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

Abstract

The relatively high radiosensitivity of the lens of the eye to neutron radiation is now well established (Evans, 1948; Ham, 1953). In addition to experimental cataracts obtained in several species of neutron-irradiated laboratory animals, lens opacities have been observed in human beings exposed to cyclotron neutrons (Abelson and Kruger, 1949; Dollfus, 1950; Krause and Bond, 1951) and to fission neutrons from atomic detonations (Cogan et al., 1949; Sinskey, 1955). The growing prevalence in our environment of neutron sources such as high-voltage generators (cyclotrons) and reactors makes it increasingly important to know how much neutron radiation the human lens can tolerate. Furthermore, if neutrons are more effective than electromagnetic waves in causing lens opacities, precise comparison of the cataractogenic effects of these radiations should be made to further the knowledge of their fundamental radiobiologic differences. This investigation was undertaken in an attempt to obtain quantitative information about the relative biological effectiveness (RBE) of neutrons, x-rays, and gamma rays for the production of lens opacities in several species of mammals. Methods And Results Cyclotron Fast Neutrons: Mice, rats, and guinea-pigs were exposed in the Oak Ridge 86-inch cyclotron to neutrons having a mean effective energy of 1—2 MEV, obtained by the Be(p,n) reaction; the exposure facility and methods of dosimetry have been described in detail (Hurst et al., 1956; Upton et al, 1956). The cyclotron radiations were administered at a rate of 60 to 125 rep/min., contaminating gamma rays constituting approximately 5 to 15 per cent of the total dose in rep. Male and female mice of the RF strain, eight to fourteen weeks old, rats of a Wistar subline fifteen to eighteen weeks of age, and guinea-pigs5 weighing 350 to 450 gm. were exposed in groups of 10 to 25 per dose level. After irradiation, the survivors were examined periodically for lens opacities with a Zeiss-Opton slit lamp, and the observations were scored according to a grading system described previously (Christen-berry and Furth, 1951). For purposes of comparison, animals of corresponding age, sex, species, and strain were exposed to x-rays, with the following irradiation factors: 250 kvp, 30 ma, T.S.D. 93.7 cm., 70 to 80 r/min., filtration 3 mm. Al (Be window), h.v.l. 0.55 mm. Cu. RF mice were also exposed to Co60 gamma rays (1.1–1.3 MEV) at a rate of approximately 57 r/min.; the methods of exposure have been described (Upton et al., 1956). Unless otherwise indicated, doses of radiation were administered in a single brief exposure of the whole body. Lens opacities first became detectable within three to four months following irradiation, the rapidity of development and final severity of the opacities varying with the dose (Figs. 1–3).