Dose-response of initial G2-chromatid breaks induced in normal human fibroblasts by heavy ions

Purpose: To investigate initial chromatid breaks in prematurely condensed G 2 chromosomes following exposure to heavy ions of different LET. Material and methods: Exponentially growing human fibroblast cells AG1522 w ere irradiated with gamma -rays, energetic carbon (13 keV/mum, 80 keV/mum) ions. Chromosomes were prematurely condensed using calyculin-A. Initial chr omatid-type and isochromatid breaks in G2 cells were scored. Results: The dose-response curves for total chromatid breaks were linear re gardless of radiation type. The relative biological effectiveness (RBE) sho wed a LET-dependent increase, peaking around 2.7 at 55 80 keV/mum and decre asing at higher LET. The dose-response curves for isochromatid-type breaks were linear for high-LET radiations, but linear-quadratic for gamma -rays a nd 13 keV/mum carbon ions. The RBE for the induction of isochromatid breaks obtained from linear components increased rapidly between 13 keV/mum (abou t 7) and 80 keV/mum carbon (about 71), and decreased gradually until 440 ke V/mum iron ions (about 66). Conclusions: High-LET radiations are more effective at inducing isochromati d breaks, while low-LET radiations are more effective at including chromati d-type breaks. The densely ionizing track structures of heavy ions and the proximity of sister chromatids in G2 cells result in an increase in isochro matid breaks.