Radiosensitivity varies depending on the cell type; highly differentiated cells typically exhibit greater radioresistance. We recently demonstrated that human macrophages derived from THP-1 monocytic cells, which lack TP53, are highly resistant to radiation-induced apoptosis compared with undifferentiated THP-1 cells. However, the mechanisms by which THP-1 cells acquire radioresistance during differentiation remain unknown. Herein, we investigated the mechanisms by which THP-1-derived macrophages develop p53-independent radioresistance by analyzing DNA damage responses and apoptotic pathways. Analysis of γ-H2AX foci, which indicates the formation of DNA double-strand breaks (DSB), suggested that a capacity to repair DSB of macrophages is comparable to that of radiosensitive THP-1 cells. Furthermore, treatment with inhibitors against DSB repair-related proteins failed to enhance radiation-induced apoptosis in THP-1-derrived macrophages. Analysis of the apoptotic pathways showed that radiosensitive THP-1 cells undergo apoptosis through the caspase-8/caspase-3 cascade after irradiation, whereas this was not observed in the macrophages. Caspase-8 protein expression was lower in macrophages than in THP-1 cells, whereas mRNA expressions were comparable between both cell types. Co-treatment with a proteasome inhibitor and ionizing radiation effectively induced apoptosis in macrophages in a caspase-8-dependent manner. Results suggest that the regulation of caspase-8-mediated apoptosis during differentiation plays a role in the p53-independent radioresistance of THP-1-derived macrophages.

The Japanese government formulated the Nuclear Emergency Response Guidelines in response to the Fukushima Daiichi Nuclear Power Plant accident (FDNPP accident) caused by the Great East Japan Earthquake in March 2011. Under these guidelines, Japan has established its current nuclear disaster response system. This manuscript outlines the transition of Japan's nuclear disaster response system before and after the FDNPP accident and also shows the results of a questionnaire survey on the level of preparation the prefecture currently has for the evacuation of residents at the time of a nuclear disaster. About 70% of the prefectures where nuclear facilities are located or adjacent have completed or are in the process of completing evacuation plans, and all except one indicated they have the equipment needed to perform radiation contamination inspections of residents. These results suggest that activities are taking place throughout Japan to build a new disaster response system. It will be important to verify whether the evacuation manuals prepared by prefectural governments are effective through large-scale training and to develop human resources for performing radiation contamination inspections of evacuating residents.

Ten years have elapsed since the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, and the relative contribution of natural radiation is increasing in Fukushima Prefecture due to the reduced dose of artificial radiation. In order to accurately determine the effective dose of exposure to artificial radiation, it is necessary to evaluate the effective dose of natural as well as artificial components. In this study, we measured the gamma-ray pulse-height distribution over the accessible area of Namie Town, Fukushima Prefecture, and evaluated the annual effective dose of external exposure by distinguishing between natural and artificial radionuclides. The estimated median (range) of absorbed dose rates in air from artificial radionuclides as of 1 April 2020, is 133 (67–511) nGy h−1 in the evacuation order cancellation zone, and 1306 (892–2081) nGy h−1 in the difficult-to-return zone. The median annual effective doses of external exposures from natural and artificial radionuclides were found to be 0.19 and 0.40 mSv in the evacuation order cancellation zone, and 0.25 and 3.9 mSv in the difficult-to-return zone. The latest annual effective dose of external exposure discriminated into natural and artificial radionuclides is expected to be utilized for radiation risk communication.

No abstract

This paper does not necessarily reflect the views of the International Commission on Radiological Protection. Several radiation monitoring research projects are underway on dose assessment, biological analysis, and risk communication under an agreement with Namie Town. Indoor radon and thoron progeny concentrations have been measured using passive-type monitors to estimate internal doses due to inhalation. In addition, airborne radiocaesium concentrations at five points in Namie Town have been analysed using a high-purity germanium detector to estimate internal doses for comparison with radon. External radiation doses from natural and artificial radionuclides have also been estimated using an in-situ gamma-ray spectrometer. Other support activities are mentioned briefly in this article,