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. By irradiating batches of cells with different doses and types of radiation, a relationship between dose and the fraction of cells that die can be found, and then used to find the doses corresponding to some common survival rate. The ratio of these doses is the RBE of
R. Instead of 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 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 rays, 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 or
cobalt-60 gamma rays. 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. According to one source it may be much higher (6500 with X rays as the reference) on ovocytes. 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. 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 recoil of the parent-nucleus of the decaying atom typically carries only about 2% of the energy of the alpha-particle that is emitted by the decaying atom, its range is extremely short (about 2–3 angstroms), due to its high
electric charge and high
mass. The parent nucleus is required to recoil, upon emission of an
alpha particle, with a discrete kinetic energy due to
conservation of momentum. Thus, all of the ionization energy from the recoil-nucleus is deposited in an extremely small volume near its original location, typically in the cell nucleus on the chromosomes, which have an affinity for heavy metals. The bulk of studies, using sources that are external to the cell, have yielded RBEs between 10 and 20. Since most of the ionization damage from the travel of the alpha particle is deposited in the
cytoplasm, whereas from the travel of the recoil-nucleus is on the DNA itself, it is likely greater damage is caused by the recoil nucleus than by the alpha particle itself. ==History==