In current clinical radiotherapy electron, photon and ion beams are delivered (quasi-) continuously with typical dose-rates of a few Gy/min. Recent developments in dose delivery like IMRT and respiratory-gated treatment as well as accelerator techniques like flattening filter free Linacs and laser-based particle accelerators are driven towards intermittent irradiation with higher dose rates. Compared to continuous dose delivery these new techniques differ not only in dose rate, but also in the temporal sequence of pulse delivery, i.e. pulse lengths and pulse intervals. Previous studies on the influence of high dose rates were focussed on single pulse electron exposure and vary in energy, pulse duration and cell line; the impact of intermittent irradiation was rarely investigated. In the present work, the influence of dose rates of up to 1012 Gy/min and of intermittent irradiation with pulse lengths and pulse intervals in the range of seconds were studied.
The radiation source ELBE (Electron Linac for beams with high Brilliance and low Emittance) was used to mimic intermittent irradiation with asymmetric split-doses, separated by pulse intervals in the range of 10 ms to 90 s, and the quasi-continuous electron beam of a clinical LINAC. Using the HNSCC line SQ20B the impact of pulse structure was analyzed by clonogenic survival assay. Moreover, the LINAC-like electron beam and electron pulses with pulse dose rates of up to 1012 Gy/min were used to measure the kinetics of g-H2AX/53BP1 foci disappearance up to 24 h after treatment as a surrogate marker for DNA double-strand break complexity in the normal human breast epithelial cell line 184A1.
In general, the radiation response was found to be independent from electron pulse structure for the two endpoints under investigation.
These results reveal that ultra-high pulse dose rates of 1012 Gy/min and pulse intervals of 10 ms to 90 s between two pulses have no significant influence on the radiobiological effectiveness of megavoltage electrons.
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All authors have declared no conflicts of interest.