Survival of Chinese hamster V79 cells after irradiation with a mixture of neutrons and 60Co gamma rays: experimental and theoretical analysis of mixed irradiation.

Irradiation with a mixture of high-LET and low-LET radiation was investigated to determine whether the sequence of delivery had any effect. Survival of Chinese hamster V79 cells irradiated sequentially with neutrons (40%) followed by 60Co gamma rays (60%) did not differ from cells irradiated in the reverse order up to a total dose of 7.5 Gy, which indicated that the effect of mixed irradiation was independent of the sequence of delivery under the conditions adopted. These results imply that the Zaider-Rossi model of mixed irradiation, which predicts that the sequence of delivery has no effect, is superior to the additive damage model, which predicts that sequence has an effect. Based on these results, the Zaider-Rossi model was extended to fit various types of mixed irradiation (sequential and partially or completely simultaneous irradiation), and the experiments for sequential and completely simultaneous irradiations obtained by Higgins et al. (Radiat. Res. 95, 45-56, 1983) and McNally et al. (Int. J. Radiat. Biol. 45, 301-310, 1984) were simulated as detailed examples. Using this extended model, their contradictory results regarding cell survival following sequential and simultaneous irradiation were shown to be due to differences in the dose rate of the radiation delivered in each study.