The Kinetics of Recovery during the First Few Weeks after Whole-body X-irradiation of Mice

Summary

Residual damage after five different exposures to whole-body radiation has been titrated at frequent time-intervals between 6 hours and 6–7 weeks by determining the additional dose which results in 50 per cent deaths in 30 days. The results fit very well a three-phase model for recovery based on known biological and radiobiological properties. During the first few hours recovery is rapid and independent of dose size. The second phase proceeds from about 6 hours on at a constant rate of about 50 rad per day. The constancy of the rate is due to the logarithmic nature of cell division in the irradiated host and the value of the daily rate depends on D₃₇ and cell doubling time. The duration of the second phase is dose-dependent. In the third phase the level of recovery oscillates above and below a mean because of the inertia of the system which regulates cell number in the haematopoietic tissues of the complex organism. The pattern of oscillation is independent of dose size. The incompleteness of recovery after the larger priming doses and the dependence of the oscillatory pattern on the duration of the priming dose are not explained by the model. The kinetics of recovery cannot be accounted for by a single exponential hypothesis.