Predicting Human Cancer Risk for Internally Deposited Ra-226*

O.G. Raabe (University of California, Davis)

This report demonstrates the remarkable usefulness and efficacy of using animal data to predict human cancer risks associated with internally deposited radionuclides. The functionally injurious and carcinogenic responses in life-time studies of beagles having skeletal burdens of Ra-226 are represented by three-dimensional dose-rate/time/response surfaces that compete with other serious endpoints during an individual's lifetime yielding observed effects. Each radiation effect is described by a three dimensional power function with time to the effect after intake given as a function of time-weighted average dose-rate. The characteristic parameters of these relationships were determined using maximum likelihood regression methods. The risk of death from causes associated with natural aging was represented by a Gompertz function. At very high dose rates (high bone concentrations of radium), the principal deleterious responses were those associated with radiation injury, while bone cancer (sarcoma) became predominant at intermediate dose rates. Risk relationships for beagles injected with Ra-226 at Davis were scaled to people with the time variable normalized with respect to species life span, and were found to accurately predict the published observations in the U.S human radium studies. The predicted relationships of competing risks in people with respect to time after intake from radiation injury, head and skeletal cancer induction, and natural aging are graphically shown using three-dimensional illustrations. *Research supported by the National Cancer Institute under Grant No. R01 CA46296 and by the U.S. Department of Energy under Contract No. DE-AC03-76SF00472 with the University of California, Davis.

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