Recovery kinetics of radiation-induced chromosome aberrations in human G0 lymphocytes

Declining yields of radiation-induced dicentric chromosomes in human G0 lymphocytes were observed in split-dose experiments with time intervals varied up to 8 h. In agreement with microdosimetric intratrack-intertrack interaction models, only the dose-squared yield component was reduced and approached an asymptotic value equal to one half of the corresponding single exposure yield. For 150 kV X-rays and 13 MeV electrons, at total doses up to 6 Gy, the time constant tau of the approximately exponential decline was practically dose- and quality-independent within a range of 100-180 min. For 10 kV X-rays, in the presence of a dominant linear yield component, only a small split-dose effect, but with a consistent tau-value, was observed for a total dose of 5 Gy. Since tau can be interpreted as the mean life time of "primary lesions" in chromatin fibres, its independence from absorbed dose and radiation quality means that radiation damage of the split-dose recovery mechanism can be excluded for doses up to 6 Gy. By correlating the observed split-dose reduction of the acentric fragment yield to the reduction of the dicentric yield, (1.64 +/- 0.03) acentrics/dicentric for 150 kV X-rays and (1.51 +/- 0.11) acentrics/dicentric for 13 MeV electrons were obtained. Acentrics formed in the course of dicentric formation as well as in other binary interactions of "primary lesions" are represented in these ratios. Post-irradiation recovery during time intervals between irradiation and cell stimulation up to 24 h did not occur. The relations to comparable results in cell lethality experiments are discussed, and a hypothesis of "fast" and "slow" binary interactions of "primary lesions" is put forward.