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Absorbed dose is a dose quantity which is the measure of the energy deposited in
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both
radiation protection Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Expos ...
(reduction of harmful effects), and
radiology Radiology ( ) is the medical discipline that uses medical imaging to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiat ...
(potential beneficial effects for example in cancer treatment). It is also used to directly compare the effect of radiation on inanimate matter such as in
radiation hardening Radiation hardening is the process of making electronic components and circuits resistant to damage or malfunction caused by high levels of ionizing radiation ( particle radiation and high-energy electromagnetic radiation), especially for environ ...
. The SI unit of measure is the
gray Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
(Gy), which is defined as one Joule of energy absorbed per kilogram of matter. The older, non-SI CGS unit rad, is sometimes also used, predominantly in the USA.


Deterministic effects

Conventionally, in radiation protection, unmodified absorbed dose is only used for indicating the immediate health effects due to high levels of acute dose. These are tissue effects, such as in acute radiation syndrome, which are also known as deterministic effects. These are effects which are certain to happen in a short time.


Effects of acute radiation exposure


Radiation therapy

The measurement of absorbed dose in tissue is of fundamental importance in radiobiology as it is the measure of the amount of energy the incident radiation is imparting to the target tissue.


Dose computation

The absorbed dose is equal to the radiation exposure (ions or C/kg) of the radiation beam multiplied by the ionization energy of the medium to be ionized. For example, the ionization energy of dry air at 20 °C and 101.325 
kPa KPA may refer to: * Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA) * Kensington (Olympia) station, London, England, National Rail station code * Kenya Ports Authority * ''Kiln phosphoric acid'', a ...
of pressure is . (33.97 eV per ion pair) Therefore, an exposure of (1 roentgen) would deposit an absorbed dose of (0.00876 Gy or 0.876 rad) in dry air at those conditions. When the absorbed dose is not uniform, or when it is only applied to a portion of a body or object, an absorbed dose representative of the entire item can be calculated by taking a mass-weighted average of the absorbed doses at each point. More precisely, \bar=\frac Where :\bar is the mass-averaged absorbed dose of the entire item T :T is the item of interest :D(x,y,z) is the absorbed dose as a function of location :\rho(x,y,z) is the density as a function of location :V is volume


Medical considerations

Non-uniform absorbed dose is common for soft radiations such as low energy x-rays or beta radiation. Self-shielding means that the absorbed dose will be higher in the tissues facing the source than deeper in the body. The mass average can be important in evaluating the risks of radiotherapy treatments, since they are designed to target very specific volumes in the body, typically a tumour. For example, if 10% of a patient's bone marrow mass is irradiated with 10 Gy of radiation locally, then the absorbed dose in bone marrow overall would be 1 Gy. Bone marrow makes up 4% of the body mass, so the whole-body absorbed dose would be 0.04 Gy. The first figure (10 Gy) is indicative of the local effects on the tumour, while the second and third figure (1 Gy and 0.04 Gy) are better indicators of the overall health effects on the whole organism. Additional dosimetry calculations would have to be performed on these figures to arrive at a meaningful effective dose, which is needed to estimate the risk of cancer or other stochastic effects. When ionizing radiation is used to treat cancer, the doctor will usually prescribe the radiotherapy treatment in units of gray. Medical imaging doses may be described in units of coulomb per kilogram, but when
radiopharmaceutical Radiopharmaceuticals, or medicinal radiocompounds, are a group of pharmaceutical drugs containing radioactive isotopes. Radiopharmaceuticals can be used as diagnostic and therapeutic agents. Radiopharmaceuticals emit radiation themselves, which ...
s are used, they will usually be administered in units of
becquerel The becquerel (; symbol: Bq) is the unit of radioactivity in the International System of Units (SI). One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relatin ...
.


Stochastic risk - conversion to equivalent dose

For stochastic radiation risk, defined as the ''probability'' of cancer induction and genetic effects occurring over a long time scale, consideration must be given to the type of radiation and the sensitivity of the irradiated tissues, which requires the use of modifying factors to produce a risk factor in
sievert The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
s. One sievert carries with it a 5.5% chance of eventually developing cancer based on the
linear no-threshold model The linear no-threshold model (LNT) is a dose-response model used in radiation protection to estimate stochastic health effects such as radiation-induced cancer, genetic mutations and teratogenic effects on the human body due to exposure to io ...
. This calculation starts with the absorbed dose. To represent stochastic risk the dose quantities
equivalent dose Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the p ...
''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 The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
or rem which implies that biological effects have been taken into account. The derivation of stochastic risk is in accordance with the recommendations of the
International Committee on Radiation Protection The International Commission on Radiological Protection (ICRP) is an independent, international, non-governmental organization, with the mission to protect people, animals, and the environment from the harmful effects of ionising radiation. Its r ...
(ICRP) and
International Commission on Radiation Units and Measurements The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology, originally as the X-Ray Unit Committee until 1950. Its objective "is to develop concepts, de ...
(ICRU). The coherent system of radiological protection quantities developed by them is shown in the accompanying diagram. For whole body radiation, with
Gamma rays A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
or
x-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...
the modifying factors are numerically equal to 1, which means that in that case the dose in grays equals the dose in sieverts.


Development of the absorbed dose concept and the gray

Wilhelm Röntgen Wilhelm Conrad Röntgen (; ; 27 March 184510 February 1923) was a German mechanical engineer and physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achie ...
first discovered
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s on November 8, 1895, and their use spread very quickly for medical diagnostics, particularly broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques. Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary for radiation intensity and various countries developed their own, but using differing definitions and methods. Eventually, in order to promote international standardisation, the first International Congress of Radiology (ICR) meeting in London in 1925, proposed a separate body to consider units of measure. This was called the
International Commission on Radiation Units and Measurements The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology, originally as the X-Ray Unit Committee until 1950. Its objective "is to develop concepts, de ...
, or ICRU, and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of
Manne Siegbahn Karl Manne Georg Siegbahn FRS(For) HFRSE (3 December 1886 – 26 September 1978) was a Swedish physicist who was awarded the Nobel Prize in Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy". Biography Siegbahn ...
. One of the earliest techniques of measuring the intensity of X-rays was to measure their ionising effect in air by means of an air-filled
ion chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...
. At the first ICRU meeting it was proposed that one unit of X-ray dose should be defined as the quantity of X-rays that would produce one esu of charge in one
cubic centimetre A cubic centimetre (or cubic centimeter in US English) (SI unit symbol: cm3; non-SI abbreviations: cc and ccm) is a commonly used unit of volume that corresponds to the volume of a cube that measures 1 cm × 1 cm × 1 cm. One cub ...
of dry air at 0  °C and 1 standard atmosphere of pressure. This unit of radiation exposure was named the roentgen in honour of Wilhelm Röntgen, who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to gamma radiation. This approach, although a great step forward in standardisation, had the disadvantage of not being a direct measure of the absorption of radiation, and thereby the ionisation effect, in various types of matter including human tissue, and was a measurement only of the effect of the X-rays in a specific circumstance; the ionisation effect in dry air. In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with
William Valentine Mayneord William Valentine Mayneord, CBE FRS (14 February 1902 – 10 August 1988) was a British physicist and pioneer in the field of medical physics. Early life and education He was born in Redditch, Worcestershire to Walter and Elizabeth Mayneord b ...
and the radiobiologist John Read, published a paper in which a new unit of measure, dubbed the ''"gram roentgen"'' (symbol: gr) was proposed, and defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation". This unit was found to be equivalent to 88 ergs in air, and made the absorbed dose, as it subsequently became known, dependent on the interaction of the radiation with the irradiated material, not just an expression of radiation exposure or intensity, which the roentgen represented. In 1953 the ICRU recommended the rad, equal to 100 erg/g, as the new unit of measure of absorbed radiation. The rad was expressed in coherent cgs units. In the late 1950s, the CGPM invited the ICRU to join other scientific bodies to work on the development of the International System of Units, or SI. It was decided to define the SI unit of absorbed radiation as energy deposited per unit mass which is how the rad had been defined, but in MKS units it would be J/kg. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was equal to 100 rad, the cgs unit.


Other uses

Absorbed dose is also used to manage the irradiation and measure the effects of ionising radiation on inanimate matter in a number of fields.


Component survivability

Absorbed dose is used to rate the survivability of devices such as electronic components in ionizing radiation environments.


Radiation hardening

The measurement of absorbed dose absorbed by inanimate matter is vital in the process of
radiation hardening Radiation hardening is the process of making electronic components and circuits resistant to damage or malfunction caused by high levels of ionizing radiation ( particle radiation and high-energy electromagnetic radiation), especially for environ ...
which improves the resistance of electronic devices to radiation effects.


Food irradiation

Absorbed dose is the physical dose quantity used to ensure irradiated food has received the correct dose to ensure effectiveness. Variable doses are used depending on the application and can be as high as 70 kGy.


Radiation-related quantities

The following table shows radiation quantities in SI and non-SI units: Although the United States Nuclear Regulatory Commission permits the use of the units
curie In computing, a CURIE (or ''Compact URI'') defines a generic, abbreviated syntax for expressing Uniform Resource Identifiers (URIs). It is an abbreviated URI expressed in a compact syntax, and may be found in both XML and non-XML grammars. A CURIE ...
, rad, and rem alongside SI units, the
European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been de ...
European units of measurement directives As of 2009, the European Union had issued two units of measurement directives: In 1971 it issued Directive 71/354/EEC which required EU member states to standardise on the International System of Units (SI) rather than use a variety of CGS ...
required that their use for "public health ... purposes" be phased out by 31 December 1985.


See also

*
Kerma (physics) In radiation physics, kerma is an acronym for "kinetic energy released per unit mass" (alternately, "kinetic energy released in matter", "kinetic energy released in material",, or "kinetic energy released in materials"), defined as the sum of the ...
*
Mean glandular dose Mean glandular dose (MGD) is a quantity used to describe the absorbed dose of radiation to the breast in mammography. It is based on a measurement of air kerma and conversion factors. MGD can be calculated from measurements made with PMMA blocks. ...
* :Units of radiation dose


Notes


References


Literature

*


External links


''Specific Gamma-Ray Dose Constants for Nuclides Important to Dosimetry and Radiological Assessment'', Laurie M. Unger and D. K . Trubey, Oak Ridge National Laboratory, May 1982
- contains gamma-ray dose constants (in tissue) for approximately 500 radionuclides. {{Authority control Physical quantities Radiobiology Radiation protection