The Biological Roentgen1

Abstract

MESUREMENT of the inten-I sity of x- and gamma-rays by means of living organisms and tissues as indicators of dosage has been a common practice for many years. As early as 1915, Ritter correlated the amount of effect of x-rays on growing seedlings with doses measured by the pastilles then in use. Later Jüngling employed both beans and mice for similar measurements. The mice and other large laboratory animals proved unsatisfactory for the purpose because of complications which were not then understood. To simplify the experiment and to obtain clear- cut results, cells and small organisms have been found more useful. In these, scattered radiation can be disregarded; the entire organism is uniformly irradiated, and the complications met with when a small part of a large body is exposed are absent. The variety of biological materials which have been used in these experiments is surprising. Bacteria, yeasts, and spores of various kinds; algæ and seeds of different sorts; protozoa, the eggs of worms, insects, frogs, and salamanders; tadpoles, chick embryos, and larval tissues; water fleas; tissue cultures and tumor particles—all of these have yielded valuable information for the solution of radiological problems. The reaction which is most often used as a criterion of the effect of radiations involves a fundamental characteristic of living protoplasm, namely, the power of growth and repair. A sufficient dose alters the rate and direction of growth. Cell division is slowed down, with the result that development is stunted or abnormal; or it may be completely stopped by death. Even in adult tissues the effect which is measured is of the same order. Radiations retard the process of regeneration, so that damaged cells cannot be replaced by healthy new growth, and injury ensues. This change in the rate and direction of growth is most easily measured in small, actively growing cells and organisms. The relation between dose and amount of effect is illustrated by experiments on the eggs of the fruit fly, Drosophila. The criterion of effect is the proportion of eggs in a sample which survive and hatch as larvse. In Figure 1 are shown the results of more than two hundred tests in which various doses, measured in roentgens, were administered under definite experimental conditions. It is clear that the percentage of eggs surviving after each dose is fairly constant. From the curve, drawn to fit the data, one may determine the survival percentages which may be expected to follow any dose within the range shown in the figure. The same procedure is carried out when other kinds of test objects are used. Thus, the ratios between the number of mitoses in irradiated tissue culture preparations and in the controls, or the proportion of bacteria which form colonies after receiving definite doses of radiation can be expressed by a similar graph. The curve shows that the amount of effect differs greatly among similar individuals. Some are killed or injured by small doses, while others remain alive even after heavy doses.