Michele Maria Bianchi

Lens Opacification in Mice Exposed to 14-MeV Neutrons

The induction of lens opacification in mice exposed to 14-MeV neutrons was evaluated by counting the number of opacities observed in the posterior region of the lens using a slit-lamp microscope. The incidence of cataract formation in the anterior region of the lens was also observed. Serial observations were carried out from 3 up to 25 months after irradiation. Male hybrid mice (C54B1 X C3H), 75 days old at the time of the exposure, have been used. A linear progression of opacities was observed with age at a rate depending on the dose. By comparison with 250-kVp X rays the RBE was found to increase, with decreasing dose, from 9 to 21. The incidence of opacification in the anterior region of the lens gives an RBE value of 8. Split-dose irradiations were carried out with an interval of 24 hr between doses. There was very little difference in the number of opacities observed after single or fractionated neutron doses of approximately the same total amount. In contrast, fractionation of the X-ray dose reduced the effectiveness of the irradiation.

Biological responses to various neutron energies from 1 to 600 mev. II. Lens opacification in mice.

Eye lens opacification in mice has been used to evaluate the RBE of neutrons of 1, 5, and 600 MeV. The experimental observations were based on counting the points of opacities appearing in the posterior region of the lens. RBE values have been calculated with reference to the equal effects observed after exposure to 250-kVp X rays. Data previously reported for 15- and 400-MeV neutrons have also been included. For all neutron energies, the RBE increases with decreasing dose. In addition, at very low doses the RBE appears to reach about the same value independently of the neutron energy used. In contrast, for higher dose values the RBE decreases with increasing neutron energy from about 24 for 1 MeV to about 6 for the 600-MeV neutrons.

Biological responses to various neutron energies from 1 to 600 mev. I. Testes weight loss in mice.

Testes weight loss in mice has been used as a test system to determine the relative biological efficiency (RBE) of neutrons ranging in energy from fission to a beam of 600 MeV nominal energy. The RBE values, calculated as equal effect dose ratios show a maximum at the neutron energy of 1 MeV. No significant dependence was found on the level of absorbed dose. Analyses of the dose-effect relations obtained suggest the existence of a radiation-insensitive component of the testes that accounts for 32.5% of the total weight. By considering only the radiosensitive component, the X-ray data show an exponential dose-effect relation. When the same correction is applied to the neutron data, dose-effect curves with an extrapolation number less than one are obtained for some neutron energies.

Observations on Late Effects in Mice Exposed to 400 MeV Neutrons

Life-long observations on mortality and pathology at death were carried out on groups of mice irradiated with 250 kV X-rays or exposed to a 400 MeV neutron beam, both directly and after attenuation corresponding to the maximum dose build-up region, at comparable dose-rates. Doses up to 84 rad of 400 MeV neutrons and up to 200 rad of X-rays showed no effect on the longevity of the animals, which suggest an upper limit to the r.b.e. for life-shortening of approximately 2-5. Similar conclusions were drawn from the data on all types of leukemias. For all other neoplasms, the age-specific death-rate showed a similar shortening of the latency times for groups of mice irradiated with 0-84 rad of 400 MeV direct neutrons and 0-400 rad of X-rays, also suggesting an upper limit to the r.b.e. slightly higher than that previously indicated for life-shortening. No definite effect was observed after exposure to the attenuated neutron beam at the doses used in these experiments.