Biological Dosimetry of Ionizing Radiation

A worsening of the accidental hazards linked to the use of ionizing radiation is currently being observed for four reasons. First, the increasing need for radiation sources in numerous industrial applications (food sterilization, construction, engineering...) leads to an increasing probability of loss of the sources or abnormal/unsuitable use and storage. Second, advances in medicine generate new protocols and tools that are more efficient but also much more complex to carry out, increasing the risk of accidental overexposure. Third, the possibility of a terrorist attack using radiological or nuclear devices has to be taken into account. Finally, recent events in Fukushima (Japan) highlight the risks of exposure in the case of nuclear power plant accidents. All these issues could lead to the accidental exposure of one to several thousand individuals not wearing dosimeters. Thus, it is essential to be able to estimate the exposure level of victims. Nowadays, this evaluation is based on clinical diagnosis (mainly irradiation symptoms and hematological variations) supplemented with biological dosimetry and physical dose reconstruction. Biological dosimetry is especially important when the personal dosimeter is lacking or when the accidental context is unclear. All this information should help the medical staff to deliver appropriate medical care and to manage the long-term medical follow-up, if required. It has been known since the last century that ionizing radiation causes DNA damage and that DNA misrepair can induce chromosome aberrations: stable (translocations, deletions, insertions) or unstable (dicentrics, centric rings, acentric fragments). These aberrations are observed in metaphase cells. A misrepair can be also observed after anaphase in the form of micronuclei. The applicability of the available assays of biodosimetry is based on the analysis of the chromosome damage present in peripheral blood lymphocytes, which is a convenient because its collection is non-invasive and it is easy to obtain. Dicentric assay is currently the gold standard method for classic biodosimetry in cases of recent accidental exposure. The scoring of dicentrics allows assessment of the whole-body dose received by the individual. Moreover, in numerous accident contexts the exposure of victims is heterogeneous. In these cases, the fraction of the body irradiated and the dose received by this fraction can be estimated by dicentric scoring. In the case of large-scale accidents the dicentric aberration is always used as well as the micronuclei because its analysis is faster and easier. However, dicentrics and micronuclei are unstable aberrations and their rate decreases with time. Consequently, for past accidental exposure, the analysis of stable chromosome aberrations like translocations is required (Figure 1).