Elke Beyreuther

Dose-dependent biological damage of tumour cells by laser-accelerated proton beams

We report on the first irradiation of in vitro tumour cells with laser-accelerated proton pulses showing dose-dependent biological damage. This experiment, paving the way for future radiobiological studies with laser-accelerated protons, demonstrates the simultaneous availability of all the components indispensable for systematic radiobiological studies: a laser-plasma accelerator providing proton spectra with maximum energy exceeding 15MeV and applicable doses of a few Gy within a few minutes; a beam transport and filtering system; an in-air irradiation site; and a dosimetry system providing both online dose monitoring and absolute dose information applied to the cell sample and the full infrastructure for analysing radiation-induced damage in cells.

Dose rate dependence for different dosimeters and detectors: TLD, OSL, EBT films, and diamond detectors

PURPOSEnThe use of laser accelerators in radiation therapy can perhaps increase the low number of proton and ion therapy facilities in some years due to the low investment costs and small size. The laser-based acceleration technology leads to a very high peak dose rate of about 10(11) Gy∕s. A first dosimetric task is the evaluation of dose rate dependence of clinical dosimeters and other detectors.nnnMETHODSnThe measurements were done at ELBE, a superconductive linear electron accelerator which generates electron pulses with 5 ps length at 20 MeV. The different dose rates are reached by adjusting the number of electrons in one beam pulse. Three clinical dosimeters (TLD, OSL, and EBT radiochromic films) were irradiated with four different dose rates and nearly the same dose. A faraday cup, an integrating current transformer, and an ionization chamber were used to control the particle flux on the dosimeters. Furthermore two diamond detectors were tested.nnnRESULTSnThe dosimeters are dose rate independent up to 4●10(9) Gy∕s within 2% (OSL and TLD) and up to 15●10(9) Gy∕s within 5% (EBT films). The diamond detectors show strong dose rate dependence.nnnCONCLUSIONSnTLD, OSL dosimeters, and EBT films are suitable for pulsed beams with a very high pulse dose rate like laser accelerated particle beams.

Establishment of technical prerequisites for cell irradiation experiments with laser-accelerated electrons.

PURPOSEnIn recent years, laser-based acceleration of charged particles has rapidly progressed and medical applications, e.g., in radiotherapy, might become feasible in the coming decade. Requirements are monoenergetic particle beams with long-term stable and reproducible properties as well as sufficient particle intensities and a controlled delivery of prescribed doses at the treatment site. Although conventional and laser-based particle accelerators will administer the same dose to the patient, their different time structures could result in different radiobiological properties. Therefore, the biological response to the ultrashort pulse durations and the resulting high peak dose rates of these particle beams have to be investigated. The technical prerequisites, i.e., a suitable cell irradiation setup and the precise dosimetric characterization of a laser-based particle accelerator, have to be realized in order to prepare systematic cell irradiation experiments.nnnMETHODSnThe Jena titanium:sapphire laser system (JETI) was customized in preparation for cell irradiation experiments with laser-accelerated electrons. The delivered electron beam was optimized with regard to its spectrum, diameter, dose rate, and dose homogeneity. A custom-designed beam and dose monitoring system, consisting of a Roos ionization chamber, a Faraday cup, and EBT-1 dosimetry films, enables real-time monitoring of irradiation experiments and precise determination of the dose delivered to the cells. Finally, as proof-of-principle experiment cell samples were irradiated using this setup.nnnRESULTSnLaser-accelerated electron beams, appropriate for in vitro radiobiological experiments, were generated with a laser shot frequency of 2.5 Hz and a pulse length of 80 fs. After laser acceleration in the helium gas jet, the electrons were filtered by a magnet, released from the vacuum target chamber, and propagated in air for a distance of 220 mm. Within this distance a lead collimator (aperture of 35 mm) was introduced, leading, along with the optimized setup, to a beam diameter of 35 mm, sufficient for the irradiation of common cell culture vessels. The corresponding maximum dose inhomogeneity over the beam spot was less than 10% for all irradiated samples. At cell position, the electrons posses a mean kinetic energy of 13.6 MeV, a bunch length of about 5 ps (FWHM), and a mean pulse dose of 1.6 mGy/bunch. Cross correlations show clear linear dependencies for the online recorded accumulated bunch charges, pulse doses, and pulse numbers on absolute doses determined with EBT-1 films. Hence, the established monitoring system is suitable for beam control and a dedicated dose delivery. Additionally, reasonable day-to-day stable and reproducible properties of the electron beam were achieved.nnnCONCLUSIONSnBasic technical prerequisites for future cell irradiation experiments with ultrashort pulsed laser-accelerated electrons were established at the JETI laser system. The implemented online control system is suitable to compensate beam intensity fluctuations and the achieved accuracy of dose delivery to the cells is sufficient for radiobiological cell experiments. Hence, systematic in vitro cell irradiation experiments can be performed, being the first step toward clinical application of laser-accelerated particles. Further steps, including the transfer of the established methods to experiments on higher biological systems or to other laser-based particle accelerators, will be prepared.

DNA double-strand break signalling: X-ray energy dependence of residual co-localised foci of γ-H2AX and 53BP1

Purpose:u2003The application of ionising radiation for medical purposes requires the investigation of induced and persistent DNA damages, especially for soft X-rays that are assumed to be more effective than higher energy photons. Therefore, we examined the energy dependent time and dose response of residual DNA damage foci for soft X-rays in comparison to 200 kV photons. Materials and methods:u2003DNA damage present in cell line 184A1 within 48u2009h after irradiations with 10 kV, 25 kV and 200 kV photons was analysed by immunochemical detection of co-localised γ-H2AX (phosphorylated histone H2AX) and 53BP1 (tumour protein 53 binding protein) foci. Results:u2003The dose dependencies of the colocated foci revealed significant energy dependent differences with increasing amounts of residual foci at decreasing X-ray energy independent on postirradiation time. Dose-dependent RBE (relative biological effectiveness) values ranging from 4 to 7 were determined for 10 kV relative to 200 kV X-rays based on the 24 hour dose responses. For 25 kV photons, ratios considerably higher than one were obtained only for doses above 2 Gy. Conclusions:u2003The expected energy dependence with increasing DNA damage at decreasing photon energy was confirmed for the residual co-localised foci measured at different time points after irradiation.

Establishment of a small animal tumour model for in vivo studies with low energy laser accelerated particles

BackgroundThe long-term aim of developing a laser based acceleration of protons and ions towards clinical application requires not only substantial technological progress, but also the radiobiological characterization of the resulting ultra-short pulsed particle beams. Recent in vitro data showed similar effects of laser-accelerated versus conventional protons on clonogenic cell survival. As the proton energies currently achieved by laser driven acceleration are too low to penetrate standard tumour models on mouse legs, the aim of the present work was to establish a tumour model allowing for the penetration of low energy protons (~ 20xa0MeV) to further verify their effects in vivo.MethodsKHT mouse sarcoma cells were injected subcutaneously in the right ear of NMRI (nu/nu) mice and the growing tumours were characterized with respect to growth parameters, histology and radiation response. In parallel, the laser system JETI was prepared for animal experimentation, i.e. a new irradiation setup was implemented and the laser parameters were carefully adjusted. Finally, a proof-of-principle experiment with laser accelerated electrons was performed to validate the tumour model under realistic conditions, i.e. altered environment and horizontal beam delivery.ResultsKHT sarcoma on mice ears showed a high take rate and continuous tumour growth after reaching a volume ofu2009~u20095xa0mm3. The first irradiation experiment using laser accelerated electrons versus 200xa0kV X-rays was successfully performed and tumour growth delay was evaluated. Comparable tumour growth delay was found between X-ray and laser accelerated electron irradiation. Moreover, experimental influences, like anaesthesia and positioning at JETI, were found to be negligible.ConclusionA small animal tumour model suitable for the irradiation with low energy particles was established and validated at a laser based particle accelerator. Thus, the translation from in vitro to in vivo experimentation was for the first time realized allowing a broader preclinical validation of radiobiological characteristics and efficacy of laser driven particle accelerators in the future.

Relative biological effectiveness of 25 and 10 kV X-rays for the induction of chromosomal aberrations in two human mammary epithelial cell lines

Administration of ionizing radiation for diagnostic purposes can be associated with a risk for the induction of tumors. Therefore, particularly with regard to general screening programs, e.g. with mammography, cost-benefit considerations must be discussed including risk estimation depending upon the radiation quality administered. The present study was initiated to investigate the in vitro X-ray energy dependence for the induction of chromosomal aberrations in the two mammary epithelial cell lines, 184A1 and MCF-12A. The induced excess fragments, dicentric chromosomes and centric rings were analyzed and the relative biological effectiveness (RBE) was determined for 10 and 25xa0kV X-rays relative to 200xa0kV X-rays. The assumed energy dependence with higher values for 10xa0kV X-rays was confirmed for the excess fragments, with RBEM values of 1.92xa0±xa00.26 and 1.40xa0±xa00.12 for 10xa0kV X-rays and 1.17xa0±xa00.12 and 0.97xa0±xa00.10 for 25xa0kV photons determined for cell lines 184A1 and MCF-12A, respectively. Meaningful results for the induction of dicentric chromosomes and centric rings were obtained only for higher doses with RBE values of 1.31xa0±xa00.21 and 1.70xa0±xa00.29 for 184A1 and 1.08xa0±xa00.08 and 1.43xa0±xa00.12 for MCF-12A irradiated with 25 and 10xa0kV X-rays, respectively.

Radiobiological response to ultra-short pulsed megavoltage electron beams of ultra-high pulse dose rate.

Abstract Purpose: In line with the long-term aim of establishing the laser-based particle acceleration for future medical application, the radiobiological consequences of the typical ultra-short pulses and ultra-high pulse dose rate can be investigated with electron delivery. Materials and methods: The radiation source ELBE (Electron Linac for beams with high Brilliance and low Emittance) was used to mimic the quasi-continuous electron beam of a clinical linear accelerator (LINAC) for comparison with electron pulses at the ultra-high pulse dose rate of 1010 Gy min−1 either at the low frequency of a laser accelerator or at 13 MHz avoiding effects of prolonged dose delivery. The impact of pulse structure was analyzed by clonogenic survival assay and by the number of residual DNA double-strand breaks remaining 24 h after irradiation of two human squamous cell carcinoma lines of differing radiosensitivity. Results: The radiation response of both cell lines was found to be independent from electron pulse structure for the two endpoints under investigation. Conclusions: The results reveal, that ultra-high pulse dose rates of 1010 Gy min−1 and the low repetition rate of laser accelerated electrons have no statistically significant influence (within the 95% confidence intervals) on the radiobiological effectiveness of megavoltage electrons.

Comparison study of in vivo dose response to laser-driven versus conventional electron beam

AbstractnThe long-term goal to integrate laser-based particle accelerators into radiotherapy clinics not only requires technological development of high-intensity lasers and new techniques for beam detection and dose delivery, but also characterization of the biological consequences of this new particle beam quality, i.e. ultra-short, ultra-intense pulses. In the present work, we describe successful in vivo experiments with laser-driven electron pulses by utilization of a small tumour model on the mouse ear for the human squamous cell carcinoma model FaDu. The already established in vitro irradiation technology at the laser system JETI was further enhanced for 3D tumour irradiation in vivo in terms of beam transport, beam monitoring, dose delivery and dosimetry in order to precisely apply a prescribed dose to each tumour in full-scale radiobiological experiments. Tumour growth delay was determined after irradiation with doses of 3 and 6xa0Gy by laser-accelerated electrons. Reference irradiation was performed with continuous electron beams at a clinical linear accelerator in order to both validate the dedicated dosimetry employed for laser-accelerated JETI electrons and above all review the biological results. No significant difference in radiation-induced tumour growth delay was revealed for the two investigated electron beams. These data provide evidence that the ultra-high dose rate generated by laser acceleration does not impact the biological effectiveness of the particles.n

Towards ion beam therapy based on laser plasma accelerators

Abstract Only few ten radiotherapy facilities worldwide provide ion beams, in spite of their physical advantage of better achievable tumor conformity of the dose compared to conventional photon beams. Since, mainly the large size and high costs hinder their wider spread, great efforts are ongoing to develop more compact ion therapy facilities. One promising approach for smaller facilities is the acceleration of ions on micrometre scale by high intensity lasers. Laser accelerators deliver pulsed beams with a low pulse repetition rate, but a high number of ions per pulse, broad energy spectra and high divergences. A clinical use of a laser based ion beam facility requires not only a laser accelerator providing beams of therapeutic quality, but also new approaches for beam transport, dosimetric control and tumor conformal dose delivery procedure together with the knowledge of the radiobiological effectiveness of laser-driven beams. Over the last decade research was mainly focused on protons and progress was achieved in all important challenges. Although currently the maximum proton energy is not yet high enough for patient irradiation, suggestions and solutions have been reported for compact beam transport and dose delivery procedures, respectively, as well as for precise dosimetric control. Radiobiological in vitro and in vivo studies show no indications of an altered biological effectiveness of laser-driven beams. Laser based facilities will hardly improve the availability of ion beams for patient treatment in the next decade. Nevertheless, there are possibilities for a need of laser based therapy facilities in future.

Ultrashort Pulse Laser Accelerated Proton Beams for First Radiobiological Applications

We report on the generation of proton pulses with maximum energies exceeding 15 MeV by means of the irradiation of few micron thick metal foils by ultrashort (30 fs) laser pulses at a power level of 100 TW. In contrast to the well known situation for longer laser pulses, here, a near linear scaling of the maximum proton energy with laser power can be found. Aiming for radiobiological applications the long and short term stability of the laser plasma accelerator as well as a compact energy selection and dosimetry system is presented. The first irradiation of in vitro tumour cells showing dose dependent biological damage is demonstrated paving the way for systematic radiobiological studies.