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The induction of intracellular DNA strand breaks by X rays and various heavy charged particles was measured by the alkaline unwinding and alkaline and neutral filter elution techniques. No variations in strand break induction were found between the different cell lines under investigation. For a given particle, both the LET and the particle energy determined the efficiency to induce DNA lesions. RBE values for the total amount of induced strand breaks were always less than 1. For DNA double-strand breaks (DSBs), RBE values only slightly greater than 1 were determined for particle radiation with an LET around 300 keV/microns. Intracellular DSB/SSB ratios were found to be equivalent to data reported for in vitro systems using radioprotective conditions [Christensen et al., Int. J. Radiat. Biol. 22, 457-477, 1972; Taucher-Scholz et al., Adv. Space Res. 12(2-3), (2)73-(2)80, 1992]. Strand break rejoining as an indicator of cellular repair processes was detected even after high-LET irradiation (LET < or = 10,000 keV/microns). However, both the half-times of rejoining and the fraction of residual DNA breaks increased with the atomic number of the particle. After particle irradiation with LET values beyond 10,000 keV/microns, no rejoining of DNA strand breaks was found.

No abstract

Radiation-induced DNA double-strand breaks (dsbs) were measured in CHO-K1 cells by means of an experimental approach involving constant-field gel electrophoresis and densitometric scanning of ethidium bromide stained gels. For X-irradiation, an induction efficiency of 36 +/- 5 dsbs (Gy x cell)-1 was determined. With this set-up, the induction of dsbs was investigated in CHO-K1 cells after irradiation with accelerated carbon ions with specific energies ranging from 2.7 to 261 MeV/u. This set of particle beams covers the important linear energy transfer (LET) range between 17 and 400 keV/microns, where maximum efficiencies have been reported for other cellular endpoints like inactivation or mutation induction. For LETs up to 100 keV/microns, RBEs of approximately 1 have been determined, while efficiencies per unit dose decline for higher LETs. No RBE maximum > 1 was found. Data are compared with published results on dsb induction in mammalian cells by radiations of comparable LET.

An experimental setup using static-field gel electrophoresis (SFGE) was developed to determine radiation-induced DNA double-strand breaks (DSBs) in CHO-K1 cells after exposure to X-rays or heavy charged particles. The fraction of DNA eluted into the gel matrix depends on the quantity of DSBs introduced. In agreement with a recent report, SFGE and pulsed-field electrophoresis were found to be equally sensitive in DSB detection. With radiolabeled DNA from cell cultures, the absolute amount of DNA migrating out of agarose plugs into the gel was quantified by determining the radioactivity in the gel lane. Alternatively, relative measurements of the amount of DNA released into the gel were achieved with a standardized protocol for both SFGE and a subsequent densitometric scanning of photographic negatives from gels stained with ethidium bromide. After calibration with the radioactive method, the fractions of DNA retained could be calculated directly from the data obtained with the densitometric assay to set up classical dose-effect curves. This procedure was validated for its application with heavy ions using an 500 MeV/u lead beam.