Induction of sister chromatid exchange as a function of charged-particle linear energy transfer

Frequencies of sister chromatid exchanges (SCEs) were evaluated in chromosome spreads of CHO-AA8 cells at the second mitosis after irradiation with charged particles of 10, 40, 80, and 120 keV/microns. At each LET there was a dose-dependent increase in the frequency of SCEs. In contrast to the majority of end points where relative biological effectiveness increases as LET increases up to an optimum and then declines, it was found that the most biologically effective particles were protons at 10 keV/microns, followed by deuterons at 40 keV/microns, then alpha particles at 80 and 120 keV/microns. Nuclear cross-sectional areas of these cells were log-normally distributed with a mean of 77 microns 2. When induced SCEs per chromosome were related to charged particles per cell nucleus (down to a mean of 0.5 particles per nucleus), results at low fluences favored a linear relationship between SCEs and particles which was relatively independent of LET. These observations are not compatible with the origin of radiation-induced SCEs being DNA double-strand breaks and favor the notion that they may be consistent with DNA single-strand breaks.