ASN Report 2020

1.3  Scientific uncertainties and vigilance The action taken in the fields of nuclear safety and radiation protection to prevent accidents and limit detrimental effects has led to a reduction in risks but not to zero risk, whether in terms of the doses received by workers or those associated with discharges and releases from BNIs. Many uncertainties persist; they induce ASN to remain attentive to the results of scientific work in progress in radiobiology and radiopathology for example, with possible consequences for radiation protection, particularly with regard to management of risks associated with low doses. One can mention, for example, several areas of uncertainty concerning radiosensitivity, the effects of low doses according to age, the existence of signatures (specific mutations of DNA) that could be observed in radiation-induced cancers and certain non-cancerous diseases observed after radiotherapy. 1.3.1 The individual response to ionising radiation The effects of ionising radiation on personal health vary from one individual to the next. It is known for example, since it was stated for the first time by Bergonié and Tribondeau in 1906, that a given dose does not have the same effect when received by a growing child or by an adult. Furthermore, the variability in individual radiosensitivity to high doses of ionising radiation has been extensively documented by radiotherapists and radiobiologists. High levels of radiosensitivity have been observed in persons suffering from genetic diseases affecting the repair of DNA and cellular signalling; in these individuals they can lead to “radiological burns”. Such abnormal responses are also observed in people suffering from neuro­ degenerative diseases. At low doses, there is both cell radiosensitivity and individual radiosensitivity, which could concern about 5 to 10% of the population. Thanks to the lowering of detection thresholds, recent methods of immunofluorescence of molecular targets for signalling and repairing DNA damage enable the effects of ionising radiation at low doses to be better documented. The biochemical and molecular effects of a simple X-ray examination then become visible and measurable. The results of the research work conducted using these new investigation methods must still be confirmed in the clinical environment before being integrated into medical practices. This research work indicates that an abnormal response to ionising radiation can be expressed in three clinical forms: radiosensitivity at high doses of radiotherapy, radiosusceptibility to radiation-induced cancer, and radiation-induced degeneration (cataract or radiation- induced cardiovascular effects, for example). Progress in research and the validation of clinical results should rapidly make it possible to define the optimum conditions for highlighting the individual response to ionising radiation in patients and to take this into account in personalised medical management. Further to the work of the European research group on low doses (Multidisciplinary European Low Dose Initiative – MELODI) and review documents published in 2019 addressing the clinical and epidemiological aspects of the individual response to ionising radiation and the available screening tests and their robustness, the ICPR working group (TG111) dedicated to this subject is continuing its work to summarise knowledge on the individual response to ionising radiation with a view to developing international radiation protection recommendations. 4. The radioactive dose rate determines the absorbed dose (energy absorbed by the material per unit mass and time). It is measured in Gray per second (Gy/s) in the International System of Units (SI). It is used in physics and in radiation protection. The individual response to ionising radiation is thus gradually being recognised as an important subject of research and application in radiobiology and radiation protection, while at the same raising ethical and societal questions. 1.3.2 Effects of low doses The Linear No-Threshold relationship The hypothesis of this relationship, adopted to model the effects of low doses on health (see point 1.2), albeit practical from the regulatory standpoint and albeit conservative from the health standpoint, is not as scientifically well-grounded as might be hoped for. Some feel that the effects of low doses could be higher, while others believe that these doses could have no effect below a certain threshold, and some others even assert that low doses have a beneficial effect. Research in molecular and cellular biology is progressing, as are epidemiological surveys of large cohorts. But faced with the complexity of the DNA repair and mutation phenomena, and the methodological limitations of epidemiology, uncertainties remain and the public authorities must exercise caution. Dose, dose rate and duration of exposure The epidemiological studies performed on individuals exposed to the Hiroshima and Nagasaki bombings have given a clearer picture of the effects of radiation on health, concerning exposure due to external irradiation (external exposure) received in a few fractions of a second at high dose and high dose rate (4) . The studies carried out in the countries most affected by the Chernobyl accident (Belorussia, Ukraine and Russia) were also able to improve our understanding of the effects of radiation on health caused by exposure through internal contamination (internal exposure), more specifically through radioactive iodine. Studies on nuclear industry workers have given a clearer picture of the risk associated with chronic exposures at low doses established over many years, whether as a result of external exposure or internal contamination. Radiography room in the Léon Bourgeois clinic (Paris) in 1916 ASN Report on the state of nuclear safety and radiation protection in France in 2020 105 01 – NUCLEAR ACTIVITIES: IONISING RADIATION AND HEALTH AND ENVIRONMENTAL RISKS 01