Induction and repair of DNA double-strand breaks assessed by gamma-H2AX foci after irradiation with pulsed or continuous proton beams

Abstract

In particle tumor therapy including beam scanning at accelerators, the dose per voxel is delivered within about 100ms. In contrast, the new technology of laser plasma acceleration will produce ultimately shorter particle packages that deliver the dose within a nanosecond. Here, possible differences for relative biological effectiveness in creating DNA double-strand breaks in pulsed or continuous irradiation mode are studied. HeLa cells were irradiated with 1or 5Gy of 20-MeV protons at the Munich tandem accelerator, either at continuous mode (100ms), or applying a single pulse of 1-ns duration. Cells were fixed 1h after 1-Gy irradiation and 24h after 5-Gy irradiation, respectively. A dose-effect curve based on five doses of X-rays was taken as reference. The total number of phosphorylated histone H2AX (gamma-H2AX) foci per cell was determined using a custom-made software macro for gamma-H2AX foci counting. For 1h after 1-Gy 20-MeV proton exposures, values for the relative biological effectiveness (RBE) of 0.97±0.19 for pulsed and 1.13±0.21 for continuous irradiations were obtained in the first experiment 1.13±0.09 and 1.16±0.09 in the second experiment. After 5Gy and 24h, RBE values of 0.99±0.29 and 0.91±0.23 were calculated, respectively. Based on the gamma-H2AX foci numbers obtained, no significant differences in RBE between pulsed and continuous proton irradiation in HeLa cells were detected. These results are well in line with our data on micronucleus induction in HeLa cells.