Fig. 5.

High linear energy transfer (LET) protons directly induce persistent complex DNA damage (CDD) formation, which contributes to increased radiosensitivity. (A) HeLa cells were irradiated with 4 Gy protons at low or high LET and CDD measured at various time points after ionizing radiation (IR) by the enzyme-modified neutral comet assay in the absence or presence (indicated as modified) of APE1, NTH1, and OGG1. Shown is the percent tail DNA ± standard deviation. *P < .02, **P < .01, ***P < .005, ****P < .001, as analyzed by a one-sample t test. Clonogenic survival of (B) HeLa, (C) UMSCC74A, and (D) UMSCC6 cells after protons at low or high LET. Shown is the surviving fraction ± standard error. A comparison of the surviving fraction at 2 Gy (SF2) is statistically significantly different by one-way analysis of variance (P < .02 for HeLa and UMSCC74A; P < .01 for UMSCC6).