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

The experiment was performed in support of a Japanese initiative to investigate the biological effects 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 first 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 different 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 neutron-activated 56MnO2 powder. The following findings 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 significantly 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 effects in experimental mice and rats that result from dispersion of various levels of neutron-activated 56MnO2 powder, which is the subject of separate studies.


Introduction
Our experiments were performed in support of a Japanese initiative to investigate the biological effects of irradiation from residual neutron-activated radioactivity that resulted from the A-bombing (Hoshi 2020). During nuclear 1 3 explosions that take place in the atmosphere, neutronactivated radionuclides are distributed in surface layers of the soil, contributing to the beta and gamma irradiation that results from residual radioactivity. The main radionuclides are 24 Na, 28 Al, 31 Si, 32 P, 38 Cl, 42 K, 45 Ca, 46 Sc, 56 Mn, 59 Fe, 60 Co, and 134 Cs (Weitz 2014). Radionuclide 56 Mn (T 1/2 = 2.58 h) is one of the main neutron-activated emitters during the first hours after neutron activation of soil dust particles (Tanaka et al. 2008;Weitz 2014). The purpose of this international multicenter study was to extend our previous work Stepanenko et al. 2017) to estimate internal doses for laboratory animals (mice and rats) with different exposures to 56 MnO 2 in the form of dispersed powder. The results of the internal dose assessments will be used to investigate the biological effects that result from this type of exposure, which will be the subject of future publications. Table 1 gives details of the laboratory mice and rats used in the experiments and also the initial 56 Mn activity (100 mg 56 MnO 2 powder sprayed over the animals while they were in their cages).

Materials and methods
The total numbers of mice and rats targeted for dosimetry only were 24 and 9, respectively. Along with the animals scheduled for dosimetry, animals that were intended for subsequent biological studies were additionally placed in the same cages. As a result, the total number of animals in each cage for each irradiation was different, from 6 to 9 rats and from 3 to 10 mice per cage.
All experimental work was performed during 2018-2019 at research reactor IVG.1 ("Baikal-1") located in the territory of the Semipatinsk nuclear test site (Lanin 2013), Republic of Kazakhstan. Details of neutron activation of MnO 2 powder (Rare Metallic Co., Ltd) and exposure of animals to dispersed 56 MnO 2 particles were presented in our previous paper . Briefly, experimental animals were placed in special boxes for exposure to 56 MnO 2 powder (Fig. 1). One hundred milligrams of activated powder was used for each 56 MnO 2 exposure. Statistical distribution of MnO 2 particle sizes is presented in Fig. 2. Animals were exposed Table 1 Laboratory mice and rats (supplier: Kazakh Scientific Center of Quarantine and Zoonotic Diseases, Almaty, Kazakhstan under contract with Charles River Laboratories, Germany) and initial activity of neutron-activated 56 MnO 2 powder used for spraying over animals in each cage) As a result of irradiation of 100 mg MnO 2 by thermal neutrons with fluence F = 1.2 × 10 14 neutron/cm 2 , the yield of 56 Mn activity (Ao) is equal to 8 × 10 7 Bq. The ratio Ao/F is equal to 6.7 × 10 −7 Bq per thermal neutron/cm 2 **Numbers of animals in each cage for each irradiation were different-from 6 to 9 (rats) and from 3 to 10 (mice) animals per cage  The following organs and tissues were surgically extracted from experimental animals: lungs, heart, small intestine, large intestine, stomach, esophagus, liver, spleen, kidney, trachea, skin, eyes, and blood. To measure specific activity of 56 Mn, small pieces (about 1 ml) of each organ were weighed and subjected to gamma-spectrometry by an AMPTEC, Inc., Gamma-Rad5 spectrometer with an NaI(Tl) detector. Details of measurement conditions and calibration of the spectrometer were presented in our previous paper . A description of internal dose estimations according to the Medical Internal Radiation Dose methodology (Bolch et al. 2009) was presented in the same paper. According to MIRD methodology, internal radiation doses were assessed by taking into account accumulated activity of 56 Mn in all studied organs (which are listed above), self-irradiation of these organs, and their irradiation by all other sampled organs and tissues. Calculation of absorbed fractions of energy in studied organs from beta and gamma irradiation of 56 Mn was performed using the Monte-Carlo method (Briemeister 2000) and age-dependent mathematical phantoms of rats and mice . The spectrum of 56 Mn beta particles (Stabin et al. 2001) was accounted for internal dose calculations. Gamma irradiation from 56 Mn ) was accounted for as well.

Results
Each extracted sample of organs (lungs, heart, small intestine, large intestine, stomach, esophagus, liver, spleen, kidney, trachea, skin, eyes, and blood) from all investigated laboratory animals was subjected to gamma spectrometry in a well-shielded room. Volumes of extracted samples were small enough (about 1 ml) to consider them as radiating point sources in comparison with distance to and size of the spectrometer's detector. The highest 56 Mn specific activities were found in large and small intestine, stomach, lungs, and skin, which corresponds to our previous results obtained from similar experiments on

Discussion
In the present study, we found that under similar exposure conditions to 56 MnO 2 powder, the internal doses in mice were several times higher in comparison with rats. This can, perhaps, be explained by the following: higher breathing rate in mice versus rats and, lower organ weight in mice compared with rats (Besyadovsky et al. 1978). It should be noted that the latter circumstance leads to the fact that the specific absorbed fraction of energy (that is, fraction of absorbed energy per unit mass of the organ) is essentially higher in mice than in rats (see Figs. 6,7,8,9). Difference in doses of internal irradiation   of mice with the same activity of 56 MnO 2 powder dispersed over the experimental animals can be explained by the fact that the number of mice per cage was different during different irradiation sessions (see Table 3 with corresponding note). This can also explain the absence of a simple proportionality between the internal radiation doses and the dispersed activity of 56 MnO 2 (Tables 2 and 3). The increased doses in the lungs are explained by the fact that this organ is critical when inhaling small radioactive particles of 56 MnO 2 , which leads to an increased accumulation of activity in this organ. High doses of irradiation of the gastrointestinal tract can be explained by the fact that in the process of cleaning and grooming, experimental animals swallowed radioactive particles retained by their hair, which led to a high accumulation of activity in the stomach and intestines during exposure (1 h), as it was noted in Stepanenko et al. (2017), Shichijo et al. (2017). The retention of radioactive particles by animal hair leads to an increase in skin radiation dose.

Conclusion
This study aimed to estimate internal doses in laboratory animals (mice and rats) that had been exposed to various levels of 56 MnO 2 in the form of dispersed powder. The experiment was performed in support of the Japanese initiative to investigate the biological effects of irradiation from residual neutron-activated radioactivity that resulted from the A-bombing (Hoshi 2020;Roesch 1987;Imanaka et al. 2012;Kerr et al. 2013Kerr et al. , 2015Ohtaki et al. 2014). Radionuclide 56 Mn (T 1/2 = 2.58 h) is one of the main neutron-activated emitters during the first hours after neutron activation of soil dust particles.
In our previous studies Shichijo et al. 2017) related to irradiation of male Wistar rats after dispersion of 56 MnO 2 powder, the internal doses in rats were found to be very inhomogeneous: distribution of doses among different 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 neutron-activated 56 MnO 2 powder.
The following findings were noted: (a) Internal radiation doses in mice were several times higher in comparison with rats under similar conditions of exposure to 56 MnO 2 powder. (b) When 2.74 × 10 8 Bq of 56 MnO 2 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 significantly lower in organs of rats with the same activity of exposure to 56 MnO 2 powder (2.74 × 10 8 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 × 10 8 Bq of 56 MnO 2 (in comparison with exposure to 2.74 × 10 8 Bq of 56 MnO 2 ). (e) Internal radiation doses in organs of mice were 7-14 times lower with the lowest 56 MnO 2 amount (8.0 × 10 7 Bq) in comparison with the highest amount, 8.0 × 10 8 Bq, of dispersed 56 MnO 2 powder.
The data obtained will be used for interpretation of biological effects in experimental mice and rats that result from dispersion of various levels of neutron-activated 56 MnO 2 powder, which is the subject of separate studies.
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