Fig. 4.

High linear energy transfer (LET) protons induce DNA single-strand breaks (SSBs) and alkali-labile site-associated DNA damage, which displays slow repair kinetics. (A) HeLa, (C) UMSCC74A, or (D) UMSCC6 cells were irradiated with 1.5 Gy X rays or protons at low or high LET and SSBs/alkali labile sites measured at various time points after ionizing radiation (IR) by the alkaline comet assay. (B) HeLa cells were irradiated with 4 Gy IR and DNA double-strand breaks measured at various time points after IR by the neutral comet assay. Shown is the percent tail DNA ± standard deviation normalized to the levels seen immediately after IR, which was set to 100%. *P < .005, **P < .002, ***P < .001, as analyzed by a one-sample t test.