CHROMOSOMAL DAMAGE AND REPAIR IN G(1)-PHASE CHINESE-HAMSTER OVARY CELLS EXPOSED TO CHARGED-PARTICLE BEAMS

Chromosomal fragmentation was examined in G(1)-phase Chinese hamster ovary cells using the premature chromosome condensation (PCC) technique. The yield and distribution of chromatin breaks, the lesions revealed by PCC, were measured in cells exposed to X rays or each of nine particle beams covering a range of LET from 0.56 to 2700 keV/mu m. The average number of breaks per cell was found to be linearly proportional to the fluence of high-LET neon ions (183 keV/mu m). Assuming a linear response for the other beams, the level of breakage per unit dose rose from a plateau at the lowest LET values to a peak in the 100-200 keV/mu m range and then declined continuously thereafter, eventually falling well below the low-LET plateau. The maximum breakage RBE was 1.5. The average number of breaks per particle traversal rose steadily from 0.006 to 11 breaks/cell as the LET increased from 0.56 to 2700 keV/mu m. The breaks were distributed randomly within the cell population after low-LET irradiation, but became progressively overdispersed with increasing LET. Rejoining of prematurely condensed chromosomes plus fragments was followed for up to 5 h for four particle beams having LET values between 0.56 and 183 keV/mu m. An LET-dependent trend toward higher levels of residual fragments was observed.