The term radiation refers to any part of the electromagnetic spectrum, including infra-red light, visible light, UV light and radio waves. However, in medicine, radiation is often taken to mean radiation at the higher wave lengths such as x-rays and gamma rays. The amount of energy carried by these higher frequencies is much higher than say radio waves and the term ionising radiation is sometimes used to refer to the ability of the radiation to displace electrons.
Ionising radiation can be used diagnostically. In these situations, the dangers of ionising radiation must be minimised (see radiation exposure and Ionising Radiation (medical exposure) regulations 2000).
However, when used therapeutically, it is the damaging effects of ionising radiation are exploited. Radiotherapy causes double-stranded breaks in DNA, causes cell damage and death. Rapidly dividing cells such as malignant cells are particularly susceptible to radiation damage.