Relative biological effectiveness

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 tissues. 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 angstroms 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" (W_R) that enters in the conversion of units of absorbed energy (such as rads and grays), a simple measureable physical quantity, to units of biological equivalent dose for radiation exposure (such as rems and sieverts, 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 weapons, uranium mining, 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 ratio

where D_X is a reference absorbed dose of radiation of a standard type X, and D_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.

Experimental methods

Typically the evaluation of relative biological effectiveness is done on various types of living cells grown in 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. 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-rays and gamma radiation (both consisting of photons), alpha radiations (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.

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, 4–6 for simple eukaryotic cells, 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^[1]. 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.

Standardization

To bypass the complexity of tissue dependence, the International Commission on Radiological Protection (ICRP) defined standard RBE values, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry.^[2] 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.

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.

See also

• Background radiation

References

External links

• Relative Biological Effectiveness in Ion Beam Therapy