- Relative biological effectiveness
Relative Biological Effectiveness (RBE) is a
health physicsconcept introduced in the 1950s, after it was noted that different types of radiation might affect living organisms differently. A higher RBE equates with greater biological damage for equivalent radiation exposure.
The four primary types of
ionizing radiationthat were of concern to scientists working in laboratories were photonradiation, beta radiation, neutron radiation, and alpha radiation.
Sources that produced those types of radiations were used to irradiate various types of living cells grown in
culture medium. Each type was given a known and measured amount of radiation exposure, delivering an exact amount of ionizing energyto each culture dish.
The cells grown were
prokaryoticcells such as bacteria, simple eukaryoticcells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats. The cells were irradiated until they each reached the LD-50point, that is, the point at which a lethal dosewas delivered to half of the cells. A lethal dose was defined as one which created the inability to engage in mitotic division(or, for bacteria, binary fission), effectively sterilizing the cell, even if it still engaged in some cellular functions.
It was found that for all cell types, photon radiation and beta radiation were essentially equivalent, and they were assigned the base value of 1 for their RBE.
For neutron radiation it was found that it took 2–3 times more beta or photon radiation to cause an LD-50 compared to neutron radiation in bacteria; about 4–6 times more beta or photon radiation to cause an LD-50 compared to neutron radiation in simple eukaryotic cells; and about 6–8 times more beta or photon radiation to cause an LD-50 compared to neutron radiation for the higher eukaryotic cells. Accordingly, to be safe, the RBE for neutron radiation was given the value of 10 in governmental regulations.
For alpha radiation it was found that it took 4–6 times more beta or photon radiation to cause an LD-50 compared to alpha radiation in bacteria, about 8–12 times more beta or photon radiation to cause an LD-50 compared to alpha radiation in simple eukaryotic cells; and about 12–16 times more beta or photon radiation to cause an LD-50 compared to alpha radiation for the higher eukaryotic cells. Accordingly, to be safe, the RBE for alpha radiation was given the value of 20 in governmental regulations.
These RBE values are applied by health physicists so that they can compare various types of radiation exposure in a more meaningful way.Thus, one rad is one rem (röntgen equivalent in man) if delivered from, for example, an X-ray procedure involving photons. However, one rad of neutron exposure is thus 10 rem of exposure, and one rad of internally deposited alpha exposure is 20 rem of exposure. In the more current
SIusage, the units are not the rad and rem, but the gray (Gy) and sievert(Sv), though the RBE values remain the same for calculating sieverts from grays of exposure.
While photon, beta and neutron are all relatively low LET (
linear energy transfer) radiations, in which the ionizations caused by the radiation are separated by many thousands of ångströms (Å), alpha radiation is a high LET radiation, with the ionizations occurring essentially about every ångström of travel of the alpha particle.
In the studies conducted in the 1950s for determination of these RBE values, the sources of radiation were all external to the cells that were irradiated. However, in real life, external irradiation of tissue is essentially impossible from alpha radiation, because the alpha particle cannot traverse the dead layer of skin that surrounds people. Accordingly, alpha radiation is only 'meaningful' if it comes from internally deposited alpha emitters, which then allows for intimate proximity to the alpha radiation. The range of an alpha particle is typically about the diameter of a single eukaryotic cell.
This distinction has raised a serious concern that the RBE for alpha-emitters might be substantially underestimated [Winters-TH, Franza-JR, Radioactivity in Cigarette Smoke, New England Journal of Medicine, 1982; 306(6): 364–365] , as it neglects the small, but potentially significant, ionization caused by the recoil of the parent nucleus during the alpha decay. While this energy of the recoil nucleus is typically only about 2% of the energy of the alpha particle, the range of the recoil nucleus is extremely short, i.e. about 2–3 Å, due to its high
electric chargeand high mass. Thus, all of the ionization energy is deposited in an extremely small volume wherever the parent nucleus happens to be located, which is often on the chromosomesbecause alpha emitters are typically heavy metalswhich preferentially collect on (stain) chromosome material. This has the effect of 'wiping out' that region of the chromosome, whereas the alpha particle only causes a few ionizations, losing most of its ionization energy in the cytoplasm. Studies conducted with intratracheal instillation of polonium-210, an alpha emitter, in hamsters have yielded RBEs as high as 1,000 in some studies.
Due to the uncertainty associated with the RBE for alpha radiation, and the lack of neutron exposure to personnel in almost all settings, it is strongly discouraged to use rems or sieverts as measures of comparative radiation exposure, and instead to use the actual exposure in rads or grays, followed with a description of the type of exposure (almost always from photon or beta in which the RBE is 1 anyway, though rarely from alpha).
Notes and references
Wikimedia Foundation. 2010.
Look at other dictionaries:
relative biological effectiveness — relative biological effectiveness. См. относительная биологическая эффективность. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
Relative biological effectiveness — Die relative biologische Wirksamkeit, kurz RBW (engl.: relative biological effectiveness, RBE), ist in der Strahlenbiologie ein Unterscheidungsfaktor für Strahlenarten hinsichtlich ihrer biologischen Effekte. Definition Die relative biologische… … Deutsch Wikipedia
relative biological effectiveness — The ratio of the number of rads of gamma (or X) radiation of a certain energy that will produce a specified biological effect to the number of rads of another radiation required to produce the same effect measures the “relative biological… … Military dictionary
relative biological effectiveness — santykinis biologinis efektyvumas statusas T sritis Gynyba apibrėžtis Gama ar rentgeno spindulių ir kitokios jonizuojančiosios spinduliuotės, sukeliančios vienodą biologinį efektą, sugertųjų dozių santykis. atitikmenys: angl. relative biological… … NATO terminų aiškinamasis žodynas
relative biological effectiveness — santykinis biologinis efektyvumas statusas T sritis apsauga nuo naikinimo priemonių apibrėžtis Atitinkamos energijos gama ir rentgeno spindulių sugertosios dozės santykis su kitos jonizuojančiosios spinduliuotės sugertąja doze, kuri sukuria tokį… … Apsaugos nuo naikinimo priemonių enciklopedinis žodynas
relative biological effectiveness — rel·a·tive biological effectiveness rel ət iv n the relative capacity of a particular ionizing radiation to produce a response in a biological system abbr. RBE * * * (RBE) an expression of the effectiveness of other types of radiation in… … Medical dictionary
relative biological effectiveness — noun : the relative capacity of a particular ionizing radiation to produce a response in a biological system abbreviation RBE … Useful english dictionary
relative biological effectiveness of the radiation — santykinis biologinis spinduliuotės veiksmingumas statusas T sritis Standartizacija ir metrologija apibrėžtis Eksperimentiškai randamas dydis, išreiškiamas tam tikros rūšies norminės spinduliuotės sugertosios dozės ir spinduliuotės, kuri tomis… … Penkiakalbis aiškinamasis metrologijos terminų žodynas
relative biological effectiveness of the radiation — santykinis biologinis spinduliuotės veiksmingumas statusas T sritis fizika atitikmenys: angl. relative biological effectiveness of the radiation vok. relative biologische Wirksamkeit der Strahlung, f rus. относительная биологическая эффективность … Fizikos terminų žodynas
relative biologische Wirksamkeit — relative biologische Wirkung (f), relative biologische Wirksamkeit (f), RBW eng relative biological effectiveness, RBE … Arbeitssicherheit und Gesundheitsschutz Glossar