Cancer risk following alpha-emitter exposure.

The International Commission on Radiological Protection (ICRP) mandated a task group (Task Group 64) to review recently published epidemiological studies related to cancer risk and incorporated alpha emitters, and to evaluate whether the results might consolidate or challenge assumptions underlying the current radiation protection system. Three major alpha emitters were considered: radon and its decay products, plutonium, and uranium. Results came mainly from cohorts of workers, while for radon, major studies of the general population contributed to a better understanding of the risk of lung cancer at low and chronic exposure. Selection criteria for the review were: assessment of individual exposure of the target organ, long duration of the health survey, availability of attained age at end of follow-up, and adjustment for major co-factors. Task Group 64 is composed of members from ICRP Committees 1 and 2 (because epidemiological and dosimetric expertise were needed) and external experts. A first report (ICRP Publication 115) considered the risk of lung cancer related to inhalation of radon and its decay products. As the estimated excess risk per unit of exposure was higher by a factor of 2 compared with a previous ICRP estimate in 1993, Task Group 64 suggested a reconsideration of the reference levels for the workplace and for the general population. A second report, using the same standardised methodology (lung cancer baselines, population, life expectancy), will include estimation of the cancer risk of nuclear workers exposed to plutonium, focusing on the risk of lung cancer. A comparison of these risks with those of populations exposed to external gamma radiation alone will be made in the near future. For uranium, the results related to the organ-specific dose were too sparse to draw reliable conclusions, despite a recent publication. More research is needed on this topic.