Radiation dose may be expressed in terms of absorbed dose, equivalent dose, and effective dose. An absorbed dose is the amount of radiation energy absorbed per unit mass of an organ or tissue. In 1976 the gray was adopted as the International System of Units (SI) unit of absorbed dose; one gray is equal to one joule of radiation energy per kilogram of matter. Absorbed dose is also sometimes reported in a unit known as the rad, which is an acronym for radiation absorbed dose. One rad is equal to 1/100th of one gray.
A measure of equivalent dose to an organ or tissue is a quantity that takes into account differences in ionization density for different types of radiation (e. g., alpha particles, beta particles, and gamma rays). Equivalent dose is calculated by weighting the absorbed dose to an organ or tissue by a radiation weighting factor that reflects the relative biological effectiveness of the photon or particle that produced the ionization within the tissue (it is assumed that the biological effect associated with a photon or particle is related to the ionization density). Values for this radiation weighting factor are assumed to be independent of the organ or tissue that is exposed as well as the biological end point under consideration. For outcomes that are assessed at the cellular or molecular level, such as DNA damage, it has been observed that a given absorbed dose of ionization radiation produces a greater amount of damage when the dose is due to alpha radiation than photons of electromagnetic radiation. For human health outcomes, such as cancer, however, there is uncertainty about the relative biological effectiveness of different forms of ionizing radiation because of the limitation of available empirical evidence about the magnitude of radiation-cancer associations for absorbed doses of different types of ionizing radiation. The International Commission on Radiological Protection currently recommends a radiation weighting factor of 1 for photons and electrons and a radiation weighting factor of 20 for alpha particles. While absorbed dose is a physical quantity, equivalent dose estimates may change as recommendations about weighting factors are revised. Equivalent dose is expressed in the unit sievert (Sv). An earlier unit used to express equivalent dose was the rem, which is an acronym for radiation equivalent dose; one rem is equal to 1/100th of one Sv.
A measure of effective dose is intended to express the biological effect associated with irradiation, taking into account differences in radiosensitivity of the body’s various organs and tissues. Effective dose is calculated by weighting the equivalent dose in a tissue or organ by a tissue weighting factor that reflects the contribution of the tissue to the detriment of health when the body is uniformly irradiated. Doses to tissues and organs such as the lung are assigned a greater weight than doses to tissues and organs that are assumed to be relatively insensitive to adverse effects of ionizing radiation. Effective dose is expressed in the unit Sv, and, like equivalent dose estimates, effective dose estimates may change as recommendations about weighting factors are revised.