Dosimetry of laser-accelerated particle beams used for in-vitro cell irradiation experiments

Introduction: Before the novel technology of laser-based particle acceleration can be used for clinical applications, several requirements have to be fulfilled. These are the supply of stable and reliable particle beams with reproducible properties, sufficient particle intensity and useable energy spectra. Additionally, a precise dose delivery and the exposure of a desired irradiation field are needed. Beside these demands, consequences on dosimetry as well as on the radiobiological effect have to be investigated for ultra-short and ultra-intense pulsed laser-accelerated particle beams.
Material and Methods: Before conducting experiments with laser-accelerated particles, the response of different solid state dosimeters (TLD, OSL, diamond ionization chamber, EBT radiochromic films) to short, intense pulses of MeV electrons have been measured. After these successful characterizations, the worldwide first systematic radiobiological cell irradiations with laser-accelerated electrons have been performed with the JeTi laser system at the FSU Jena. The electron beam was controlled and monitored by means of a Roos ionization chamber (PTW Freiburg, Freiburg, Germany) and an in-house Faraday Cup for a defined dose delivery of the prescribed dose. Moreover, the precise absolute dose delivered to each cell sample was determined by an EBT film (ISP Corp., Wayne, NJ, USA) positioned in front of the cell sample. Furthermore, the energy spectrum of the laser-accelerated electron beam was determined both from measurements with an electromagnetic spectrometer and calculated from depth dose distributions measured with EBT film stacks.
In a next step, cell irradiation experiments with laser-accelerated protons have been prepared. Therefore, an Integrated Dosimetry and Cell Irradiation Device (IDCID) was developed. It can be installed at different laser and conventional accelerators and is used for both dosimetry and cell irradiations. The device consists of an ionization chamber made of ultra-thin foils (total thickness of 22.5 µm) for online dose information and a Faraday Cup inset for absolute dosimetry that can be replaced with a cell holder inset for cell irradiations. Moreover, EBT films or CR-39 solid state track detectors can be placed in the cell holder inset matching the plane of the cell mono layer. The IDCID was thoroughly tested and characterized with monoenergetic protons of energies between 5-9 MeV at a conventional Tandem accelerator. The design of the Faraday Cup (FC) was adopted from Cambria et al. Its absolute calibration (signal to charge correlation) was performed on 3 independent ways: (1) electronic calibration by applying a defined charge to the FC amplifier, (2) dose calibration against an established absolute dosimetry at a clinical proton facility (Helmholtz Zentrum Berlin, Germany) and (3) a calibration with CR-39. As the device presented excellent working capabilities, first cell irradiations using the IDCID were performed at the DRACO 150 TW laser system at the FZD.
As radiochromic films were used for different dosimetric aspects and beam qualities, both EBT and EBT2 films have been calibrated for several beam qualities, e.g. proton, electron and photon beams of different energies, partially presented in Richter et al.
Results: Both a laser-accelerated electron and proton beam have been optimized, monitored and controlled in terms of dose homogeneity, stability and absolute dose delivery and were used for sys-tematic radiobiological cell experiments. A combi-nation of different dosimetric components were used to provide both a online beam- and dose-monitoring and a precise absolute dosimetry. These dosimeters have been thoroughly tested, characterized, precisely calibrated, and finally applied successful.
Conclusion: A precise dosimetric characterization, optimization and control of laser-accelerated and therefore ultra-short pulsed, intense particle beams is possible, allowing radiobiological experiments and meeting all necessary requirements like homogeneity, stability and precise dose delivery. In order to fulfill the much higher dosimetric requirements for clinical application, several improvements concerning i.e. proton energy, spectral shaping and patient safety have been identified.
References:
Cambria, R., Hérault, J., Brassart, N., Silari, M. and Chauvel, P., 1997, Phys Med Biol 42 1185-1196
Richter, C., Karsch, L., Woithe, J. and Pawelke, J., 2009, Med Phys 36, 5506-5514