Worldwide first systematic in vitro cell irradiation experiments with laser accelerated electrons

The novel technology of particle acceleration based on high intensity laser systems promises accelerators of compact size and reasonable costs and may significantly contribute to a widespread use of high precision hadron radiotherapy. Although some basic properties of laser acceleration are reasonably well known from theory, simulations and fundamental physical experiments, several further requests have to be fulfilled for its medical application such as supply of a stable and reliable particle beam with reproducible properties and precise delivery of dose in an appropriate irradiation time with required exposure of a desired irradiation field. Moreover, the ultra-short pulsed (in the region of 100 fs) particle beams with resulting high pulse dose-rate (in the order of 1012 Gy/min) have to be characterized with regard to their radiobiological properties.
First in-vitro cell irradiations with laser accelerated electrons have been performed at the Jena Titanium:Sapphire (JeTi) 10 terawatt laser system and dose-effect-curves were obtained for four cell lines and two endpoints. Laser pulses (80 fs duration, 2.5 Hz repetition rate) were focused into a helium gas jet, accelerating electrons to energies of up to 20 MeV. Before irradiation, the JeTi system was optimized for cell experiments: the electron spectrum was limited to a minimum energy of 3 MeV, the beam spot size was adjusted and the dose rate and homogeneity were improved. Each cell sample was equipped with two Gafchromic EBT radiochromic films, one in front and one behind the cell monolayer, used for retrospective precise dose determination. A Roos ionization chamber and a Faraday Cup monitored the beam providing on-line dose information necessary for irradiation control. Moreover the energy spectrum was measured both with an electromagnetic spectrometer and by analyzing film stack measurements. Following to the irradiation the cell survival fraction was determined using clonogenic survival assay. In addition, DNA double strand breaks present in cell 24 h after irradiation were analyzed.
Normally used for physical single-shot experiments, the JeTi was customized for a long-time cell irradiation. 163 Samples were irradiated at 13 experiment days over a period of 10 weeks with doses between 0.3 and 10 Gy. A reasonably stable and reproducible beam was achieved. Dose homogeneity was examined for all samples within the target area and the inhomogeneity obtained was less than 10 % for all days and all applied doses. Although still preliminary, the dose-effect-curves obtained show in general a lower biological effectiveness for the laser accelerated electron beams in comparison with conventional x-rays.