The Relative Biological Effectiveness of X-rays and Gamma Rays

Introduction THE effect of ionizing radiations in a living cell depends on three physical factors: (1) The intensity of the radiation at the cell (considered as a point); (2) the time during which the cell is exposed to the radiation, and (3) the quality of the radiation reaching the cell. Theoretically the biological effect should be influenced by changes in any one, any two, or all of the above factors. If this conclusion is to be tested experimentally, one must be able to measure the biological effect as well as the intensity, duration of exposure, and quality of the radiation. Furthermore, if it is desired to investigate the influence of only one of the three factors, it is essential that the other two be kept constant, and that all other conditions remain the same. This principle was strictly adhered to in the biological experiments to be described in the present paper, only the quality of radiation being changed. “Biological effect” is a general expression which should be used with considerable care. Ionizing radiations are capable of producing an infinite number of biological effects. Any quantitative relationships which may be obtained experimentally refer only to the particular biological effect investigated, and are not necessarily true for all biological effects. For this reason our experiments were carried out with more than one biological material, and the observed effect, or “end point,” was also different. In attempting to determine the influence of one of the three physical factors on a particular biological effect, it is advantageous to vary the factor within the widest practical limits. It is then possible to detect differences which otherwise might be masked by experimental errors, which, unfortunately, are always of considerable magnitude in biological experiments. For this reason gamma rays, and X-rays produced at 165 K.V., with a filter of 2.04 mm. Cu, were used in our experiments. The quality factor in this case is very different, and yet no complications due to uneven distribution of radiation in the material are introduced. In order to fulfill the requirement of equality of the other physical factors, it is necessary to base the measurements of the intensity of X-rays and gamma rays on some effect which both are capable of producing (other than the one under investigation). As such an effect, one might choose the blackening of a photographic plate, the change in a chemical compound, or even a biological change. But in view of the universal adoption of air ionization as the effect by which X-rays are measured, it is desirable to use this effect for both qualities of radiation. Furthermore, it is desirable to express the measurements in terms of the international unit of X-radiation. The measurement of gamma rays in roentgens by direct means involves certain experimental difficulties which account for the fact that no data can be obtained from the literature.