Sequential Exposures of Mammalian Cells to Low- and High-LET Radiations: I. Lethal Effects Following X-Ray and Neon-Ion Irradiation

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Abstract
Experiments were designed to test the independent action hypothesis for cell killing between low- and high-LET [linear energy transfer] radiations. Asynchronous Chinese hamster V79 cells [lung] were irradiated at a Bevalac with Bragg-peak neon ions alone or in combination with X-rays. The survival curve for cells exposed to neon ions was exponential in appearance, with no evidence of split-dose repair. The survival rate curve for cells irradiated with graded doses of X-rays shortly after a priming dose of neon ions had a reduced shoulder (Dq) relative to the X-ray only survival curve. Conversely, the survival curve for cells irradiated with graded doses of neon ions shortly after a priming dose of X-rays showed a reduced mean lethal dose (D0) compared to the neon-only survival curve. Both of these effects increased with the size of the priming dose. The charged particles and X-rays each produce damage which interacts with the other, and suggest that low- and high-LET radiations act by a synergistic mode. In addition, experiments with selective fractionation intervals demonstrated that the interaction effects described diminish with increased fractionation time, regardless of the radiation sequence. Apparently, even the high-LET-induced damage which is involved in the interaction process can be repaired by cells.