Continuous irradiation of beagles with gamma rays.

The effects of exposure to 60Co gamma rays for the duration of life on mortality patterns in beagles were investigated. Doses of 3, 7.5, 18.8, 37.5, 75, 127.5, 262.5, 375, or 540 mGy were delivered over a 22-h day 7 days a week. Hazard models with time-dependent covariates were used to identify radiation-related trends in mortality. Hematopoietic failure occurring early in life was positively associated with the accumulated dose and the rate at which the dose was delivered. The risk of death from causes other than cancer that occurred later in the life span also depended on accumulating dose and dose rate but was lower than the cancer risk. Once an animal survived long enough to die from cancer, failure times depended only on the accumulating dose. The slope 1 and slope 2 boundary in a Gompertz plot historically used to identify the transition between dose rate and total dose effects was also found to distinguish between late-occurring deaths associated with tumor or nontumor events.