公益財団法人 放射線影響協会 放射線影響協会疫学センター

REA Views on the BMJ Article by Dr. E. Cardis at IARC and co-authors(2005/07)

Title of article:
Risk of cancer after low doses of ionizing radiation-retrospective cohort study in 15 countries

  1. Dr. E. Cardis at International Agency for Research on Cancer (IARC), Lyon, and coauthors have recently published, in an article in the British Medical Journal (BMJ), the results of epidemiological analyses on the potential effects of external radiation exposure on cohorts of nuclear industry workers in 15 countries.


  2. The objectives of the study included providing direct estimates of risk of cancer death after repeated or protracted occupational exposures to low doses of ionizing radiation, thereby comparing them with estimates derived from Japanese atomic bomb survivors with acute exposure to high doses. This is important, as it is mainly the latter which have been used for establishing radiation protection standards. Concluding that “1-2% of deaths from cancer among workers in this cohort may be attributable to radiation,” they further maintained that the results suggest that “there is a small excess risk of cancer, even at the low doses and dose rates typically received by nuclear workers in this study.”

    data/information submitted from Japan were included in analyses of leukaemia.


  3. REA, however, deems these conclusions inappropriate at this stage, and believes that one should not make a hasty judgment or interpretation that evident health effects by low-dose radiation have been found, on the basis of the following considerations:


  4. The present multi-national epidemiological analysis, though constituting the largest cohort ever studied, produced no statistically significant estimated excess risks for death from leukaemia, in contrast to the 3-country Canadian, UK and US joint research results and the data from atomic bomb survivors.


  5. On the other hand, the excess relative risk (ERR) per Sv (sievert: exposure dose) for all cancers excluding leukaemia was statistically significant, which was not the case for the 3-country joint research. The central risk estimate was about three times higher than that for the atomic bomb survivors. In this particular analysis, however, data from Japan, Idaho National Engineering Laboratory (US), and Ontario Hydro Canada were excluded on the basis that information on socioeconomical status (SES) for these cohorts was either unavailable or incomplete.


  6. Though the IARC research analyzed a much larger cohort than the 3-country joint research, it had less statistical power, contrary to general expectations. The decrease in statistical power is presumably due to the fact that nearly 60,000 workers with internal contamination or neutron exposures were excluded, though they evidently have had relatively high external exposure doses as well.


  7. No definitive results were obtained with respect to the possibility of confounding effects of lifestyle-related factors such as smoking, because of lack of information in most of the participating cohorts. The IARC article only stated, based on analyses of causes of cancer deaths related or unrelated to smoking, that confounding effect of smoking “is unlikely to explain all of this increased risk.”


  8. The present IARC article contains no figure/table illustrating the relationship between the cumulative doses received and ERRs. Therefore, we cannot exclude the possibility that the published results were largely influenced by exceptionally high values accidentally obtained only in a certain dose area.


  9. Regarding differences in risk between the participating cohorts, analyses excluding one country (or cohort) at a time were conducted to produce excess relative risk estimates per Sv. These were no longer significantly different from zero when Canada was excluded. It may be strongly suggested that the data from Canada, which were particularly in the higher range, have affected and shifted the overall results.


  10. Detailed information such as the excess relative risk estimate of mortality when incorporating the three cohorts (including Japan) for which SES information was either unavailable or incomplete, as well as the results of analyses when including the workers with internal contamination or neutron exposures, and the estimated dose-response relationships between cumulative doses and excess relative risks, etc., is not currently available to the public, though all those analyses have already been completed within the multi-national project.


It is indeed regrettable that the article has been published in such a way that the public has little accessibility to more detailed information of the 15 country study, presumably due to space limitations, considering the fact that the results of the study have tremendous implications.