Comparison of chromosomal damage induced by X-rays and Ar ions with an LET of 1840 keV/micrometer in G1 V79 cells.

Synchronous V79 Chinese hamster cells were exposed in G1 to either X-rays or 4.6 MeV/u Ar-ions (LET = 1840 keV/micrometer) and the induction of chromosomal damage was measured at five sampling times ranging from 14 to 30 h after treatment. To distinguish between cells in the first and second post-irradiation cycle the fluorescence-plus-Giemsa technique was applied. The experiment showed that the time-course of the appearance of damaged cells was markedly influenced by radiation-induced cell cycle delays and depended on both radiation quality and dose. The yield of aberrant metaphases and the number of aberrations per metaphase was found to increase with sampling time, but this increase was more pronounced for Ar ions. These differences in yield-time profiles of X-ray and Ar ion induced chromosomal damage are particularly important for an accurate determination of the RBE for particles. Our data clearly indicate that meaningful RBEs can only be obtained if chromosomal damage is analysed at several post-irradiation sampling times and the complete time-course of the expression of chromosomal damage is taken into account. Besides these quantitative differences, differences in the spectrum of chromosomal lesions were observed for X-rays and Ar ions. Following particle exposure more breaks and less exchange-type aberrations were formed compared with X-irradiation and, despite irradiation in G(1), a significant number of chromatid-type aberrations occurred in Ar-irradiated samples. The experimental results are interpreted on the basis of the different pattern of energy deposition by sparsely and densely ionizing radiation. In addition, a statistical analysis based on the Neyman type A distribution is performed, which takes into account the specific stochastic properties of particle irradiation.