Planning and Acting Network for Low Dose Radiation Research (PLANET)
Notice: Recent activities of PLANET are summarized and published in Journal of Radiation Research.
Establishment of PLANET
The mission of the Institute for Radiological Science (NIRS), a division of the National Institutes for Quantum Science and Technology (QST), includes radiation effects research, radiation protection research, and radiation emergency medicine in response to nuclear disasters and other situations. In radiation effects research, an important issue is to reduce the uncertainty in risk estimation, especially for low-dose and low-dose-rate radiation exposure.
To steadily and continuously conduct research for this issue, it is necessary to establish an all-Japan network of academia and research institutions that facilitates collaboration among relevant parties, including regulatory authorities. NIRS established a Preparatory Committee on such a network in 2016, and the Preparatory Committee compiled a report (here) on the network. The network is named the Planning and Acting Network for Low Dose Radiation Research (PLANET). PLANET prioritizes research needs taking into account the potential of Japan and proposes strategies to improve the estimation of low-dose and low-dose-rate radiation risks. PLANET also proposes a support system for cooperation and collaboration with relevant researchers and research institutions in Japan and aims to promote collaboration with international organizations.
Establishment of the Working Group on Dose-Rate Effects of
Animal Experiments (WG1)
The PLANET Steering Committee was established in 2017 and is considering specific research tasks with the aim of advancing the five priority issues identified by the Preparatory Committee. The tasks include epidemiological studies of the health effects of low-dose and low-dose-rate radiation, elucidation of biological mechanisms, bridging animal experiments and epidemiology, modifying effects of age, sex, genetic predisposition, and lifestyle, and archiving of data. The Steering Committee established the Working Group on Dose-Rate Effects of Animal Experiments (WG1) to extract and summarize specific findings on dose-rate effects from animal experiments.
WG1 Output 1; Estimation of DDREF from animal experimental data
To obtain estimates of the dose rate effectiveness factor (DREF), WG1 analyzed cancer mortality data of B6C3F1 female mice chronically and acutely exposed to Cs-137 γ-rays, as reported by NIRS and the Institute for Environmental Sciences, Japan. This study applied a model that accounts for differences in risk depending on age at exposure, suggesting a significantly reduced risk in mice exposed to 21 mGy per day compared to mice exposed to acute irradiation. Consequently, under the conditions of the present analysis, the DREF was greater than 1 and approximately 3 when adjusted for age dependence of susceptibility (Doi et al., Radiat. Res., 2020).
WG1 Output 2; Review of experimental studies on low dose rate radiation effects
WG1 also reviews papers on low dose rate effect studies in animal models. Epidemiological data have contributed significantly to the estimation of dose and dose rate effectiveness factor (DDREF) for human populations. Studies using animal models, on the other hand, have contributed significantly to providing qualitative and quantitative data on the magnitude and mechanisms of risk. This review examines animal model studies related to cancer development and organizes the findings with a focus on how the underlying biological mechanisms of carcinogenesis are involved in the dose-rate effects of radiation-induced carcinogenesis (Suzuki et al., J. Radiat. Res., 2023).
WG1 Output 3; Risk analysis using mathematical models describing carcinogenic mechanisms
WG1 recently employed a mathematical model of multistage carcinogenesis to analyze existing radiation-related cancer mortality data from Japanese atomic bomb survivors and lifespan mouse experiments. Analyses show that radiation exposure chronologically shifts the age-related increase in cancer risk forward. Time shift per dose was ~100-fold greater in humans, which was explained by the ~100-fold interspecies difference in somatic mutation rates. Confirming the generality of this finding will make it possible to use the data obtained in mice more rationally for human risk assessment (Imaoka et al., Int. J. Cancer, 2024).
Updating strategic research agenda and establishment of new working groups
Taking into account international trends in the field of low-dose radiation effects research and the situation in Japan, PLANET published a report (find here [only in Japanese]) and summarised the priority research areas in Japan into (1) characterization of low-dose and low-dose-rate radiation risk, (2) factors to be considered for individualization of radiation risk, (3) biological mechanisms of low-dose and low-dose-rate radiation effects and (4) integration of epidemiology and biology in 2023. PLANET has established the following three working groups to examine issues in these research areas; 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 ICRP.
Steering Committee (Updated in September 2024)
| Kai, Michiaki (Head) | Nippon Bunri University |
| Imaoka, Tatsuhiko | QST |
| Iwasaki, Toshiyasu | Central Research Institute of Electric Power Industry |
| Kobayashi, Junya | International University of Health and Welfare |
| Kobayashi, Toshiyuki | Institute for Environmental Sciences |
| Misumi, Munechika | Radiation Effects Research Foundation |
| Ohira, Tetsuya | Fukushima Medical University |
| Sakai, Kazuo | Tokyo University of Health Sciences, Radiation Effects Association |
| Sugihara, Takashi | Institute for Environmental Sciences |
| Suzuki, Keiji | Nagasaki University |
| Tauchi, Hiroshi | Ibaraki University |
| Yasuda, Hiroshi | Hiroshima University |
| Yoshinaga, Shinji | Hiroshima University |
WG1 (Dose Rate Effects in Animal Experiments) 2024
| Suzuki, Keiji (Head) | Nagasaki University |
| Doi, Kazutaka | QST |
| Imaoka, Tatsuhiko | QST |
| Sasatani, Megumi | Hiroshima University |
| Tomita, Masanori | Central Research Institute of Electric Power Industry |
WG2 (Dose and Dose-Rate Mapping for Radiation Risk Studies) 2024
| Sugihara, Takashi (Head) | Institute for Environmental Sciences |
| Imaoka, Tatsuhiko | QST |
| Kohda, Jun | Institute for Environmental Sciences |
| Sasatani, Megumi | Hiroshima University |
| Tauchi, Hiroshi | Ibaraki University |
| Matsumoto, Yoshihisa | Tokyo Institute of Technology |
| Yasuda, Hiroshi | Hiroshima University |
WG3 (Species- and Organ-Specific Dose-Rate Effects) 2024
| Suzuki, Keiji (Head) | Nagasaki University |
| Iizuka, Daisuke | QST |
| Ojima, Mitsuaki | Oita University of Nursing and Health Sciences |
| Daino, Kazuhiro | QST |
| Nakamura, Asako | Ibaraki University |
| Fujimichi, Yuki | Central Research Institute of Electric Power Industry |
WG4 (Research Mapping for Radiation-Related Carcinogenesis) 2024
| Iwasaki, Toshiyasu (Head) | Central Research Institute of Electric Power Industry |
| Ohtsuka, Kensuke | Central Research Institute of Electric Power Industry |
| Kobayashi, Junya | International University of Health and Welfare |
| Tsuruoka, Chizuru | QST |
| Noda, Asao | Radiation Effects Research Foundation |
| Hirouchi, Tokuhisa | Institute for Environmental Sciences |
| Yamada, Yutaka | QST |
