The sequential irradiation of mammalian cells with X rays and charged particles of high LET.

Chinese hamster V79 cells, synchronized in late-S phase, were irradiated with high-LET charged particles or X rays, or exposed sequentially to a single dose of charged particles followed by graded doses of X rays. The charged-particle irradiations consisted of deuterons (LET, 50 keV/microns) or 3He ions (96 or 160 keV/microns). The survival data obtained following the sequential irradiations show a synergistic effect compared with exposure to the low-LET radiation. An enhancement ratio is defined in order to quantify this effect. On the basis of this concept the present data show an enhanced interaction effect as the LET of the primary dose is increased or the size of this dose is increased. The data are also discussed in terms of a recent theoretical formulation which predicts synergism as a result of the interaction between the sublethal damage produced by the two radiations.