Suzuki, M., Piao, C., Hall, E. J. and Hei, T. K. Cell Killing and Chromatid Damage in Primary Human Bronchial Epithelial Cells Irradiated with Accelerated ⁵⁶Fe Ions

We examined cell killing and chromatid damage in primary human bronchial epithelial cells irradiated with high-energy ⁵⁶Fe ions. Cells were irradiated with graded doses of ⁵⁶Fe ions (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The survival curves for cells plated 1 h after irradiation (immediate plating) showed little or no shoulder. However, the survival curves for cells plated 24 h after irradiation (delayed plating) had a small initial shoulder. The RBE for ⁵⁶Fe ions compared to ¹³⁷Cs γ rays was 1.99 for immediate plating and 2.73 for delayed plating at the D₁₀. The repair ratio (delayed plating/immediate plating) was 1.67 for ¹³⁷Cs γ rays and 1.22 for ⁵⁶Fe ions. The dose–response curves for initially measured and residual chromatid fragments detected by the Calyculin A-mediated premature chromosome condensation technique showed a linear response. The results indicated that the induction frequency for initially measured fragments was the same for ¹³⁷Cs γ rays and ⁵⁶Fe ions. On the other hand, ∼85% of the fragments induced by ¹³⁷Cs γ rays had rejoined after 24 h of postirradiation incubation; the corresponding amount for ⁵⁶Fe ions was 37%. Furthermore, the frequency of chromatid exchanges induced by γ rays measured 24 h after irradiation was higher than that induced by ⁵⁶Fe ions. No difference in the amount of chromatid damage induced by the two types of radiations was detected when assayed 1 h after irradiation. The results suggest that high-energy ⁵⁶Fe ions induce a higher frequency of complex, unrepairable damage at both the cellular and chromosomal levels than ¹³⁷Cs γ rays in the target cells for radiation-induced lung cancers.