations. The DNA damage response (DDR) is a highly complex and coordinated system that determines the cellular outcome of DNA damage caused by radiation. The DDR is not a single pathway, but rather a group of highly interrelated signalling pathways, each of which controls different effects on the cell. This system can be divided into several parts, the sensors of DNA damage and the transducers and effectors of damage response.
