Zero Tumor Incidence in Mice after Repeated Lifetime Exposures to 0.5 Gy of Beta Radiation

A final series of experiments on tumor induction by repeating ^{90}{\rm Sr}-{}^{90}{\rm Y}$ β irradiation of the back of ICR mice three times weekly throughout the life span showed no tumor incidence with repeated doses of 0.5 Gy per exposure. The whole dose-response relationship for repeated irradiation with 0.5 to 11.8 Gy per exposure, summarized along with data obtained previously, showed a unique nonlinear dose response with saturation of tumor incidence at doses of over 2.5 Gy per exposure and a threshold-like value around 1.5 Gy per exposure.