Cellular low-dose effects of ionizing radiation

The most important physical quantity used to define the deposition of energy in organs and tissues from ionizing radiation is the absorbed dose, but this measure does not characterize the fluctuation of energy absorption resulting from the stochastic nature of the energy deposition events in individual cells. The fluctuation in the energy deposition between cells in a tissue is generally disregarded, but can be significant when the possible effects of ionizing radiation on cells at low doses are considered. Current models of radiation effects in cellular systems are based on direct damage to nuclear DNA being an initiating event in the carcinogenic process. For a given type of radiation, DNA damage is induced in proportion to dose, which implies a linear relationship between cancer induction and dose in the low-dose region. These models have been challenged with a range of studies showing effects in the absence of direct DNA damage due to energy deposition. A range of new observed non-targeted effects of biological responses to radiation shows other mechanisms of action: genomic instability, low dose hypersensitivity, adaptive responses, inverse dose-rate and bystander effects. They are important in determining the biological responses at low doses of radiation and have the potential to influence the shape of the dose-effect relationship by a saturating response above a threshold dose. There is convincing epidemiological evidence that doses of ionizing radiation above about a few tens of mGy cause a small but significant increase in cancer risk.