Internal dose assessment of a hibakusha survivor via autoradiography of paraffin-embedded specimens

Background: Evaluation of radiation doses for Hiroshima and Nagasaki atomic bomb (A-bomb) survivors focused primarily on external exposure due to the lack of reliable evidence regarding internal exposure. However, the contribution of internal exposure remains uncertain. This study aimed to detect persistent uranium-235 alpha radiation in lung tissue of a Hiroshima survivor and assess its pathological significance following inhalation exposure. Methods: Autoradiography was performed on paraffin-embedded specimens from a Hiroshima A-bomb survivor with lung cancer who was not in Hiroshima at the time of the bombing but moved to the city 3 days later (at 8 years of age) and survived for 70 years after the bombing. After a 10-month photoemulsion exposure, we confirmed that alpha tracks were emitted from deposited uranium-235 (235U) by comparing track lengths with those of 8.785-MeV polonium-212 alpha particles from the thorium series. Radioactivity concentration was determined by counting alpha tracks. Tumor tissue-absorbed doses from alpha particles were estimated. Results: Paraffin-embedded sections from the survivor, processed 70 years post-bombing, revealed internal exposure to alpha particles emitted from 235U, the fissile material of the Hiroshima bomb. The highest internal exposure was observed in tumor tissue with fibrosis, followed by tumors adjacent to charcoal deposition, tumors without fibrosis, and normal tissue. Cavities with diameters approximately twice the length of the 235U tracks were identified in the lung tumors. 235U persisted in the survivor's body for 70 years. The organ dose from 235U was lower than that from external exposure; however, individual cell nuclei may still have been significantly affected. Radioactive materials persisted in the body, contributing to internal radiation exposure. Conclusions: The study findings indicate the importance of analyzing the relationship between the impact of internal exposure at the tumor cellular level and organ dose. Pathological specimens from survivors provide invaluable insights into internal exposure and carcinogenesis.