Internal exposure to neutron-activated ⁵⁶Mn dioxide powder in Wistar rats-Part 2: pathological effects

To fully understand the radiation effects of the atomic bombing of Hiroshima and Nagasaki among the survivors, radiation from neutron-induced radioisotopes in soil and other materials should be considered in addition to the initial radiation directly received from the bombs. This might be important for evaluating the radiation risks to the people who moved to these cities soon after the detonations and probably inhaled activated radioactive "dust." Manganese-56 is known to be one of the dominant radioisotopes produced in soil by neutrons. Due to its short physical half-life, ⁵⁶Mn emits residual radiation during the first hours after explosion. Hence, the biological effects of internal exposure of Wistar rats to ⁵⁶Mn were investigated in the present study. MnO₂ powder was activated by a neutron beam to produce radioactive ⁵⁶Mn. Rats were divided into four groups: those exposed to ⁵⁶Mn, to non-radioactive Mn, to ⁶⁰Co γ rays (2 Gy, whole body), and those not exposed to any additional radiation (control). On days 3, 14, and 60 after exposure, the animals were killed and major organs were dissected and subjected to histopathological analysis. As described in more detail by an accompanying publication, the highest internal radiation dose was observed in the digestive system of the rats, followed by the lungs. It was found that the number of mitotic cells increased in the small intestine on day 3 after ⁵⁶Mn and ⁶⁰Co exposure, and this change persisted only in ⁵⁶Mn-exposed animals. Lung tissue was severely damaged only by exposure to ⁵⁶Mn, despite a rather low radiation dose (less than 0.1 Gy). These data suggest that internal exposure to ⁵⁶Mn has a significant biological impact on the lungs and small intestine.