Radiobiological response to ultra-short pulsed megavoltage electron beams of ultra-high pulse dose rate

Purpose: In line with the long-term aim of establishing the laser-based particle acceleration for future medical application, the radiobiological consequences of the typical ultra-short pulses and ultra-high pulse dose rate can be investigated with electron delivery. Materials and methods: The radiation source ELBE (Electron Linac for beams with high Brilliance and low Emittance) was used to mimic the quasi-continuous electron beam of a clinical linear accelerator (LINAC) for comparison with electron pulses at the ultra-high pulse dose rate of 10(10) Gy min(-1) either at the low frequency of a laser accelerator or at 13 MHz avoiding effects of prolonged dose delivery. The impact of pulse structure was analyzed by clonogenic survival assay and by the number of residual DNA double-strand breaks remaining 24 h after irradiation of two human squamous cell carcinoma lines of differing radiosensitivity. Results: The radiation response of both cell lines was found to be independent from electron pulse structure for the two endpoints under investigation. Conclusions: The results reveal, that ultra-high pulse dose rates of 10(10) Gy min(-1) and the low repetition rate of laser accelerated electrons have no statistically significant influence (within the 95% confidence intervals) on the radiobiological effectiveness of megavoltage electrons.