Nature of ionising radiation and its interaction with tissue

Ionising radiation may be electromagnetic, such as X-rays or gamma rays, or it may be particulate, for example neutrons, beta or alpha particles. Absorption of ionising radiation in matter is a complex process, and depends on the energy and nature of the radiation as well as the composition of the target. Absorbed dose (Gy) measures the energy deposited in unit mass of tissue. Equivalent dose (Sv) takes into account the characteristics of the radiation. Effective dose (also Sv) is the hypothetical dose that would have the same effect if applied uniformly to the whole body. At high doses of radiation, deterministic effects occur: there is a definite threshold above which the severity of the effects increase with the dose. At low doses there may be stochastic effects: these do not have a threshold for occurrence and are difficult to predict in a particular individual. (Author)