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

Purpose: The novel technology of laser based particle acceleration promises accelerators of compact size and reasonable costs that may significantly contribute to a widespread use of ion radiotherapy. Although some basic properties of laser acceleration are reasonably well known from theory, simulations and fundamental experiments, several demands have to be fulfilled for its medical application. Moreover, the ultra-short pulsed particle beams with resulting high pulse dose-rate have to be characterized with regard to their radiobiological properties. Therefore, a precise dosimetry is necessary that considers the special characteristics of the laser accelerated protons. Special attention has to be drawn on the low energy (<10 MeV), the exponential energy spectrum, the low repetition rate and the pulse-to-pulse fluctuations of the available laser accelerated proton beams.

Material & Methods: An Integrated Dosimetry and Cell Irradiation Device (IDCID) for systematic in vitro and in vivo experiments with laser accelerated protons was developed and characterized. The IDCID (Fig. 1) consists of a kapton vacuum window, an ionisation chamber consisting of ultra-thin foils (22.5 µm) for online dose information and a Faraday cup inset for absolute dosimetry that can be replaced by an inset for cell or even mouse model irradiation. Radiochromic films, i.e. GafChromic EBT, or CR-39 solid state track detectors can be included matching the plane of the cell monolayer. For the use of the Faraday Cup (FC) for absolute dosimetry and cross-calibration of the ionisation chamber, an accurate FC calibration is necessary. Therefore, three independent ways were chosen: (1) electronic calibration by applying a defined charge to the FC amplifier, (2) dose calibration against a clinical established absolute dosimetry and (3) calibration with CR-39 solid state track detectors. The dose calibration was performed at the proton therapy facility at the Helmholtz Zentrum Berlin, where also EBT and EBT2 films were calibrated for determination of 2D dose distribution. Moreover, the FC calibration with the help of CR-39 was carried out at the Tandem accelerator of the Forschungszentrum Dresden-Rossendorf.

Results: Successfully, the functional capability of the IDCID was thoroughly tested and precisely calibrated with three independent methods. EBT / EBT2 films were calibrated for several proton beam qualities with mean energies between 5 - 62 MeV. As next step, first systematic in vitro cell irradiations were performed for a human squamous cell carcinoma (FaDu) cell line irradiated with 7 MeV monoenergetic protons.

Conclusion: Both the dosimetric and radiobiological requirements for systematic cell irradiations with laser accelerated protons have been fulfilled.

Supported by the BMBF, grant 03ZIK445

Fig. 1: Integrated dosimetry and cell irradiation device (a) with Faraday cup inset, (b) with cell holder inset