S. Reinhardt

Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams

PURPOSE Dose verification in highly conformal radiation therapy such as IMRT or proton therapy can benefit from the high spatial resolution offered by radio-chromic films such as Gafchromic EBT or EBT2. Recently, a new generation of these films, EBT3, has become available. The composition and thickness of the sensitive layer are the same as for the previous EBT2 films. The most important change is the symmetric layer configuration to eliminate side orientation dependence, which is reported for EBT2 films. METHODS The general film characteristics such as sensitivity to read-out orientation and postexposure darkening evolution of the new EBT3 film are evaluated. Film response has been investigated in clinical photon and proton beams and compared to former EBT2 films. Quenching effects in the proton Bragg peak region have been studied for both, EBT2 and EBT3 films. RESULTS The general performance of EBT3 is comparable to EBT2, and the orientation dependence with respect to film side is completely eliminated in EBT3 films. Response differences of EBT2 and EBT3 films are of the same order of magnitude as batch-to-batch variations observed for EBT2 films. No significant difference has been found for both generations of EBT films between photon and proton exposure. Depth dose measurements of EBT2 and EBT3 show an excellent agreement, though underestimating dose by up to 20% in the Bragg peak region. CONCLUSIONS The symmetric configuration of EBT3 presents a major improvement for film handling. EBT3 has similar dosimetric performance as its precursor EBT2 and can, thus, be applied to dose verification in IMRT in the same way. For dose verification in proton therapy the underresponse in the Bragg peak region has to be taken into account.

A laser-driven nanosecond proton source for radiobiological studies

Ion beams are relevant for radiobiological studies and for tumor therapy. In contrast to conventional accelerators, laser-driven ion acceleration offers a potentially more compact and cost-effective means of delivering ions for radiotherapy. Here, we show that by combining advanced acceleration using nanometer thin targets and beam transport, truly nanosecond quasi-monoenergetic proton bunches can be generated with a table-top laser system, delivering single shot doses up to 7 Gy to living cells. Although in their infancy, laser-ion accelerators allow studying fast radiobiological processes as demonstrated here by measurements of the relative biological effectiveness of nanosecond proton bunches in human tumor cells.

Ionoacoustic characterization of the proton Bragg peak with submillimeter accuracy.

PURPOSE Range verification in ion beam therapy relies to date on nuclear imaging techniques which require complex and costly detector systems. A different approach is the detection of thermoacoustic signals that are generated due to localized energy loss of ion beams in tissue (ionoacoustics). Aim of this work was to study experimentally the achievable position resolution of ionoacoustics under idealized conditions using high frequency ultrasonic transducers and a specifically selected probing beam. METHODS A water phantom was irradiated by a pulsed 20 MeV proton beam with varying pulse intensity and length. The acoustic signal of single proton pulses was measured by different PZT-based ultrasound detectors (3.5 and 10 MHz central frequencies). The proton dose distribution in water was calculated by Geant4 and used as input for simulation of the generated acoustic wave by the matlab toolbox k-WAVE. RESULTS In measurements from this study, a clear signal of the Bragg peak was observed for an energy deposition as low as 10(12) eV. The signal amplitude showed a linear increase with particle number per pulse and thus, dose. Bragg peak position measurements were reproducible within ±30 μm and agreed with Geant4 simulations to better than 100 μm. The ionoacoustic signal pattern allowed for a detailed analysis of the Bragg peak and could be well reproduced by k-WAVE simulations. CONCLUSIONS The authors have studied the ionoacoustic signal of the Bragg peak in experiments using a 20 MeV proton beam with its correspondingly localized energy deposition, demonstrating submillimeter position resolution and providing a deep insight in the correlation between the acoustic signal and Bragg peak shape. These results, together with earlier experiments and new simulations (including the results in this study) at higher energies, suggest ionoacoustics as a technique for range verification in particle therapy at locations, where the tumor can be localized by ultrasound imaging. This acoustic range verification approach could offer the possibility of combining anatomical ultrasound and Bragg peak imaging, but further studies are required for translation of these findings to clinical application.

The effects of ultra-high dose rate proton irradiation on growth delay in the treatment of human tumor xenografts in nude mice.

The new technology of laser-driven ion acceleration (LDA) has shown the potential for driving highly brilliant particle beams. Laser-driven ion acceleration differs from conventional proton sources by its ultra-high dose rate, whose radiobiological impact should be investigated thoroughly before adopting current clinical dose concepts. The growth of human FaDu tumors transplanted onto the hind leg of nude mice was measured sonographically. Tumors were irradiated with 20 Gy of 23 MeV protons at pulsed mode with single pulses of 1 ns duration or continuous mode (∼100 ms) in comparison to controls and to a dose-response curve for 6 MV photons. Tumor growth delay and the relative biological effectiveness (RBE) were calculated for all irradiation modes. The mean target dose reconstructed from Gafchromic films was 17.4 ± 0.8 Gy for the pulsed and 19.7 ± 1.1 Gy for the continuous irradiation mode. The mean tumor growth delay was 34 ± 6 days for pulsed, 35 ± 6 days for continuous protons, and 31 ± 7 days for photons 20 ± 1.2 Gy, resulting in RBEs of 1.22 ± 0.19 for pulsed and 1.10 ± 0.18 for continuous protons, respectively. In summary, protons were found to be significantly more effective in reducing the tumor volume than photons (P < 0.05). Together with the results of previous in vitro experiments, the in vivo data reveal no evidence for a substantially different radiobiology that is associated with the ultra-high dose rate of protons that might be generated from advanced laser technology in the future.

Ionoacoustic tomography of the proton Bragg peak in combination with ultrasound and optoacoustic imaging

Ions provide a more advantageous dose distribution than photons for external beam radiotherapy, due to their so-called inverse depth dose deposition and, in particular a characteristic dose maximum at their end-of-range (Bragg peak). The favorable physical interaction properties enable selective treatment of tumors while sparing surrounding healthy tissue, but optimal clinical use requires accurate monitoring of Bragg peak positioning inside tissue. We introduce ionoacoustic tomography based on detection of ion induced ultrasound waves as a technique to provide feedback on the ion beam profile. We demonstrate for 20 MeV protons that ion range imaging is possible with submillimeter accuracy and can be combined with clinical ultrasound and optoacoustic tomography of similar precision. Our results indicate a simple and direct possibility to correlate, in-vivo and in real-time, the conventional ultrasound echo of the tumor region with ionoacoustic tomography. Combined with optoacoustic tomography it offers a well suited pre-clinical imaging system.

CCD detector development for the eROSITA space telescope

The German X-ray telescope eROSITA is the core instrument on the Russian satellite Spectrum-Roentgen-Gamma (SRG). Its scientific goal is the exploration of the X-ray Universe in the energy band from about 0.3 keV up to 10 keV with excellent energy, time and spatial resolution and large effective telescope area. The launch of the SRG satellite is scheduled for 2013. The observational program divides the planned mission duration of seven years into an all-sky survey and pointed observations. For detection of the single X-ray photons with high resolution, adequate frame transfer pnCCDs and the associated front-end electronics have been developed. The back-illuminated, 450 μm thick and fully depleted pnCCDs with a 3 cm × 3 cm large image area have been produced in the MPI Halbleiterlabor in the course of further development of the XMM-Newton X-ray pnCCDs. By means of the concept of back-illumination and full depletion of the chip thickness, high quantum efficiency is obtained over the entire energy band of interest. The performance of each eROSITA CCD was tested on chip level using a so-called ‘cold chuck probe station’. A special feature of this setup is that it allows spectroscopic measurements with a 55Fe source. Based on these results, we will select the seven best CCDs for the eROSITA focal plane cameras. An analog signal processor with 128 parallel channels has been developed for readout of the pnCCD signals. This ASIC permits fast and low-noise signal filtering. For a detailed characterization of the CCD detectors an appropriate control, supply and data acquisition electronics system was developed. We achieve a read noise of 2 electrons rms and an energy resolution of 135 eV FWHM for photons with energy of 5.9 keV. Even at the low X-ray energy of 280 eV, we measure a spectrum of Gaussian shape with a FWHM of 52 eV. However, the energy resolution will degrade during the seven years in space due to radiation damage caused by protons. The radiation damage effect was studied and quantified for the eROSITA CCDs in an experiment. After successful development and verification of the CCD and its signal processor chip, we have started to assemble a flight-like eROSITA camera.

WE-D-BRF-02: Acoustic Signal From the Bragg Peak for Range Verification in Proton Therapy

PURPOSE Range verification in ion beam therapy relies to date on nuclear imaging techniques which require complex and costly detector systems. A different approach is the detection of thermoacoustic signals that are generated due to localized energy loss of ion beams. Aim of this work is to study the feasibility of determining the ion range with sub-mm accuracy by use of high frequency ultrasonic (US) transducers and to image the Bragg peak by tomography. METHODS A water phantom was irradiated by a pulsed 20 MeV proton beam with varying pulse intensity, length and repetition rate. The acoustic signal of single proton pulses was measured by different PZT-based US detectors (3.5 MHz and 10 MHz central frequencies). For tomography a 64 channel US detector array was used and moved along the ion track by a remotely controlled motor stage. RESULTS A clear signal of the Bragg peak was visible for an energy deposition as low as 1012 eV. The signal amplitude showed a linear increase with particle number per pulse and thus, dose. Range measurements were reproducible within +/- 20 micrometer and agreed well with Geant4 simulations. The tomographic reconstruction does not only allow to measure the ion range but also the beam spot size at the Bragg peak position. CONCLUSION Range verification by acoustic means is a promising new technique for treatment modalities where the tumor can be localized by US imaging. Further improvement of sensitivity is required to account for higher attenuation of the US signal in tissue, as well as lower energy density in the Bragg peak in realistic treatment cases due to higher particle energy and larger spot sizes. Nevertheless, the acoustic range verification approach could offer the possibility of combining anatomical US imaging with Bragg Peak imaging in the near future. The work was funded by the DFG cluster of excellence Munich Centre for Advanced Photonics (MAP).

32P-haltige Folien als Implantate für die LDR-Brachytherapie gutartiger Stenosen in der Urologie und Gastroenterologie☆

For LDR-brachytherapy, a limited number of implant geometries and materials are available. To avoid wound healing related hyper-proliferation (stenosis, keloids) a novel radioactive foil system was developed based on beta emitting (32)P, which can be easily integrated in existing implants such as urethral catheters or bile duct stents. As substrate material for these foils PEEK (polyetherethercetone) was chosen because of its radiation hardness during neutron activation of (32)P. The activity was determined by liquid scintillation counting and gamma spectroscopy, dose distributions were measured with scintillation detectors and radiochromic films. The correlation between activity and dose was checked by Monte-Carlo-simulations (Geant4). Prototypes of the (32)P-implants have shown in wash-out tests the required tightness for sealed radioactive sources. In animal tests on urethra and bile duct, the uncomplicated and save application of (32)P-foils mounted on standard implants has been demonstrated, which is almost unchanged due to the simple radiation protection with plexiglass. This concept of radioactive implants with integrated (32)P-foils could extend essentially the application possibilities of LDR-brachytherapy.

Divergence of laser-driven ion beams from nanometer thick foils

We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2° are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from μm thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle in cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.

Particle detection with PNCCDs

A PNCCD is successfully operating as one of the focal plane CCDs aboard the satellite XMM-Newton. An advanced version of this kind of CCDs will be the sensing devices for the eROSITA X-ray astronomy mission. These fully depleted CCDs are developed and manufactured at the MaxPlanck-Institute Semiconductor Lab together with the company PNSensor. Their performance features make them useful in a variety of measurement situations in addition to the astronomical ones. Applications range from photon detection (e.g. optical wave front sensors, cameras for X-ray free electron lasers) to charged particle detection of e.g. electrons, protons, and alpha particles. First tests have been performed for using a PNCCD in a transmission electron microscope (TEM). Alpha particles of an 241Am radioactive source are used for generating large signal electron clouds for diagnostic purposes. Finally, protons have been used in a radiation hardness test. These three applications will be described and discussed. The prospect of resolving space and energy of each particle in combination with ∼100% efficiency makes the PNCCD especially suitable for low flux applications e.g. examining sensitive samples in a TEM.