Bystander and other delayed effects and multi-organ involvement and failure following high dose exposure to ionising radiation

There is no doubt that ionising radiation damages DNA and that certain organs in the body are more vulnerable than others to the effects of radiation. However, the reasons for the different sensitivities vary and there are now known to be many late expressed effects of exposure that cannot simply be explained on the basis of direct DNA damage. Examples include transmissible genomic instability, bystander effects and adaptive responses, which seem to be interrelated phenomena occurring even at low doses and affecting very high numbers of cells in the exposed organ or organism. Clinically, systemic effects such as fatigue and mental confusion are well known side effects of radiotherapy. Accident victims are also known to exhibit systemic effects that involve general system failure and cannot easily be attributed to effects on particular targeted tissues or organs. There is also evidence for emergent properties of systems that involve communication within and between organs, and concerted responses that are not predictable using reductionist approaches in radiotoxicology. This paper reviews and analyses data regarding delayed or late expressed effects, with particular reference to how they might impact on the understanding and treatment of multi-organ involvement and failure following exposure to accidental or deliberate releases of radiation.