Induction and repair of DNA double-strand breaks assessed by gamma-H2AX foci after irradiation with pulsed or continuous proton beams
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In particle tumor therapy including beam scanning at accelerators, the dose per voxel is delivered within about 100 ms. 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 1 or 5 Gy of 20-MeV protons at the Munich tandem accelerator, either at continuous mode (100 ms), or applying a single pulse of 1-ns duration. Cells were fixed 1 h after 1-Gy irradiation and 24 h 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 1 h 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 5 Gy and 24 h, 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.
KeywordsRBE Protons DNA DSB repair Gamma-H2AX foci Pulsed irradiation
This work was supported by the DFG Cluster of Excellence: Munich-Centre for Advanced Photonics, by the EU-project EuroDyna, by the Maier Leibnitz Laboratory Munich, by the German Federal Ministry of Education and Research (BMBF, PtJ-Bio, 0313909), by the Deutsche Forschungsgemeinschaft (DFG, SFB824), BMBF (MOBITUM, 01EZ0826; Kompetenzverbund Strahlenforschung, 03NUK007E) and by the European Union (EU-CARDIORISK, FP7-21103).
Conflict of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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