Dose-effect relationships and estimation of the carcinogenic effects of low doses of ionizing radiation - March 2005

Executive Summary The assessment of carcinogenic risks associated with doses of ionizing radiation from 0.2 Sv to 5 Sv is based on numerous epidemiological data. However, the doses which are delivered during medical X-ray examinations are much lower (from 0.1 mSv to 20 mSv). Doses close to or slightly higher than, these can be received by workers or by populations in regions of high natural background irradiation. Epidemiological studies have been carried out to determine the possible carcinogenic risk of doses lower than 100 mSv, and they have not been able to detect statistically significant risks even on large cohorts or populations. Therefore, these risks are at worse low since the highest limit of the confidence interval is relatively low. It is highly unlikely that putative carcinogenic risks could be estimated or even established for such doses through case-control studies or the follow-up of cohorts. Even for several hundred thousands of subjects, the power of such epidemiological studies would not be sufficient to demonstrate the existence of a very small excess in cancer incidence or mortality adding to the natural cancer incidence which, in non-irradiated populations, is already very high and fluctuates according to lifestyle. Only comparisons between geographical regions with high and low natural irradiation and with similar living conditions could provide valuable information for this range of doses and dose rates. The results from the ongoing studies in Kerala (India) and China need to be carefully analyzed. Because of these epidemiological limitations, the only method for estimating the possible risks of low doses (< 100 mSv) is extrapolation from carcinogenic effects observed between 0.2 and 3 Sv. A linear no-threshold relationship (LNT) describes well the relation between the dose and the carcinogenic effect in this dose range where it could be tested. However, the use of this relationship to assess by extrapolation the risk of low and very low doses deserves great caution. Recent radiobiological data undermine the validity of estimations based on LNT in the range of doses lower than a few dozen mSv which leads to the questioning of the hypotheses on which LNT is implicitly based: 1) constancy of the probability of mutation (per unit dose) whatever the dose or dose rate, 2) independence of the carcinogenic process which after the initiation of a cell evolves similarly whatever the number of lesions present in neighboring cells and the tissue.