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First in vitro cell experiments with laser-accelerated protons

Naumburger, D.; Baumann, M.; Beyreuther, E.; Bock, S.; Cowan, T. E.; Dammene, Y.; Enghardt, W.; Helbig, U.; Karsch, L.; Kraft, S. D.; Laschinsky, L.; Lessmann, E.; Metzkes, J.; Richter, C.; Sauerbrey, R.; Schramm, U.; Schürer, M.; Sobiella, M.; Woithe, J.; Zeil, K.; Pawelke, J.


Background: The novel technology of laser particle acceleration, which promises ion radiotherapy accelerators of compact size and reasonable costs, generates ultra-short pulsed particle beams (~ 100 fs) with very high pulse dose rate (more than 1012 Gy/min). The development of this new technology for radiotherapy application is the aim of the joint research project onCOOPtics – High intensity lasers for radiooncology. One important step before potential medical application is the radiobiological characterization of this new radiation quality starting with in vitro cell experiments.

Material and Methods: Cell irradiations have been performed with protons generated at the 150 TW laser system Draco installed at the FZD. Before starting irradiation experiments the laser particle accelerator had to be optimized with respect to intensity, energy distribution, spot size, stability and reliability of the proton beam. Furthermore, beam filtering and transport to an in-air irradiation site and a dosimetry system were developed and realised. For a precise dose measurement, low energy protons were filtered out by a permanent magnet system. At cell irradiation site the proton beam possessed an energy spectrum of 6 to 18 MeV with a maximum at 7 MeV resulting in a minimal penetration depth in water of approximately 500 µm. To avoid stopping of protons within the bottom of the cell culture vessel or the cell monolayer, dedicated material for cell cultivation and irradiation had to be established to minimize material on the proton path. Cells of the radiosensitive tumour cell line SKX were seeded at thin biofilm (50 µm thickness) in place of conventional cultivation vessels. To quantify the irradiation damage, residual DNA double strand breaks were detected using the immunofluorescence H2AX/53BP1 staining technique 24 h after irradiation, optimized for the special cell carrier material. Cell samples were irradiated with three different doses applied by different proton pulse numbers and controlled by online dose measurement. As online dosimetry system an ionization chamber was used, cross-calibrated against Gafchromic EBT radiochromic films and a Faraday cup to provide precise dose determination at the cell site.

Results: The successful in vitro cell irradiation by laser-accelerated protons represents an important milestone on the long term development of laser ion acceleration for clinical radiotherapy. The laser accelerator generated a stable and reproducible proton beam over the whole experiment time. The measurement of the ionization chamber as online dosimeter showed a stable mean dose per pulse of (0.1370.039) Gy during all irradiations. The EBT films verified a homogeneous dose distribution at the cell location. The irradiated tumour cells demonstrated a clear trend in the number of DNA double-strand breaks in accordance with delivered dose.

Conclusion: Systematic cell irradiation experiments with laser-accelerated protons has been started determining dose-effect curves for both tumour and normal tissue cell lines and also including the cell surviving assay as second biological endpoint. In addition to the experiments with laser-accelerated proton pulses reference cell irradiations are performed using a continuous proton beam at a conventional tandem accelerator.

Funding: This work was supported by the BMBF (no. 03ZIK445).

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