The relevance of mean dose rate and energy spectrum for the different biological effectiveness of laser accelerated electrons

The new technology of laser acceleration, which promises radiotherapy accelerators of compact size and reasonable costs, results in ultra-short pulsed particle beams (in the region of 100 fs) with very high pulse dose rate (more than 1012 Gy/min). One important step before potential medical application is the radiobiological characterisation of this new radiation quality. Therefore, in vitro cell irradiations with laser accelerated electrons have been performed at the Jena Titanium:Sapphire (JETI) laser system. The obtained dose-effect-curves have been measured for two cell lines (tumor cell line FaDu, normal tissue cell line184A1) and biological endpoints (residual H2AX/53BP1 foci and clonogenic survival assay) revealing a reduced biological effectiveness of laser accelerated MeV electrons in comparison to continuous 200 kV X-ray reference irradiation.
In addition to the ultra-high pulse dose rate of the JETI electron beam this radiation quality differs also in energy spectrum and average dose rate from the 200 kV X-rays.
A negligible influence of the mean dose rate on biological effectiveness was confirmed by measuring the same cell response to 200 kV X-ray irradiation at 0.35 Gy/min (mean dose rate of the JETI electron beam) and at 1.4 Gy/min applied for reference irradiation. The influence of the energy spectrum was investigated by cell irradiation at a medical linear accelerator (LINAC). The two cell lines already studied at JETI have been irradiated with monoenergetic 6 MeV electrons according to the mean energy of the JETI electron beam. Clonogenic survival and residual DNA-DSB were determined for the LINAC electrons and compared to 200 kV X-rays, both radiations being similar in time-structure and mean dose rate. No difference in the number of residual H2AX/53BP1 foci after 24 h has been measured for both (quasi) continuous beams. In contrast increased clonogenic survival was found for the LINAC electrons compared to 200 kV X-rays as expected by the known increase of biological effectiveness with decreasing secondary electron energy. However, the decreased biological effectiveness is less pronounced for the LINAC electrons in comparision to the JETI laser electrons. Therefore, the remaining influence of the ultra-high pulse dose rate of JETI electrons is currently investigated.

This work has been supported by BMBF (no. 03ZIK445).