The Nagasaki Atomic Bomb Survivors' Tumor Tissue Bank

Establishment and activity of the planning and acting network for low dose radiation research in Japan (PLANET): 2016-2023

The Planning and Acting Network for Low Dose Radiation Research in Japan (PLANET) was established in 2017 in response to the need for an all-Japan network of experts. It serves as an academic platform to propose strategies and facilitate collaboration to improve quantitative estimation of health risks from ionizing radiation at low-doses and low-dose-rates. PLANET established Working Group 1 (Dose-Rate Effects in Animal Experiments) to consolidate findings from animal experiments on dose-rate effects in carcinogenesis. Considering international trends in this field as well as the situation in Japan, PLANET updated its priority research areas for Japanese low-dose radiation research in 2023 to include (i) characterization of low-dose and low-dose-rate radiation risk, (ii) factors to be considered for individualization of radiation risk, (iii) biological mechanisms of low-dose and low-dose-rate radiation effects and (iv) integration of epidemiology and biology. In this context, PLANET established Working Group 2 (Dose and Dose-Rate Mapping for Radiation Risk Studies) to identify the range of doses and dose rates at which observable effects on different endpoints have been reported; Working Group 3 (Species- and Organ-Specific Dose-Rate Effects) to consider the relevance of stem cell dynamics in radiation carcinogenesis of different species and organs; and Working Group 4 (Research Mapping for Radiation-Related Carcinogenesis) to sort out relevant studies, including those on non-mutagenic effects, and to identify priority research areas. These PLANET activities will be used to improve the risk assessment and to contribute to the revision of the next main recommendations of the International Commission on Radiological Protection.

Autoradiographic analysis of internal plutonium radiation exposure in Nagasaki atomic bomb victims

Background

Radiation doses received by Hiroshima and Nagasaki atomic bomb survivors has been evaluated from data related only to external exposure because there was no reliable evidence for internal exposure in atomic bomb victims. However, we assumed that the contribution of internal exposure cannot be ruled out.

Methods

Autoradiography was carried out with the 70-year old paraffin-embedded specimens taken from Nagasaki atomic bomb victims who died within 5 months after the bombing. After exposure to photo emulsion for 6 months alpha-tracks were revealed in the specimens. We confirmed the alpha-tracks were emitted from deposited plutonium (Pu) in reference to the track length of the 8.787 MeV alpha-particle of thorium series from Polonium-212. Radioactivity concentration of Pu was obtained by counting alpha-tracks. The absorbed dose of each cell nucleus penetrated by an alpha-particle was estimated by calculating the absorbed energy from the particle.

Results

Using old paraffin embedded sections processed about 70 years ago, we demonstrated for the first time that conditions in the aftermath of the bombing led to internal exposure to alpha-particles emitted from Pu, the fissile material of the Nagasaki atomic bomb. Dose rate of internal exposure was higher in the victims exposed outdoors than those indoors. Radioactivity concentration was relatively uniform among organs examined in a victim.

Conclusion

Pu was deposited in the bodies of the Nagasaki A-bomb victims presumably via various routes. Organ dose from Pu of the Nagasaki A-bomb victims studied was during their surviving period, which is lower compared with external exposure. However, the impact to the individual cell nucleus by a single alpha-particle might not be negligible, It would be meaningful; to analyze the relationship of the impact of internal exposure at the cellular level and organ dose. The 70-year old pathological specimens utilized in our study are an invaluable source for understanding internal radiation exposure and are crucial in elucidating experimentally unreproducible phenomena.