Relative biological effectiveness
Encyclopedia
In radiology
, the relative biological effectiveness (often abbreviated as RBE) 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 tissue
s. The higher that number, more damaging is the type of radiation, for the same amount of absorbed energy
.
Different types of radiation have different biological effectiveness mainly because they transfer their energy to the tissue in different ways. Photons and beta particles have a low linear energy transfer
coefficient, meaning that they ionize atoms in the tissue that are spaced by several thousand angstrom
s apart along their path. In contrast, alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one angstrom apart.
The relative biological effectiveness is sometimes referred to as the "radiation weighting factor" (WR) that enters in the conversion of units of absorbed energy (such as rad
s and gray
s), a simple measureable physical quantity, to units of biological equivalent dose
for radiation exposure (such as rems and sievert
s, respectively). However although RBE acts as a type of weighting factor to go from physical energy to biological effect, it must not be confused with the defined tissue weighting factors used to go on to convert an equivalent dose
to a given tissue in the body, to an effective dose (radiation safety)
, which provides an estimation of total danger to the whole organism, as a result of the radiation dose (which may only be to part of the body, or to all of it).
The concept of RBE is relevant in medicine, such as in radiology
and radiotherapy, and to the evaluation of risks and consequences of radioactive contamination
in various contexts, such as nuclear power plant
operation, nuclear fuel
disposal and reprocessing, nuclear weapon
s, uranium mining
, and ionizing radiation safety.
where DX is a reference absorbed dose
of radiation of a standard type X, and DR is the absorbed dose of radiation of type R that causes the same amount of biological damage. Both doses are quantified by the amount of energy
absorbed in the cells.
cells such as bacteria
, simple eukaryotic cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats
. The doses are adjusted to the LD-50 point; that is, to the amount that will cause 50% of the cells become unable to undergo mitotic division (or, for bacteria, binary fission), thus being effectively sterilized — even if they can still carry out other cellular functions.
The types R of ionizing radiation most considered in RBE evaluation are X-ray
s and gamma radiation (both consisting of photon
s), alpha radiations (helium-4
nuclei), beta radiation (electron
s and positron
s), neutron radiation
, and heavy nuclei
, including the fragments of nuclear fission
. For some kinds of radiation, the RBE is strongly dependent on the energy of the individual particles.
For other radiation types, the RBE is not a well-defined physical quantity, since it varies somewhat with the type of tissue and with the precise place of absorption within the cell. Thus, for example, the RBE for alpha radiation is 2–3 when measured on bacteria
, 4–6 for simple eukaryotic cell
s, and 6–8 for higher eukaryotic cells. The RBE of neutrons is 4–6 for bacteria, 8–12 for simple eukaryotic cells, and 12–16 for higher eukariotic cells.
For this reason, it has been suggested that the health impact of contamination by alpha emitters might have been substantially underestimated. Measurements of RBE with external sources also neglect the ionization caused by the recoil of the parent nucleus due to the alpha decay. While the nucleus typically carries only about 2% of the energy of the alpha particle, its range is extremely short (about 2–3 angstroms), due to its high electric charge
and high mass
. Thus, all of the ionization energy is deposited in an extremely small volume near its original location. Alpha emitters are typically heavy metals
that have chemical affinity for the chromosomes; these facts should increase the probability of a recoiling nucleus causing irreparable chromosome damage. In contrast, the emitted alpha particle will lose most of its ionization energy in the cytoplasm
, where the damage need not be lethal. Indeed, some studies conducted with intratracheal instillation of the alpha-emitter polonium-210 in hamsters have yielded RBEs as high as 1,000.
(ICRP) defined standard RBE values, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry. These values are conservatively chosen to be greater than the experimental values observed for the most sensitive cell types. The ICRP 1991 standard values for relative effectiveness are given below.
Thus, for example, a given amount of energy absorbed in the form of 15 keV neutrons should be assumed to produce 10 times the damage caused by an equal amount of energy absorbed as X-rays or gamma rays.
Radiology
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...
, the relative biological effectiveness (often abbreviated as RBE) is a number that expresses the relative amount of damage that a fixed amount of ionizing radiation
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...
of a given type can inflict on biological tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
s. The higher that number, more damaging is the type of radiation, for the same amount of absorbed energy
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...
.
Different types of radiation have different biological effectiveness mainly because they transfer their energy to the tissue in different ways. Photons and beta particles have a low linear energy transfer
Linear energy transfer
Linear energy transfer is a measure of the energy transferred to material as an ionizing particle travels through it. Typically, this measure is used to quantify the effects of ionizing radiation on biological specimens or electronic devices....
coefficient, meaning that they ionize atoms in the tissue that are spaced by several thousand angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
s apart along their path. In contrast, alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one angstrom apart.
The relative biological effectiveness is sometimes referred to as the "radiation weighting factor" (WR) that enters in the conversion of units of absorbed energy (such as rad
Rad (unit)
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...
s and gray
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 ....
s), a simple measureable physical quantity, to units of biological equivalent dose
Equivalent dose
The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation...
for radiation exposure (such as rems and 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, respectively). However although RBE acts as a type of weighting factor to go from physical energy to biological effect, it must not be confused with the defined tissue weighting factors used to go on to convert an equivalent dose
Equivalent dose
The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation...
to a given tissue in the body, to an effective dose (radiation safety)
Effective dose (radiation safety)
The concept of effective dose is used in radiation protection as an estimate of the stochastic effect that a non-uniform radiation dose has on a human. It should not be confused with equivalent dose, which is a related but more general concept. For whole-body radiation, the sum of effective doses...
, which provides an estimation of total danger to the whole organism, as a result of the radiation dose (which may only be to part of the body, or to all of it).
The concept of RBE is relevant in medicine, such as in radiology
Radiology
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...
and radiotherapy, and to the evaluation of risks and consequences of radioactive contamination
Radioactive contamination
Radioactive contamination, also called radiological contamination, is radioactive substances on surfaces, or within solids, liquids or gases , where their presence is unintended or undesirable, or the process giving rise to their presence in such places...
in various contexts, such as nuclear power plant
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
operation, nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
disposal and reprocessing, nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s, uranium mining
Uranium mining
Uranium mining is the process of extraction of uranium ore from the ground. The worldwide production of uranium in 2009 amounted to 50,572 tonnes, of which 27% was mined in Kazakhstan. Kazakhstan, Canada, and Australia are the top three producers and together account for 63% of world uranium...
, and ionizing radiation safety.
Definition
The relative biological effectiveness for radiation of type R on a tissue of type T is traditionally defined as the ratiowhere DX is a reference absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...
of radiation of a standard type X, and DR is the absorbed dose of radiation of type R that causes the same amount of biological damage. Both doses are quantified by the amount 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...
absorbed in the cells.
Experimental methods
Typically the evaluation of relative biological effectiveness is done on various types of living cells grown in culture medium, including prokaryoticProkaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
cells such as bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, simple eukaryotic cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats
RATS
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
. The doses are adjusted to the LD-50 point; that is, to the amount that will cause 50% of the cells become unable to undergo mitotic division (or, for bacteria, binary fission), thus being effectively sterilized — even if they can still carry out other cellular functions.
The types R of ionizing radiation most considered in RBE evaluation are 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 and gamma radiation (both consisting of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s), alpha radiations (helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
nuclei), beta radiation (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 and 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), neutron radiation
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
, and 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...
, including the fragments of nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
. For some kinds of radiation, the RBE is strongly dependent on the energy of the individual particles.
Dependence on tissue type
Early on it was found that X-rays, gamma radiation, and beta radiation were essentially equivalent for all cell types. Therefore, the standard radiation type X is generally an X-ray beam with 250 KeV photons. As a result, the relative biological effectiveness of beta and photon radiation is essentially 1.For other radiation types, the RBE is not a well-defined physical quantity, since it varies somewhat with the type of tissue and with the precise place of absorption within the cell. Thus, for example, the RBE for alpha radiation is 2–3 when measured on bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, 4–6 for simple eukaryotic cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s, and 6–8 for higher eukaryotic cells. The RBE of neutrons is 4–6 for bacteria, 8–12 for simple eukaryotic cells, and 12–16 for higher eukariotic cells.
Dependence on source location
In the early experiments, the sources of radiation were all external to the cells that were irradiated. However, since alpha particles cannot traverse the outermost dead layer of human skin, they can do significant damage only if they come from the decay of atoms inside the body. Since the range of an alpha particle is typically about the diameter of a single eukaryotic cell, the precise location of the emitting atom in the tissue cells becomes significant.For this reason, it has been suggested that the health impact of contamination by alpha emitters might have been substantially underestimated. Measurements of RBE with external sources also neglect the ionization caused by the recoil of the parent nucleus due to the alpha decay. While the nucleus typically carries only about 2% of the energy of the alpha particle, its range is extremely short (about 2–3 angstroms), due to its high electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
and high 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:...
. Thus, all of the ionization energy is deposited in an extremely small volume near its original location. Alpha emitters are typically heavy metals
Heavy metals
A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...
that have chemical affinity for the chromosomes; these facts should increase the probability of a recoiling nucleus causing irreparable chromosome damage. In contrast, the emitted alpha particle will lose most of its ionization energy in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
, where the damage need not be lethal. Indeed, some studies conducted with intratracheal instillation of the alpha-emitter polonium-210 in hamsters have yielded RBEs as high as 1,000.
Standardization
To bypass the complexity of tissue dependence, the International Commission on Radiological ProtectionInternational Commission on Radiological Protection
The International Commission on Radiological Protection is an advisory body providing recommendations and guidance on radiation protection; It was founded in 1928 by the International Society of Radiology and was then called the ‘International X-ray and Radium Protection Committee’...
(ICRP) defined standard RBE values, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry. These values are conservatively chosen to be greater than the experimental values observed for the most sensitive cell types. The ICRP 1991 standard values for relative effectiveness are given below.
| Radiation | Energy | wR (also RBE or Q) |
|---|---|---|
| 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 |
Thus, for example, a given amount of energy absorbed in the form of 15 keV neutrons should be assumed to produce 10 times the damage caused by an equal amount of energy absorbed as X-rays or gamma rays.