Gamma rays biological effects

Relative Biological Effectiveness (RBE)—The RBE is a factor used to compare the biological effectiveness of absorbed radiation doses (i.e., rad) due to different types of ionizing radiation. More specifically, it is the experimentally determined ratio of an absorbed dose of a radiation in question to the absorbed dose of a reference radiation (typically 60Co gamma rays or 200 keV x rays) required to produce an identical biological effect in a particular experimental organism or tissue (see Quality Factor). 

RBE is used to denote the experimentally determined ratio of the absorbed dose from one radiation type to the absorbed dose of a reference radiation required to produce an identical biologic effect under the same conditions. Gamma rays from cobalt-60 and 200-250 keV x-rays have been used as reference standards. The term RBE has been widely used in experimental radiobiology, and the term quality factor used in calculations of dose equivalents for radiation safety purposes (ICRP 1977 NCRP 1971 UNSCEAR 1982). RBE applies only to a specific biological end point, in a specific exposure, under specific conditions to a specific species. There are no generally accepted values of RBE. 

A unit of dose equal to the amount of ionizing radiation that produces in humans the same biological effect as one rad of X-rays or gamma rays. One rem is equal to 0.01 sievert. 

The RBE of a given radiation quality relative to Co gamma rays is not a unique value but varies to a large extent with dose, biological system, and effect. 

In the case of radium, as well as any radionuclide, it is important to note that, in addition to the usual routes of exposure that must be considered (inhalation, oral, dermal, and occasionally parenteral) for toxic chemicals, there is also external and internal exposure to emissions of alpha and beta particles and gamma rays and it is these radioactive emissions which are considered to be responsible for most of the biologically deleterious effects observed in exposed persons. Further information about radionuclides is presented in Appendix B. 

Rem This older term is the short abbreviation for roentgen equivalent man. It is defined as the dose of radiation absorbed, multiplied by a modifying factor called quality factor Q, or relative biological effectiveness, RBE that takes into account the type of radiation (of any kind) and how the radiation is absorbed. Thus, for beta, gamma and X-rays, the quality factor is 1 for alpha radiation it may be as high as 20, and for neutrons it varies from 3 to 10. 

The relative biological effectiveness of neutrons. X-rays, and gamma-rays in the induction of opacities of the lens has been reported by Upton et al. Several species and strains were compared. Their results on the effects of 250-kVp X-rays or cyclotron neutrons (contaminated with gamma-rays to the extent of 5-15% of the dose) have been used. This set was chosen since... 

A. C. Upton, K. W. Christenberry, G. S. Melville, J. Furth, and G. S. Hurst, The Relative Biological Effectiveness of Neutrons, X-rays, and Gamma Rays for the Production of Lens Opacities Observations on Mice, Rats, Guinea-Pigs, and Rabbits, Radiol., 67, 686 (1956). 

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