Delayed effects of chronic low-dose high linear energy transfer (LET) radiation on mice in vivo

In vivo radioadaptive response: a review of studies relevant to radiation-induced cancer risk

Radioadaptive response (RAR) describes phenomena where small conditioning doses of ionizing radiation (IR) reduce detrimental effects of subsequent higher IR doses. Current radiation protection regulations do not include RAR because of the large variability in expression among individuals and uncertainties of the mechanism. However, RAR should be regarded as an indispensable factor for estimation and control of individual IR sensitivity. In this article, RAR studies relevant to individual cancer risk are reviewed. Using various stains of mice, carcinogenic RAR has been demonstrated. Consistently much in vivo evidence for RAR with end points of DNA and chromosome damage is reported. Most in vivo RAR studies revealed efficient induction of RAR by chronic or repeated low-dose priming irradiation. Chronic IR-induced RAR was observed also in human individuals after environmental, occupational, and nuclear accident radiation exposure. These observations may be associated with an intrinsically distinct feature of in vivo experimental systems that mainly consist of nonproliferating mature cells. 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. Regulation by endocrine factors is also a plausible mechanism for RAR at an individual level. Emerging evidence suggests that glucocorticoids, known as stress hormones, participate in in vivo RAR induction following long-term low-dose-rate exposure to IR.

Uncomfortable issues in radiation protection posed by low-dose radiobiology

This paper aims to stimulate discussion about the relevance for radiation protection of recent findings in low-dose radiobiology. Issues are raised which suggest that low-dose effects are much more complex than has been previously assumed. These include genomic instability, bystander effects, multiple stressor exposures and chronic exposures. To date, these have been accepted as being relevant issues, but there is no clear way to integrate knowledge about these effects into the existing radiation protection framework. A further issue which might actually lead to some fruitful approaches for human radiation protection is the need to develop a new framework for protecting non-human biota. The brainstorming that is being applied to develop effective and practical ways to protect ecosystems widens the debate from the narrow focus of human protection which is currently about protecting humans from radiation-induced cancers.