Internal doses in experimental mice and rats following exposure to neutron‑activated 56MnO2 powder: results of an international, multicenter study

Abstract

The experiment was performed in support of a Japanese initiative to investigate the biological efects of irradiation from residual neutron-activated radioactivity that resulted from the A-bombing. Radionuclide 56Mn (T1/2=2.58 h) is one of the main neutron-activated emitters during the frst hours after neutron activation of soil dust particles. In our previous studies (2016–2017) related to irradiation of male Wistar rats after dispersion of 56MnO2 powder, the internal doses in rats were found to be very inhomogeneous: distribution of doses among diferent organs ranged from 1.3 Gy in small intestine to less than 0.0015 Gy in some of the other organs. Internal doses in the lungs ranged from 0.03 to 0.1 Gy. The essential pathological changes were found in lung tissue of rats despite a low level of irradiation. In the present study, the dosimetry investigations were extended: internal doses in experimental mice and rats were estimated for various activity levels of dispersed neutronactivated 56MnO2 powder. The following fndings were noted: (a) internal radiation doses in mice were several times higher in comparison with rats under similar conditions of exposure to 56MnO2 powder. (b) When 2.74×108 Bq of 56MnO2 powder was dispersed over mice, doses of internal irradiation ranged from 0.81 to 4.5 Gy in the gastrointestinal tract (small intestine, stomach, large intestine), from 0.096 to 0.14 Gy in lungs, and doses in skin and eyes ranged from 0.29 to 0.42 Gy and from 0.12 to 0.16 Gy, respectively. Internal radiation doses in other organs of mice were much lower. (c) Internal radiation doses were signifcantly lower in organs of rats with the same activity of exposure to 56MnO2 powder (2.74×108 Bq): 0.09, 0.17, 0.29, and 0.025 Gy in stomach, small intestine, large intestine, and lungs, respectively. (d) Doses of internal irradiation in organs of rats and mice were two to four times higher when they were exposed to 8.0×108 Bq of 56MnO2 (in comparison with exposure to 2.74×108 Bq of 56MnO2). (e) Internal radiation doses in organs of mice were 7–14 times lower with the lowest 56MnO2 amount (8.0×107 Bq) in comparison with the highest amount, 8.0×108 Bq, of dispersed 56MnO2 powder. The data obtained will be used for interpretation of biological efects in experimental mice and rats that result from dispersion of various levels of neutron-activated 56MnO2 powder, which is the subject of separate studies.