Biological effectiveness of laser accelerated protons: In vitro dose response studies

The development of proton and ion acceleration by ultra-high intensity lasers for cancer therapy promises the realisation of compact and economic particle accelerators that can be integrated in already existing clinics. However, particle acceleration with high intensity lasers leads in comparison to the conventional used acceleration technique to ultra short beam pulses, generated with low pulse frequency, that apply a very high pulse dose. Prior to a clinical application the radiobiological consequences of laser accelerated and therewith ultra short pulsed particle beams have to be investigated.
For this in vitro dose effect curves have been determined, which required a high power laser system with a stable and reproducible acceleration of protons, precise beam monitoring and the technical ability to apply a prescribed dose to a cell sample and to determine the absolute dose received by the cells.
Systematic cell irradiations were performed at the 150 TW Ti:Sapphire laser system DRACO at HZDR that delivered laser pulseswith an energy of 3.5 J, a pulse duration of 30 fs and a frequency of 0.1 Hz. By focusing the laser on a 2 μm thin Ti foil, protons were accelerated from the target rear surface. The generated exponential energy spectrum was limited downwards to 6-20 MeV. An in-house developed integrated dosimetry and cell irradiation system was tested and calibrated, allowing precise dosimetry as well as the exact positioning of each cell sample.
In the present experiment radiosensitive head and neck tumour cells (SKX) were irradiated in a dose range from 0.5-4 Gy with an average pulse dose of 80 mGy and a mean dose rate of 0.5 Gy/min.
Investigated biological endpoints were the clonogenic cell survival and residual DNA-double strand breaks (DSB) 24 h post irradiation via γ-H2AX /p53BP1 assay.
Reference irradiation was performed with continuous, conventional accelerated 7.2 MeV proton beams at the Tandem accelerator at HZDR with a dose rate of 1.1 Gy/min.
The measured dose effect curves show no difference in biological effectiveness between laser accelerated ultra short pulsed and conventional continuous proton beams in clonogenic cell survival and residual DNA DSB.