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{{Distinguish2|the tissue sensitivity weighting factor W<sub>f</sub> using in calculating the [[effective dose (radiation safety)]]}}
{{refimprove|date=October 2013}}
{{Technical|date=July 2012}}
[[File:SI Radiation dose units.png|thumb|400px|Graphic showing relationship of SI radiation dose units]]
In [[radiology]], the '''relative biological effectiveness''' (often abbreviated as '''RBE''') is the ratio of biological effectiveness of one type of [[ionizing radiation]] relative to another, given the same [[absorbed dose|amount of absorbed energy]]. The RBE is an empirical value that varies depending on the particles, energies involved, and which biological effects are deemed relevant. It is a set of experimental measurements.


In [[dosimetry]] (the practical attempt to apply RBE realistically and uniformly to human and animal experience), the RBE is represented in regulations by the [[radiation weighting factor]], ('''W<sub>R</sub>'''), or formerly, the [[quality factor]] ('''Q''').<ref name="icrp92">{{cite journal|last=Sinclair|first=Dr. W. K. et al|title=Relative biological effectiveness (RBE), quality factor (Q) and radiation weighting factor (Wr)|journal=Annals of the ICRP|date=January 2003|volume=33|series=ICRP Publication 92|issue=4|url=http://www.elsevier.ca/ISBN/9780080443119/ICRP-Publication-92-Relative-Biological-Effectiveness-RBE-Quality-Factor-Q-and-Radiation-Weighting-Factor-wR|isbn=978-0-08-044311-9}}</ref> These weighting factors, arrived at by consensus of governments, industry, and regulators, convert [[absorbed dose]] (measured in units of [[gray (unit)|gray]]s or [[rad (unit)|rad]]s) into formal biological [[equivalent dose]] for radiation exposure (measured in units of [[sievert]]s or [[rem (unit)|rem]]).
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The higher the RBE or weighting factor numbers for a type of radiation, the more damaging is the type of radiation, per unit of energy deposited in biological tissues.
 
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 hundred [[nanometer]]s (several tenths of a [[micrometre|micrometer]]) apart, along their path. In contrast, the much more massive alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one tenth of a nanometer apart (i.e., less than one-thousandth of the typical distance between ionizations for photons and beta particles).
 
Radiation weighting factors that go from physical energy to biological effect must not be confused with [[tissue weighting factor]]s. The tissue weighting factors are used to convert an [[equivalent dose]] to a given tissue in the body, to an [[effective radiation dose]], a number that provides an estimation of total danger to the whole organism, as a result of the radiation dose to part of the body.
 
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 [[ionizing radiation#Monitoring and controlling exposure|radiation safety]].
 
However, the various RBEs are scientific numbers that represent raw data, and as raw data they go into regulatory consensus weighting factors which represent the best guess as to how relatively dangerous various types of radiation are, in practice. The radiation weighting factors will be approximately the same as the RBEs that result from some experiments, but may be quite different from RBEs that result from other experiments.
 
==Definition==
The relative biological effectiveness for radiation of type ''R'' on a tissue of type ''T'' is traditionally defined as the ratio
:<math>RBE= \frac{D_X}{D_R}</math>
where ''D''<sub>''X''</sub> is a reference [[absorbed dose]] of radiation of a standard type ''X'', and ''D''<sub>''R''</sub> 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.
 
==Experimental methods ==
[[File:Rbe definition.png|thumb|right|300px|LD-50 limit for CHO-K1 cell line irradiated by photons (blue curve) and by carbon ions (red curve).]]
Typically the evaluation of relative biological effectiveness is done on various types of living cells grown in [[culture medium]], including [[prokaryote|prokaryotic]] cells such as [[bacteria]], simple [[Eukaryote|eukaryotic]] cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as [[rat]]s. 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 radiation]]s ([[helium-4]] nuclei), [[beta radiation]] ([[electron]]s and [[positron]]s), [[neutron radiation]], and heavy [[atomic nucleus|nuclei]], including the fragments of [[nuclear fission]]. 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&nbsp;[[electron-volt|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]], 4–6 for simple [[Eukaryote|eukaryotic]] [[cell (biology)|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 eukaryotic 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.<ref>Winters-TH, Franza-JR, Radioactivity in Cigarette Smoke, New England Journal of Medicine, 1982; 306(6): 364–365</ref> 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. The bulk of studies have yielded RBEs between 10 and 20.<ref>{{cite journal|last=Chambersa|first=Douglas B.|coauthors=Richard V. Osborneb, Amy L. Garva|title=Choosing an alpha radiation weighting factor for doses to non-human biota|journal=Journal of Environmental Radioactivity|year=2006|volume=87|issue=1|pages=pp 1–14|url=http://web.engr.oregonstate.edu/~higley/rbe%20references/RBE%20Copy.Data/PDF/choosing%20an%20alpha%20radiation%20weighting%20factor%20for%20doses%20to%20non%20human%20biota%20chanbers%20et%20al%20-3700075285/choosing%20an%20alpha%20radiation%20weighting%20factor%20for%20doses%20to%20non%20human%20biota%20chanbers%20et%20al%202006.pdf|accessdate=8 May 2012}}</ref>
 
===Standardization===
[[File:Neutron radiation weighting factor as a function of kinetic energy.gif|thumb|350px|The radiation weighting factor for neutrons has been revised over time and remains controversial.]]
To bypass the complexity of tissue dependence, the [[International Commission on Radiological Protection]] (ICRP) defined standard radiation weighting factors, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry.<ref name="icrp92" /> These values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types. The ICRP 1991 standard values for relative effectiveness are given below.
 
{| class="wikitable"
|+Weighting factors W<sub>R</sub> (formerly termed Q factor)<br/> used to calculate equivalent dose<br/>according to ICRP report 92<ref name="icrp92"/>
|-
! Radiation !! Energy !! w<sub>R</sub> (also Q)
|-
| [[x-ray]]s, [[gamma ray]]s,<br>[[beta ray]]s, [[muon]]s ||&nbsp; || 1
|-
| rowspan=5 |[[neutron]]s || < 10&nbsp;keV        || 5
|-
| 10&nbsp;keV - 100&nbsp;keV || 10
|-
| 100&nbsp;keV - 2&nbsp;MeV  || 20
|-
| 2&nbsp;MeV - 20&nbsp;MeV  || 10
|-
| >&nbsp;20&nbsp;MeV        || 5
|-
| [[proton]]s, charged [[pion]]s  || >&nbsp;2&nbsp;MeV          || 2
|-
| [[alpha particle]]s, [[nuclear fission product]]s,<br/>heavy [[atomic nucleus|nuclei]] || &nbsp; || 20
|}
 
Thus, for example, a given amount of energy absorbed in the form of 15&nbsp;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.
 
==History==
The concept was introduced in the 1950s, at a time when the deployment of nuclear weapons and nuclear reactors spurred research on the biological effects of artificial radiaoctivity. It had been noticed that those effects depended both on the type and [[energy spectrum]] of the radiation, and on the kind of living tissue. The first systematic experiments to determine the RBE were conducted in that decade.{{Citation needed|date=May 2011}}
 
==See also==
* [[Background radiation]]
* [[Theory of dual radiation action]]
 
==References==
{{reflist|2}}
 
== External links ==
*[http://www-pub.iaea.org/MTCD/publications/PDF/trs461_web.pdf Relative Biological Effectiveness in Ion Beam Therapy]
 
[[Category:Radiation health effects]]

Latest revision as of 20:52, 8 December 2014

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