 x Equivalent dose Encyclopedia
The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

. It is therefore a less fundamental quantity than the total radiation energy absorbed per mass (the absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

), but is a more significant quantity for assessing the health risk of radiation exposure. For a further estimate of damage on different types of tissue (which vary in sensitivity to radiation) a further quantity called effective dose
Effective dose
Effective dose may refer to:*Effective dose the dose of pharmacologic agent which will have a therapeutic effect in some fraction of the population receiving the drug...

can be calculated (see article for more). However, the latter requires additional input information about various organs, and also fractionation of radiation to different parts of the body, if any.

Equivalent dose is dimensionally
Dimensional analysis
In physics and all science, dimensional analysis is a tool to find or check relations among physical quantities by using their dimensions. The dimension of a physical quantity is the combination of the basic physical dimensions which describe it; for example, speed has the dimension length per...

a quantity of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

per unit of mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

, and is usually measured in sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

s or rems.

## Computation

Conventional quality factors (relative biological effectiveness) to calculate equivalent doses
x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s, gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

s, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s,
positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

s, muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

s
1
neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s
< 10 keV 5
10 keV - 100 keV 10
100 keV - 2 MeV 20
2 MeV - 20 MeV 10
> 20 MeV 5
proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s
> 2 MeV 2
alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...

s, Nuclear fission products,
heavy nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

20

The equivalent dose cannot be measured directly. The dose for each tissue T and each type of radiation R (often denoted by HT,R) is calculated by the formula where DT,R is the total energy of radiation R absorbed in a unit mass of tissue T, and Q is a radiation quality factor that depends on the type and energy of that radiation. The quality factor is also known as the relative biological effectiveness
Relative biological effectiveness
In radiology, the relative biological effectiveness is a number that expresses the relative amount of damage that a fixed amount of ionizing radiation of a given type can inflict on biological tissues...

of the radiation. It connects the simple deposited energy of the radiation (a physics quantity easy to measure with a sensor) to the equivalent dose, which is —to the extent that the estimation of the Q or RBE is accurate— the biological reaction or damage from this particular amount and type of radiation.

The value of Q or RBE is 1 for x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s, gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

s and beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...

s, but higher for proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s, neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s, alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...

s etc.

To obtain the equivalent dose HT for a given mix of radiation types and energies, one simply adds the individual equivalent doses HT,R over all types of radiation energy dose R in the mix.

To obtain the average equivalent dose HR for a given combination of tissues (such as a whole organism), one may compute a weighted average of the doses HT,R over all T, with the weight of each dose being proportional to the mass of tissue T present in the combination.

In order to obtain an effective dose
Effective dose
Effective dose may refer to:*Effective dose the dose of pharmacologic agent which will have a therapeutic effect in some fraction of the population receiving the drug...

(also known as effective dose equivalent) for radiation given to only part of the body, the equivalent doses of radiation must be further weighted by a "weighting fraction" Wf specific to each tissue, which attempts to quantify the fraction of health risk for a human (or other mammal) attributable to this type of tissue. The weighting fractions for all the tissues add to 1.0. See the article on effective dose
Effective dose
Effective dose may refer to:*Effective dose the dose of pharmacologic agent which will have a therapeutic effect in some fraction of the population receiving the drug...

for this calculation.

## Units

The SI unit of measure for equivalent dose is the sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

, defined as the dose of absorbed radiation that has the same biological effect as a dose of one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

s absorbed in one kilogram of tissue. In the United States the roentgen equivalent man (rem), equal to 0.01 sievert, is still in common use, although regulatory and advisory bodies are encouraging transition to sieverts.

## Related quantities

To quantify the radiation risk of a particular environment or activity, one may use the equivalent dose received per unit of time spent in it, usually measured in sievert per year or sievert per hour.

Ionizing radiation units are standards for measuring ionizing radiation, including units for measuring the activity of radioactive sources, and for quantifying the amount of radiation striking other objects, particularly people...

The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

• Gray (unit)
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

• Banana equivalent dose
Banana equivalent dose
A banana equivalent dose is a whimsical unit of radiation exposure, informally defined as the additional dose a person will absorb from eating one banana...

• Counts per minute
Counts per minute
Counts per minute is a measure of radioactivity. It is the number of atoms in a given quantity of radioactive material that are detected to have decayed in one minute. Disintegrations per minute is also a measure of radioactivity. It is the number of atoms in a given quantity of radioactive...

• Curie
Curie
The curie is a unit of radioactivity, defined asThis is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie, for whom the unit was named. In addition to the curie, activity can be measured using an SI derived unit,...

• Becquerel
Becquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...

• Roentgen (unit)
• Roentgen equivalent man
• Sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray... 