Answer to Question #647 Submitted to "Ask the Experts"
Category: Radiation Effects — Radiation Modifiers
The following question was answered by an expert in the appropriate field:
Could you please provide an explanation of relative biological effectiveness (RBE) as used in comparing the effects of differing types of radiation?
Radiations of different qualities have different degrees of effectiveness in producing effects in biological systems. When radiation is absorbed in biological material, the energy is deposited along the tracks of charged particles in a pattern that is characteristic of the type of radiation involved. After exposure to x or gamma rays, the ionization density would be quite low. After exposure to neutrons, protons, or alpha particles, the ionization along the tracks would occur much more frequently, producing a much denser pattern of ionizations.
If it took 200 mGy of x rays but only 20 mGy of neutrons to produce the same biological effect, the RBE would be 200/20 = 10 using x rays as the reference radiation. By normalizing the results to a single reference radiation, the value of the RBE provides a quantitative index of the effectiveness per unit of absorbed dose of any radiation. In radiobiology experiments, the investigator is free to normalize the results to a radiation of choice, usually x or gamma radiation. The scientific literature contains a broad range of results of this type (NCRP 1990). For radiation protection purposes, the International Commission on Radiological Protection, ICRP, has described the effectiveness of radiations of differing qualities by a series of Quality Factors (ICRP 1977) and more recently by a series of Radiation Weighting Factors (ICRP 1991). The Radiation Weighting Factors currently being used in the ICRP's system of radiation protection are shown in the following table. The Commission chose a value of 1 for all radiations having low energy transfer (sparsely ionizing), including x and gamma radiations of all energies. The other values were selected as being broadly representative of the results observed in biological studies, particularly those dealing with cancer and hereditary endpoints.
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