Answer to Question #647 Submitted to "Ask the Experts"

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.

These differences in density of ionizations are a major reason that neutrons, protons, and alpha particles produce more biological effects per unit of absorbed radiation dose than do more sparsely ionizing radiations such as x rays, gamma rays, or electrons (Hall 1988). Other factors that contribute to these differences include the energy of radiation used, the dose received and the temporal pattern in which it was received, and the particular biological endpoint being studied. Many scientific investigations have been conducted to study the differing effectiveness of radiations under different experimental conditions. Analysis of the Relative Biological Effectiveness, RBE, is a useful way to compare and contrast the results observed in these studies. The relative biological effectiveness for a given test radiation, is calculated as the dose of a reference radiation, usually x rays, required to produce the same biological effect as was seen with a test dose, D_T, of another radiation. Thus, for the same biological endpoint,

If it took 200 mGy of x rays but only 20mGy 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.

References

1. Hall, E.J. (1988) Radiobiology for the Radiologist, J.B. Lippincott Co., Philadelphia.
   
   
2. ICRP (1977) Recommendations of the International Commission on Radiological Protection, Ann ICRP 21, Nos. 1-3.
   
   
3. ICRP (1991) Recommendations of the International Commission on Radiological Protection, Ann. ICRP 1, No. 3.
   
   
4. NCRP (1990) The Relative Biological Effectiveness of Radiations of Different Quality, NCRP Report No. 104, National Council on Radiation Protection and Measurements, Bethesda, MD.

Bruce Boecker,CHP