Adaptive response to ionising radiation and its role in influencing radiation protection standards

Sufficient evidences on the occurrence of the stimulatory effects of low dose ionizing radiation have been gathered from all over the world and presented in the report of the United Nations Scientific Committee on the Effects of ionizing Radiation (UNSCEAR). These evidences imply that the shape of the dose-response curve at low levels of ionizing radiation is not linear but indicate a threshold like trend. Nevertheless, it is still uncertain, if these findings have a practical consequence in estimating cancer risks. Parameters such as genetic variability and dose rates at which the adapting doses need to be administered still need to be ascertained in order to present a direct correlation between the phenomenon and its implication in setting-up radiation protection standards. While most studies on the phenomenon of radio-adaptive response have been conducted using adapting doses ranging between 1 and 5 cGy, results of experiments conducted in this laboratory both with in vitro and in vivo systems suggest that the ideal adapting window may lie well below 1 cGy. On comparing the extent of UV-induced UDS between in vivo and in vitro exposures, significantly higher rates of UDS were observed in the lymphocytes of radiation workers when compared to a corresponding in vitro adapting dose. We postulate that the response in vivo is much more pronounced due to cell repopulating events and extra cellular secretory factors like hormones etc. This paper highlights the need to conduct more in vivo and epidemiological studies in order to dispel ambiguities and solve problems encountered by experiments on this phenomenon which have largely been conducted with in vitro cell systems. It also brings out the significance of the dose and dose-rate parameters in determining the out come of the phenomenon. Attempts to identify genes/gene alterations, gene products and signal transduction pathways in this laboratory and elsewhere will be touched upon. Lessons learnt from studies conducted to date and those that need to be carried out before this phenomenon could influence the setting-up of standards in radiation protection will be presented. (author)