The relative effectiveness of exposure to 131I at low doses

There is much interest in estimating the thyroid cancer risks to children exposed to low doses of (131)I from nuclear fallout. Direct epidemiological studies of individuals exposed as children to low doses of (131)I have insufficient power to yield useful risk estimates. Consequently, extrapolation from risk estimates for acute exposure to kilovoltage x rays are used to generate (131)I risk estimates. There are two distinct effects that might need to be considered in modifying the x-ray generated risk estimates: first, the differing quality of the radiations and, second, the lower overall dose rates for (131)I exposure. We suggest that it is helpful to split the overall correction factor into its component parts-radiation quality and radiation dose rate-and consider them separately, in that a good deal is known separately about each. There is good radiobiological evidence that electrons emitted in (131)I decays are less biologically effective than kilovoltage x rays, and the microdosimetric/radiobiological analyses described here suggest that the low-dose relative effectiveness is around 0.6 compared to kilovoltage x rays. Use of different endpoints suggests that this estimate is quite robust, and it agrees well with available animal data. Any correction factor for dose-rate effects is likely to be close to unity.