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Geopolitical events have brought to light fears that we believed from another era. The Ukrainian context reminds us that the risk of accidental irradiation, caused by an atomic bomb explosion or an accident at a nuclear power plant, is not negligible. Regardless of the source of radiation, the appearance and severity of its effects obey a fundamental principle: the greater the energy absorbed by our cells, the greater the biological effect and the earlier and more serious the clinical consequences. This is true for both external irradiation (when the radiation source is outside the body) and internal irradiation (the radiation source is inside the body after ingestion or inhalation of radioactive material). The amount of energy received is called absorbed dose and knowing the relationship between this and its biological and clinical effect is the main task of radiobiologists. This principle has two corollaries: we must have a good understanding of both the dose and the possible effects. Knowing the radiation dose In the event of accidental irradiation, the radiation exposure of the human body is rarely homogeneous and is never measured directly. Therefore, its reconstruction a posteriori is usually complex and requires combined approaches. It requires both simulations that take into account the path of the radiation and the materials it passes through, as well as measurements on biological samples (chromosomal aberrations in blood lymphocytes, radiochemical changes in tooth enamel, etc.). The absorbed dose in our body is expressed in grays (Gy) or joules per kilogram of tissue (J/kg). Here are some orders of magnitude for our species: *0.2 mGy: average daily dose received by an astronaut on a mission *2 mGy: average dose to the breasts during a mammogram. *20-40 mGy: dose in the organ during a CT scan. *100-200 mGy (to the whole body): dose above which the risk of radiation-induced cancer is significant. *2 Gy: tumor dose applied in a radiotherapy session against cancer. *4.5 Gy (whole body): lethal dose for 50% of individuals *12 Gy (whole body): dose that causes rapid death Things are not that simple, however, as there is no There is only one type of irradiation. Depending on the situation and the radioactive material, different types of radiation are emitted: X-rays, alpha rays (emission from a helium nucleus), beta rays (emission of an electron), emission of protons or neutrons, etc. And all these radiations do not necessarily produce the same biological effects for the same absorbed dose. Taking into account these differences, the nature of each tissue and the affected surface, the effective dose is obtained, which allows different irradiations to be combined. This is the variable that the International Commission on Radiological Protection has used to calculate the risk of radiation exposure from all epidemiological observations, including those of survivors of Hiroshima and Nagasaki. The biological effects of radiation Knowing the dose of radioactivity received is not enough: the effective dose only provides information on a global risk (a probability of occurrence), but does not allow knowing the nature of the consequences for a given individual. To evaluate it, it is necessary to know the different effects induced by radiation. The energy absorbed by our body after irradiation is first absorbed by water (the main constituent of our cells) through chemical reactions called radiolysis. This gives rise to oxidative stress, since the hydrogen peroxide produced a few milliseconds after irradiation can break down the DNA contained in the cell nucleus. The fate of DNA breaks (repaired, unrepaired or poorly repaired) conditions the response at the cellular level, then at the tissue level and then at the clinical level through a succession of reactions that can extend from the first minute to several years later of irradiation. There are three main clinical consequences of irradiation: *The so-called radiosensitivity or radiotoxicity reactions: they cause tissue or organ dysfunction often associated with inflammation. The best known are radiodermitis, skin lesions that appear quickly after exposure. It is, for example, the burns of the pioneers of radiation. They are produced by the death of cells in irradiated organs or tissues, caused in turn by unrepaired DNA breaks. Depending on the dose received, they range from simple redness to necrosis and even death. *Radiosusceptibility reactions: they are cancers induced by radiation, caused by cell transformation, in turn caused by poorly repaired DNA breaks. *The so-called radiodegeneration reactions: these are the consequences of accelerated cellular aging, caused by DNA lesions that are tolerated by the cells and that accumulate in them before causing their long-term death. This is especially the case with cataracts. The severity and occurrence of these three types of radiation-induced reactions depend largely on the absorbed radiation dose. In particular, the higher the dose, the greater the probability of cell death. This is applied in a controlled way during radiotherapy. Radiation-induced cancers are more likely to occur after repeated low doses of radiation. The aging of a tissue, of late expression, is linked to its progressive transformation, for example, the lens (cataract) and the heart and blood vessels. However, after accidental irradiation, sensitivity reactions and radiation-induced cancers are considered the most serious clinical effects. Who is at higher risk? In addition to this dose dependence, there are individuals who are more radiosensitive or radiosusceptible than others. They can suffer the same effects but before and with lower doses of radiation. For example, individuals with mutations in DNA repair genes show increased morbidity and mortality when exposed to radiation. Also, carriers of mutations in cancer predisposition genes are at increased risk of developing radiation-induced cancers. Knowing the dose, one of the priorities after the irradiation of several individuals is therefore to identify potentially radiosensitive and radiosusceptible individuals. Contrary to popular belief, radiation-induced risk does not depend systematically on age or sex, although these factors are important. Three main groups of individuals can be defined: *The most hypersensitive and susceptible individuals, who represent less than 1% of the population. These are children with very rare genetic diseases whose symptoms are well described and whose life expectancy is limited to about 20 years. *At least 80% of the population have a normal response to radiation exposure. *Between 5 and 20% of the population have a pathological response to radiation, of intermediate intensity, ranging from slightly abnormal to highly pathological. These are, for example, women with persistent dermatitis after radiotherapy for breast cancer or men with long-term rectitis after radiotherapy for prostate cancer. Also people with a familial predisposition to breast cancer (carriers of a known or unknown mutation) may have a higher risk of developing it due to radiation. Note further that if we omit the risk of breast cancers, which is highly specific to women, men and women have approximately the same risk of spontaneous and radiation-induced cancers. Finally, we emphasize the fact that children, except for the genetic diseases defined above and thyroid cancers, do not necessarily have a higher risk of cancer than adults. However, it is observed that people irradiated at a young age have a higher risk of developing cancer than those who were irradiated as adults. This is because cancer takes several years to form: an adult exposed to radiation does not necessarily have “time” to develop cancer. Authors: Nicolas Foray Director de Recherche à l’Inserm, Unité U1296 « Radiations : Défense, Santé, Environnement », Inserm Michel Bourguignon Professeur émérite de Biophysique et Médecine Nucléaire, Université de Versailles Saint-Quentin-en-Yvelines (UVSQ) – Université Paris-Saclay
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