Adaptive Response – A Universal Phenomenon for Radiological Protection?

Predominantly with cells in vitro but also with whole animals in vivo it has been shown that small radiation doses like other stress factors can render cells or organisms to a stage of higher radioresistance. This has been demonstrated with chromosomal aberrations, gene mutation, cell transformation and survival. However, it is necessary to keep the appropriate conditions in a very stringent way. This implies radiation dose ranges, time factors and others. The adaptive response is transient and keeps for about three cell cycles or two to three days for mammalian cells. Most studies have been performed with low LET radiation. From the few data with high LET radiation it can be concluded that the adaptive response is much less or does not occur at all. Cellular and molecular studies indicate that the DNA repair is most important for the induction of adaptive response although the understanding of the mechanisms is certainly incomplete. In vivo other biological phenomena like the immune system also play a significant role. A high individual variability exists with respect to the extent of the adaptive response. No adaptive response apparently occurs with cells from individuals with repairand immune-deficiencies. Several experiments during the prenatal development indicate that there is no or only little adaptive response during wide developmental stages in utero. Therefore it must be concluded that the adaptive response has limitations and is not a universal principle. Due to these restrictions of the validity and strength of adaptive response it is doubtful whether adaptive response can generally be applied in the practice of radiological protection.