Impact of Local High Doses of Radiation by Neutron Activated Mn Dioxide Powder in Rat Lungs: Protracted Pathologic Damage Initiated by Internal Exposure

Abstract

Internal radiation exposure from neutron-induced radioisotopes environmentally activated following atomic bombing or nuclear accidents should be considered for a complete picture of pathologic effects on survivors. Inhaled hot particles expose neighboring tissues to locally ultra-high doses of β-rays and can cause pathologic damage. ⁵⁵MnO₂ powder was activated by a nuclear reactor to make ⁵⁶MnO₂ which emits β-rays. Internal exposures were compared with external γ-rays. Male Wistar rats were administered activated powder by inhalation. Lung samples were observed by histological staining at six hours, three days, 14 days, two months, six months and eight months after the exposure. Synchrotron radiation—X-ray fluorescence—X-ray absorption near-edge structure (SR–XRF–XANES) was utilized for the chemical analysis of the activated ⁵⁶Mn embedded in lung tissues. ⁵⁶Mn beta energy spectrum around the particles was calculated to assess the local dose rate and accumulated dose. Hot particles located in the bronchiole and in damaged alveolar tissue were identified as accumulations of Mn and iron. Histological changes showed evidence of emphysema, hemorrhage and severe inflammation from six hours through eight months. Apoptosis was observed in the bronchiole epithelium. Our study shows early event damage from the locally ultra-high internal dose leads to pathogenesis. The trigger of emphysema and hemorrhage was likely early event damage to blood vessels integral to alveolar walls.