In radiobiology, 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 amount of absorbed energy. The RBE is an empirical value that varies depending on the type of ionizing radiation, the energies involved, the biological effects being considered such as cell death, and the oxygen tension of the tissues or so-called oxygen effect.
Application The absorbed dose can be a poor indicator of the biological effect of ionizing radiation, as the biological effect can depend on many other factors, including the type of radiation, energy, and type of tissue. The relative biological effectiveness can help give a better measure of the biological effect of ionizing radiation. The relative biological effectiveness for ionizing radiation of type R on a tissue is defined as the ratio
RBE = D X D R {\displaystyle {\text{RBE}}={\frac {D_{\text{X}}}{D_{\text{R}}}}} . where D X {\textstyle D_{\text{X}}} is the reference absorbed dose of ionizing radiation of type X and D R {\textstyle D_{\text{R}}} 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. 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 (LET), meaning that they ionize atoms in the tissue that are spaced by several hundred nanometers (several tenths of a micrometer) 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). RBEs can be used for either cancer/hereditary risks (stochastic) or for harmful tissue reactions (deterministic) effects. Tissues have different RBEs depending on the type of effect. For high LET radiation (i.e., alphas and neutrons), the RBEs for deterministic effects tend to be lower than those for stochastic effects. 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 weapons, uranium mining, and ionizing radiation safety.
Relation to radiation weighting factors
For the purposes of computing the equivalent dose to an organ or tissue, the International Commission on Radiological Protection (ICRP) has defined a standard set of radiation weighting factors WR (formerly termed the quality factor Q). The radiation weighting factors convert absorbed dose (measured in SI units of grays or non-SI rads) into formal biological equivalent dose for radiation exposure (measured in units of sieverts or rem). However, ICRP states:The quantities equivalent dose and effective dose should not be used to quantify higher radiation doses or to make decisions on the need for any treatment related to tissue reactions [i.e., deterministic effects]. For such purposes, doses should be evaluated in terms of absorbed dose (in gray, Gy), and where high-LET radiations (e.g., neutrons or alpha particles) are involved, an absorbed dose, weighted with an appropriate RBE, should be used.Radiation weighting factors are largely based on the RBE of radiation for stochastic health risks. However, for simplicity, the radiation weighting factors are not dependent on the type of tissue, and the values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types, with respect to external sources (i.e., to the cell). Radiation weighting factors have not been developed for internal sources of heavy ions, such as a recoil nucleus. The ICRP 2007 standard values for relative effectiveness are given below. The higher radiation weighting factor WR for a type of radiation, the more damaging it is, and this is incorporated into the calculation to convert from units of gray to sievert.
Radiation weighting factors that go from physical energy to biological effect must not be confused with tissue weighting factors. The tissue weighting factors are used to convert an equivalent dose to a given tissue in the body, to an effective 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.
Experimental methods
Typically the evaluation of relative biological effectiveness is performed on various types of living cells grown in a culture medium, including prokaryotic cells such as bacteria, simple eukaryotic cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats. By irradiating batches of cells with different absorbed doses and types of radiation, a relationship between absorbed dose and the fraction of cells that die can be found, and then used to find the absorbed doses corresponding to some common survival rate. The ratio of these absorbed doses is the RBE of R. Instead of cell death, the endpoint might be the fraction of cells that 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-rays and gamma radiation (both consisting of photons), alpha radiation (helium-4 nuclei), beta radiation (electrons and positrons), 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.
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