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 I-131 from nuclear fallout. Direct epidemiological studies of individuals exposed as children to low doses of I-131 have insufficient power to yield useful risk estimates. Consequently, extrapolation from risk estimates for acute exposure to kilovoltage x rays are used to generate I-131 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 loner overall dose rates for I-131 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 I-131 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.