An integrated dosimetry and cell irradiation device for in vitro cell experiments withlaser accelerated protons

Background: 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 hadron radiotherapy. Although some basic properties of laser acceleration are reasonably well known from theory, simulations and fundamental physical experiments, several requests have to be fulfilled for its medical application. Moreover, the ultra-short pulsed (≈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. Therefore a precise dosimetry is necessary that takes into account the special characteristics of the laser accelerated protons. Special attention has to be drawn on the low energy (<10 MeV) of the available laser accelerated proton beams.

Material and Methods: An integrated dosimetry and cell irradiation device has been designed and optimized for the use with a laser accelerated proton beam of low energy. The device consists of a kapton vacuum window, an ultra-thin (25 µm) ionisation chamber for online dose information, a faraday cup inset for absolute dosimetry that can be replaced with a cell holder inset for cell irradiations. Moreover radiochromic films, i.e. Gafchromic EBT, can be placed in the cell holder inset matching the plane of the cell mono layer. Radiochromic films can be used to determine the two dimensional dose distribution and also for absolute dosimetry. For these purposes Gafchromic EBT films have been calibrated for different proton spectra at the eye irradiation facility of the Helmholtz Zentrum Berlin. Moreover first tests of the Faraday cup and the ionisation chamber have been carried out at the tandem accelerator of the Forschungszentrum Dresden-Rossendorf to characterize these detectors.
Fig 1: Integrated dosimetry and cell irradiation device

Results: Calibration curves for EBT films have been determined for 6 energies between 5 and 62 MeV and doses between 0.5 and 15 Gy. For energies between 10 and 62 MeV no significant energy dependence was found, whereas for 5 MeV protons the film response is reduced by up to 25 % compared to the 62 MeV proton beam quality. Moreover, this reduction in film sensitivity is dose depend. At the tandem accelerator the dependency of the Faraday cup signal from air pressure and guard ring voltage was determined. A more detailed cross calibration of Faraday Cup, ionization chamber and radiochromic films is ongoing and will be presented.

Conclusion: An integrated dosimetry and cell irradiation device for in vivo cell irradiation experiments with laser accelerated protons of low energy was designed, built and started to characterize. This includes the calibration of radiochromic films (Gafchromic EBT) and first characterizations of the different detectors.