![]() ![]() ![]() ![]() The end points are such effects as IR-induced DNA damage, chromosomal aberrations, cell transformation, cell death, and mutation in in vitro experiments and prenatal death, malformation, hematopoietic death, and carcinogenesis in in vivo experiments. 1 in 1984, RAR has been confirmed using a variety of experimental systems ranging from yeasts to animal models. Radioadaptive response (RAR) is a term describing phenomena where a small conditioning dose of IR (called the ‘priming dose’) reduces the biological effects of subsequent higher doses of IR (called the ‘challenge dose’). However, biological effects of low-dose or low-dose-rate IR remain elusive. It is widely accepted that ionizing radiation (IR) at high doses is detrimental to the exposed organism. Emerging evidence suggests that glucocorticoids, known as stress hormones, participate in in vivo RAR induction following long-term low-dose-rate exposure to IR. Regulation by endocrine factors is also a plausible mechanism for RAR at an individual level. Alternatively, induction of RAR by gap junction-mediated bystander effects suggests that multicellular systems comprising densely communicating cells may be capable of responding to long-lasting low-dose-rate priming irradiation. These observations may be associated with an intrinsically distinct feature of in vivo experimental systems that mainly consist of nonproliferating mature cells. Chronic IR-induced RAR was observed also in human individuals after environmental, occupational, and nuclear accident radiation exposure. Most in vivo RAR studies revealed efficient induction of RAR by chronic or repeated low-dose priming irradiation. Consistently much in vivo evidence for RAR with end points of DNA and chromosome damage is reported. Using various stains of mice, carcinogenic RAR has been demonstrated. In this article, RAR studies relevant to individual cancer risk are reviewed. However, RAR should be regarded as an indispensable factor for estimation and control of individual IR sensitivity. Current radiation protection regulations do not include RAR because of the large variability in expression among individuals and uncertainties of the mechanism. Radioadaptive response (RAR) describes phenomena where small conditioning doses of ionizing radiation (IR) reduce detrimental effects of subsequent higher IR doses. ![]()
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