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Absorbed dose is a physical dose quantity D representing the mean energy imparted to matter per unit mass by ionizing radiation. In the SI system of units, the unit of measure is joules per kilogram, and its special name is gray (Gy). The non-SI CGS unit rad is sometimes also used, predominantly in the USA.
The quantity absorbed dose is of fundamental importance in radiological protection for calculating radiation dose. However, absorbed dose is a physical quantity and used unmodified is not an adequate indicator of the likely health effects in humans.
It has been found that for radiation risk (defined as probability of cancer induction and genetic effects) consideration must be given to the type of radiation and the sensitivity of the irradiated tissues, which requires the use of modifying factors. Conventionally therefore, unmodified absorbed dose is not used for comparing stochastic risks but only used to compare against deterministic effects (severity of acute tissue effects that are certain to happen) such as in acute radiation syndrome.
To represent stochastic risk the equivalent dose H T and effective dose E are used, and appropriate dose factors and coefficients are used to calculate these from the absorbed dose. Equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account. These are usually in accordance with the recommendations of the International Committee on Radiation Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU). The coherent system of radiological protection quantities developed by them is shown in the accompanying diagram.
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