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Fiberoptical dosimetry uses compact solid state radioluminescence probes coupled to long to flexible light guides. Such probes are convenient, robust small. Especially in the presence of magnetic fields, such optical probes can be advantageous over the transmission of a current signal, e.g. from a photodiode directly attached to a scintillator. These characteristics make such probes attractive for emerging medical applications like particle therapy in combination with MRI. Challenging remains the discrimination of so called “stem” effect light which is generated in the fiber from the actual signal light from the probe. In this work, fiber probes attached to a single photon sensor have been exposed to a proton beam from a proton cyclotron in order to study the feasibility of fiber dosimetry in therapeutic proton fields. Probes with different luminophores have been placed in the beam within a PMMA holder of 1 cm thickness. The light guide was coupled to a Hamamatsu H12386-210 single photon detector. For timing information, the pulse was sampled and analyzed with a Serious Dynamics DAQ125 board. This is a 16 bit sampling ADC board which was running synchronous to the cyclotron. An interpolated time stamp with a resolution of 30 ps was calculated in realtime. It could be shown that a time resolved measurement of the single luminescence photons exhibits the time structure of the luminophore, e.g. the long decay which appears uncorrelated of beryllium oxide or lithium tetraborate or the decay in the ns range of plastic scintillators. Blank fiber measurements exhibit the microbunch width of the accelerator. Thereby, stem identification in therapeutic hadron fields is possible without further reference measurements. Additionally, the issue of quenching of many luminophores in hadron fields can be addressed. A further beamtime at the AGOR cyclotron in Groningen will be conducted in May 2019 and recent results will be presented at the conference.

Optically stimulated luminescence (OSL) is gaining greater importance in the field of personal dosimetry in the last few years. Its principle is based on the release of small amounts of light induced by the prior absorption of ionizing radiation. One suitable luminophore for OSL is beryllium oxide (BeO). Because of its near tissue equivalent effective atomic number of 7, it is excellent for personal dosimetry. Furthermore, the luminescence signal has a wide dose linearity ranging from the µGy region up to few Gy. For this reason, this ceramic can be used for several different areas of application. A new generation of measurement systems based on the OSL of BeO, which has a very low OSL light intensity, was developed by the radiation physics group at TU Dresden. This property allows single photon detection which is superior in contrast to other detection methods. Therefore, a single photon sensor was used as a detector. The single photon mode of the detector in combination with the so called timestamp detection method allows accessing the greatest possible information of the OSL light. This work applies the new system to dosimetry of a proton beam. Because of the LET-dependencies of the luminescence light, this presents a challenge. Common problems of solid state dosimetry are local saturation effects, which were investigated for BeO. Opportunities for correction in terms of the LET-dependency of the luminophore are being discussed. For the empirical determination of the behavior of BeO in proton beams, measurements at the medical proton therapy facility at the University Proton Therapy Dresden (UPTD) were carried out. All collected data were analyzed for LET-dependency on the response signal. For the measurements, BeO ceramics were placed at different depth infront and inside the spread out Bragg peak (SOBP). The dose read from dosimeters was analyzed with respect to the applied dose and the LET. All measurements infront of the SOBP shows no deviation of the estimated dose. The dose determination in the SOBP yielded an underestimation by 15%. This is object of current investigation.

Positron emission tomography (PET) is a means of imaging the β+-activity produced by the radiation field in ion beam therapy and therefore for treatment verification. Prompt Γ-rays that are emitted during beam application challenge the detectors and electronics of PET systems, since those are designed for low and medium count rates. Typical PET detectors operated according a modified Anger principle suffer from multiple events at high rates. Therefore, in-beam PET systems using such detectors rely on a synchronization of beam status and measurement to reject deteriorated data. In this work, a method for pile-up rejection is applied to conventional Anger logic block detectors. It allows for an in-beam data acquisition without further synchronization. Though cyclotrons produce a continuous wave beam, the radiation field shaping technique introduces breaks in the application. Time regimes mimicking synchrotrons as well as cyclotron based ones using double-scattering or pencil beam scanning field shaping at dose rates of 0.5, 1.0 and 2.0 Gy/min were investigated. Two types of inhomogeneous phantoms were imaged. The first one simulates cavity structures, the other one mimics a static lung irradiation. It could be shown that, depending on the dose rate and the beam time structure, in-beam measurement including a few seconds decay time only, yield images which revealed all inhomogeneities in the phantoms. This technique can be the basis for the development of an in-beam PET system with traditional detectors and off-the-shelf electronics.

Alternativ zu Operation und Chemotherapie bei der Behandlung von Tumoren hat die Strahlentherapie einen festen Platz in der medizinischen Behandlung eingenommen. Neben der Therapie mit Röntgenstrahlung werden auch geladene Atome als Teilchenstrahl verwendet. Diese haben eine endliche Reichweite in Materie und deponieren keine schädliche Dosis dahinter. Um diesen Vorteil nutzen zu können muss die Eindringtiefe des Strahls erfasst werden. CdZnTe als Strahlungsdetektor besitzt dieses Potential.

Proton beams are promising means for treating tumors. Such charged particles stop at a defined depth, where the ionization density is maximum. As the dose deposit beyond this distal edge is very low, proton therapy minimizes the damage to normal tissue compared to photon therapy. Nevertheless, inherent range uncertainties cast doubts on the irradiation of tumors close to organs at risk and lead to the application of conservative safety margins. This constrains significantly the potential benefits of protons over photons. In this context, several research groups are developing experimental tools for range verification based on the detection of prompt gammas, a nuclear by-product of the proton irradiation. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf, detector components have been characterized in realistic radiation environments as a step toward a clinical Compton camera. On the one hand, corresponding experimental methods and results obtained during the ENTERVISION training network are reviewed. On the other hand, a novel method based on timing spectroscopy has been proposed as an alternative to collimated imaging systems. The first tests of the timing method at a clinical proton accelerator are summarized, its applicability in a clinical environment for challenging the current safety margins is assessed, and the factors limiting its precision are discussed.

Prompt γ-ray imaging in hadron therapy is a novel approach for range verification. Due to the high energy of prompt γ-rays emitted during therapeutic irradiation in the order of MeV, Compton imaging is a feasible method. In this work, an imaging prototype together with the corresponding data handling and an image reconstruction framework are presented. Data and reconstructed images from laboratory measurements are shown and evaluated. A spatial resolution of 7 mm full width at half maximum in a distance of 7 cm has been achieved. More importantly, current limitations were identified for further work. It has been shown that an assumption on the unknown initial photon energy can considerably improve the imaging result.

A compact high-throughput gamma-ray timing and energy spectrometry systems has been characterized with respect to time and energy resolution at detector loads up to 1.5 Mcps by using 0.1-12.5 MeV bremsstrahlung and a common radioactive source. The detection system, developed for range assessment in particle therapy based on the Prompt Gamma-Ray Timing (PGT) technique, consist of a commercial ⌀2”×1” or ⌀2”×2” CeBr 3 scintillation detector with photomultiplier readout (Scionix), coupled to a digital plug-on spectrometer (U100) by Target Systemelektronik. The excellent time structure of the bremsstrahlung beam at ELBE (HZDR) allowed measuring the system time resolution simultaneously for 0.1-12 MeV photons. Also, the beam current and thus the detector load could be varied in a wide range. The energy resolution was determined in parallel with a 60 Co-source attached to the detector. Source and bremsstrahlung photons were separated by means of cuts in the timing spectra measured against the accelerator RF. The detection system could demonstrate a time resolution of < 260 ps (FWHM) for energies above 3 MeV with the smaller crystal, and an energy resolution of < 3.7% at 1.173 MeV for both crystals, up to detector loads of 1.4 Mcps and a corresponding system throughput of ~ 600 kcps. This fits well with the design goals of the detection system. Corresponding units shall be used for measuring PGT spectra during patients treatments with proton beams, in order to verify the proton beam range for single beam spots in Pencil Beam Scanning (PBS) mode. Range verification is considered a key for reducing margins and for improving precision and health outcome of particle therapy.

Positron emission tomography can yield in-vivo range information in hadron tumor therapy. Measurements are preferably done during the irradiation to minimize the loss of information due to physical decay and a metabolic transport of the nuclides. One challenge is the high number of background events due to prompt photons and neutron induced radiation. In this work, pulse shape discrimination has been applied to conventional Anger logic block detectors to enable an in-beam acquisition at a cyclotron generated beam. Images could be acquired with clinical dose rates in phantom experiments. This technique could pave the way towards more cost effective in-beam PET hardware.

A Compton camera prototype for the position-sensitive detection of prompt γ rays from nuclear reactions between proton (or ion) beams and organic targets is being commissioned in Garching. The detector system is designed to allow for reconstructing the γ-source position from the Compton scattering kinematics of the primary photon as well as from tracking the Compton-scattered electron trajectory. The camera consists of a monolithic LaBr3(Ce) scintillation absorber crystal, read out by a 256-fold segmented multi-anode PMT and preceded by a stacked array of 6 double-sided silicon strip detectors acting as scatterers. The detector system has been calibrated and characterized in the laboratory as well as at different accelerator facilities, including clinical proton beams. Results from online commissioning runs will be presented, demonstrating excellent agreement between experimental prompt-γ spectra and Monte-Carlo simulations. Particular effort was dedicated to characterize the spatial resolution achievable with the monolithic LaBr 3 (Ce) scintillator when targeting the primary photon interaction position. Intense, tightly collimated 137Cs and 60Co sources were used for 2D irradiation scans as prerequisite for studying the performance of the “k-Nearest Neighbour”-algorithm developed in Delft and extending its applicability into the energy range beyond 511 keV. Systematic results of the monolithic scintillator’s spatial resolution will be presented as a function of the k-NN parameters (e.g. events per irradiation position) and the PMT segmentation, resulting in the realization of the presently optimum spatial resolution (3.8(2) mm @1.3 MeV) already for a reduced PMT segmentation (8x8). An outline of ongoing and planned further experimental activities and upgrade plans will be presented. This work is supported by the DFG Cluster of Excellence MAP (Munich-Centre for Advanced Photonics).

(1) Purpose In order to guarantee the best outcome of a therapeutic irradiation with protons and other light ions a non-invasive in-vivo range verification is desired. One approach in this field is Prompt y-ray Imaging (PGI). A possible detection system for the prompt y-rays is the Compton camera. Several groups have been working on the construction of Compton camera prototypes (Kormoll et al., 2011; Llosá et al., 2013; Thirolf et al., 2014; Hueso-González et al., 2014). Up to now, Compton cameras have not been used in clinical practice for the monitoring of particle therapy. Therefore, by means of Geant4 simulations, we performed an end-to-end test to evaluate the clinical applicability of a Compton camera detection system and to determine the requirements regarding hardware and image reconstruction. (2) Materials/methods First, a treatment plan for a therapeutic proton irradiation for the head-neck region was prepared using XiO (Electa AB, Sweden). Based on this treatment plan, the y-ray emissions from the patient's tissue were simulated with Geant4. As a next step, the detector response was modelled, also with Geant4, for two large Compton cameras arranged around the patient in an angle of 90 degrees. Large-area detectors were already recommended (McCleskey et al., 2015). Each camera was built up from a scatter layer (CdZnTe) of dimension 10 × 10 × 0.5 cm3 and an absorber layer (LuSiO) of size 20 × 20 × 2 cm3. In practice, these cameras would be replaced by several smaller camera modules. For the simulation of the detector response a total number of previously simulated y-ray emissions were used as input corresponding to an applied dose of 1 Gy, i.e. a common dose of one field of one treatment session. After extracting the resulting coincident events, the image was reconstructed using a 3D MLEM algorithm (Schoene et al., 2015). The impact of the number of events as well as background on the image quality was also studied. (3) Results Figure 1 shows the images for the planned dose, the distribution of the y-ray emissions and the reconstructed image obtained with 128 iterations of the MLEM algorithm. For the considered number of events and the chosen voxels of 5 mm3 the runtime of the reconstruction was about two days on a cluster. (4) Conclusions For the considered large camera system and the realistic patient scenario with a dose of 1 Gy adequate images are obtained, which certainly could be applied to detect range deviations in the range of centimeters. Thus, this study demonstrates in principle the clinical applicability. The reconstruction algorithm still has potential for improvements with respect to performance. Furthermore, in practice, the costs of this complex detection system could lead to the preference of simpler methods of PGI.

CdZnTe is a semiconductor material with very good properties for gamma-ray detection. A relatively large band gap of 1.57 eV allows the use at room temperature without any additional cooling. Since holes are nearly immobile compared to electrons in CdZnTe, spectroscopic readout requires special techniques which reduce the depth dependence of the signal. One possibility is to apply an anode pattern which exploits the small pixel effect which induces a signal as the charge cloud is very close to the anode. However, a small current is already induced as the charge cloud travels the bulk. The additional drift current will be accumulated at the charge sensitive preamplifier and influences the base line determination algorithm and results in an additional energy input. We present a simple approach to eliminate the influence of this drift current with the use of pulse shape discrimination implemented in an FPGA of a sampling ADC board. With this algorithm and the known transfer function of the preamplifier a proper depth correction is possible to improve the spectral resolution.

The finite range of a proton beam in tissue opens new vistas for the delivery of a highly conformal dose distribution in radiotherapy. However, the actual particle range, and therefore the accurate dose deposition, is sensitive to the tissue composition in the proton path. Range uncertainties, resulting from limited knowledge of this tissue composition or positioning errors, are accounted for in the form of safety margins. Thus, the unverified particle range constrains the principle benefit of proton therapy. Detecting prompt gamma-rays, a side product of proton-tissue interaction, aims at an on-line and non-invasive monitoring of the particle range, and therefore towards exploiting the potential of proton therapy. Compton imaging of the spatial prompt gamma-ray emission is a promising measurement approach. Prompt gamma-rays exhibit emission energies of several MeV. Hence, common radioactive sources cannot provide the energy range a prompt gamma-ray imaging device must be designed for. In this work a benchmark measurement-setup for the production of a localized, monoenergetic 4.44MeV gamma-ray source is introduced. At the Tandetron accelerator at the HZDR, the proton-capture resonance reaction 15N(p , alpha gamma4.439)12C is utilized. This reaction provides the same nuclear de-excitation (and gamma-ray emission) occurrent as an intense prompt gamma-ray line in proton therapy. The emission yield is quantitatively described. A two-stage Compton imaging device, dedicated for prompt gamma-ray imaging, is tested at the setup exemplarily. Besides successful imaging tests, the detection efficiency of the prototype at 4.44MeV is derived from the measured data. Combining this efficiency with the emission yield for prompt gamma-rays, the number of valid Compton events, induced by gamma-rays in the energy region around 4.44MeV, is estimated for the prototype being implemented in a therapeutic treatment scenario. As a consequence, the detection efficiency turns out to be a key parameter for prompt gamma-ray Compton imaging limiting the applicability of the prototype in its current realization.

A primary subject of the present research in particle therapy is to ensure the precise irradiation of the target volume. The prompt gamma timing (PGT) method provides one possibility for in vivo range verification during the irradiation of patients. Prompt gamma rays with high energies are emitted promptly due to nuclear reactions of protons with tissue. The arrival time of these gammas to the detector reflects the stopping process of the primary protons in tissue and are directly correlated to the range. Due to the time resolution of the detector and the proton bunch time spread, as well as drifts of the bunch phase with respect to the accelerator frequency, timing spectra are smeared out and compromise the accuracy of range information intended for future clinical applications. Nevertheless, counteracting this limitation and recovering range information from the PGT measured spectra, corrections using a phase beam monitor can be performed. A first prototype of phase beam monitor was tested at GSI Darmstadt, where measurements of the energy profile of the ion bunches were performed. At the ELBE accelerator Helmholtz-Zentrum Dresden-Rossendorf (HZDR), set up to provide bremsstrahlung photons in very short pulses, a constant fraction algorithm for the incoming digital signals was evaluated, which is for optimizing the time resolution. Studies of scattering experiments with different thin targets and detector positions are accomplished at Oncoray Dresden, where a clinical proton beam is available. These experiments allow a basic characterization of the proton bunch structure and the detected yield.

In proton therapy, high dose conformality and the finite range of the projectiles are exploited to reduce the dosage to healthy tissue while increasing the dose inside the tumor volume compared to conventional radiotherapy. However, those benefits can be diminished by range uncertainties. An online range verification and in-vivo dosimetry is therefore highly desired. The prompt gamma ray timing (PGT) method utilizes the detection time of high energetic photons emitted during treatment. The time distribution of the gamma rays contains essential information about the range of the protons. However, PGT spectra are smeared by the time spread of the proton bunches. Knowledge about this time spread would help to disentangle the PGT data and to give better input parameters to simulation procedures. At the UniversitaetsProtonenTherapieDresden (Dresden, Germany), a dedicated experiment was realized to measure the time spread of a clinical proton pencil beam created by a Cyclone 230 fixed energy cyclotron from Ion Beam Applications. Two phoswich detectors each made out of plastic scintillator and BGO were placed under 90 degrees to detect coincident protons originating from elastic pp-scattering at a thin slice of PMMA. The time spread was measured for incident proton energies between 70 MeV and 225 MeV as well as for several positions of the momentum limiting slits of the energy selection system. Additionally, the absolute transmission of protons from point of extraction to beam exit was determined. Summarizing, the measured data will help to create reasonable range verification procedures using the PGT method in clinical routine.

The characteristic dose profile of accelerated ions has opened up new horizons in the context of cancer treatment. However, particle range uncertainties strongly constrain the potential of ion beam therapy. Despite of worldwide efforts, a detector system for range and dose delivery assessment in real-time is not yet available for clinical routine. Complementary to the active- and passively collimated prompt gamma ray imaging systems for range assessment under development in several research centers, the prompt gamma ray timing method has been recently proposed. Based on the measurable transit time of ions through matter, the emission times of prompt gamma rays encode essential information about the depth-dose profile. In a collaboration between OncoRay, Helmholtz-Zentrum Dresden-Rossendorf and IBA, the prompt gamma ray timing method was tested for the first time at a clinical proton accelerator (Westdeutsches Protonentherapiezentrum Essen) with different phantoms. Several fast scintillation detectors were used to acquire prompt gamma ray timing distributions at various geometries and proton energies. From the resulting distributions, particle range differences of around 5 millimeters in heterogeneous phantoms were observed. In conclusion, we proved the feasibility of the prompt gamma ray timing method for range verification in a clinical radiation environment and realistic phantoms, which reassures this novel approach as a promising alternative in the field of prompt gamma based in vivo dosimetry.

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Proton therapy is, due to the well defined penetration depth, advantageous compared to classical radiation therapy in oncology. But range uncertainties can arise easily and have to be corrected for, preferably immediately during irradiation. Prompt gammas are a good means of instantaneous determination of the dose deposition. Detection systems have to cope with the high counting rates, energy region of up to several MeV and increased background due to secondary emissions, while providing reliable information on energy, timing and location of the detected gamma ray. Various concepts utilize these prompt gammas for dose verification like collimated systems, Compton cameras or prompt gamma timing. The digital silicon photomultiplier (dSiPM), as a favorable alternative to PMTs because of good timing performances and no requirement of further electronics, has been modelled in order to understand the complex behavior when working with monolithic scintillation crystals. Especially, the selection of trigger- and validation-parameters may lead to different spectrum shapes. This model will be helpful for finding best parameter settings for the required task, because it determines the photons lost in various processes, the trigger timing information and the spatial map. Comparison of modelled spectra and measured spectra are presented.

Characterization of Scintillators with respect to their Application in Prompt Gamma Imaging Systems.

The proton therapy in oncology requires instantaneous and reliable particle range verification, which can be done using prompt gamma emissions. The characteristic requirements of prompt gamma detection include the energy range of up to several MeV, increased background due to secondary emissions and high counting rates. Different concepts make use of these prompt gammas for dose verification like collimated systems or Compton cameras. Additionally to prompt gamma imaging, the prompt gamma timing method has been proposed, utilizing the proton time-of-flight inside the body. Those approaches imply different needs on energy-, spatial- or timing-resolution of the detection system. Various scintillator materials with multiple shapes have been characterized with respect to those requirements using classical photomultiplier tubes (PMT) and different experimental setups and locations. The light output, non-linearity and energy resolution were measured using gamma sources. The timing was characterized at the ELBE facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), using the bremsstrahlung beam with photons up to 12.5 MeV. Planned measurements at the 3 MV Tandetron accelerator at HZDR will provide information at the energy resolution at therapy relevant energies of 4.4 MeV.

The characteristic dose profile of accelerated ions has opened up new horizons in the context of cancer treatment. However, particle range uncertainties strongly constrain the potentialities of ion beam therapy. Despite of worldwide efforts, a detector system for range and dose delivery assessment in real-time is not yet available for clinical routine. Complementary to the active- and passively collimated prompt gamma ray imaging systems for range assessment, the prompt gamma ray timing method has been recently proposed and tested at a research accelerator. Based on the measurable transit time of ions through matter, the emission times of prompt gamma rays encode essential information about the depth-dose profile. In a collaboration between OncoRay, Helmholtz-Zentrum Dresden-Rossendorf and IBA, the prompt gamma ray timing method was tested for the first time at a clinical proton accelerator (Westdeutsches Protonentherapiezentrum Essen) with different phantoms. Several fast scintillation detectors were used to acquire prompt gamma ray timing distributions at various geometries and proton energies. From the resulting distributions, particle range differences of around 5 millimetres in heterogeneous phantoms were observed by simple qualitative inspection. In conclusion, our preliminary analysis points out that the prompt gamma ray timing method for range verification is feasible in a clinical radiation environment and realistic phantoms, which reassures this novel approach as a promising alternative in the field of prompt gamma based in vivo dosimetry.

We present a novel approach for real time range verification in particle therapy based on prompt gamma ray timing (PGT) spectroscopy. The measurement approach relies on the fact that high-energy particles need, dependent on their range, a varying transit time through the irradiated material. This transit time is about 1-2 ns in the case of protons with a range of 5-20 cm. Timing spectroscopy of promptly emitted gamma rays, measured with an arrangement resembling a typical time-of-flight (TOF) setup, encode these transit times and thus give a real time access to the particle range. We show that statistical momenta of PGT distributions such as the center of gravity and the variance incorporate the information on the particle range with a millimeter precision, although measurement uncertainties may cover the described effect at first sight. Typical uncertainties are limited by the detector time resolution and the particle bunch spread, and are finally determined by the spectrum stability. In contrast to other prompt gamma imaging techniques, the PGT method is based on straight timing spectroscopy with a single scintillation detector. Therefore, neither passive nor electronic collimation is required. The proposed idea is verified by an analysis of experimental data taken during proton irradiation experiments at the AGOR facility at KVI-CART in Groningen (The Netherlands). The measurement results are in excellent agreement with Monte Carlo based modeling of the experimental setup. We conclude that precise range assessment is feasible within a few seconds of irradiation due to the direct measurement method.

Proton and ion beams open up new vistas for the curative treatment of tumors, but adequate technologies for monitoring the compliance of dose delivery with treatment plans in real time are still missing. Range assessment, meaning the monitoring of therapy-particle ranges in tissue during dose delivery (treatment), is a continuous challenge considered a key for tapping the full potential of particle therapies. In this context the paper introduces an unconventional concept of range assessment by prompt-gamma timing (PGT), which is based on an elementary physical effect not considered so far: therapy particles penetrating tissue move very fast, but still need a finite transit time—about 1–2 ns in case of protons with a 5–20 cm range—from entering the patient’s body until stopping in the target volume. The transit time increases with the particle range. This causes measurable effects in PGT spectra, usable for range verification. The concept was verified by proton irradiation experiments at the AGOR cyclotron, KVICART, University of Groningen. Based on the presented kinematical relations, we describe model calculations that very precisely reproduce the experimental results. As the clinical treatment conditions entail measurement constraints (e.g. limited treatment time), we propose a setup, based on clinical irradiation conditions, capable of determining proton range deviations within a few seconds of irradiation, thus allowing for a fast safety survey. Range variations of 2 mm are expected to be clearly detectable.

In the context of ion beam therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a Compton camera setup is being developed consisting of two scatter planes: two CdZnTe (CZT) cross strip detectors, and an absorber consisting of one Lu2SiO5 (LSO) block detector. The data acquisition is based on VME electronics and handled by software developed on the ROOT framework. The setup has been tested at the linear electron accelerator ELBE at HZDR, which is used in this experiment to produce bunched bremsstrahlung photons with up to 12.5 MeV energy and a repetition rate of 13 MHz. Their spectrum has similarities with the shape expected from prompt gamma rays in the clinical environment, and the flux is also bunched with the accelerator frequency. The charge sharing effect of the CZT detector is studied qualitatively for different energy ranges. The LSO detector pixel discrimination resolution is analyzed and it shows a trend to improve for high energy depositions. The time correlation between the pulsed prompt photons and the measured detector signals, to be used for background suppression, exhibits a time resolution of 3 ns FWHM for the CZT detector and of 2 ns for the LSO detector. A time walk correction and pixel-wise calibration is applied for the LSO detector, whose resolution improves up to 630 ps. In conclusion, the detector setup is suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the quantitative comparison with simulations and the test of imaging algorithms. Experiments at proton accelerators have also been performed and are currently under analysis.

Segmented Cadmium Zinc Telluride (CdZnTe) detectors are promising candidates for a high resolution radiation detection system due to their good energy and spatial resolution. They are also well suited for medical imaging in a Compton camera application. This first Compton camera prototype is set up with a double sided strip detector in the scatter layer. Such an electrode layout enables a fast timing at the expense of efficiency losses. As a Compton camera needs an adequate number of valid scattering events with precise timing, a pixelated electrode layout performs best. The analog readout electronics for a 8 x 8 pixeldetector (20 x 20 x 5mm3) has been replaced by a digital FPGA-based system. All detector signals are sampled by charge sensitive preamplifiers with 100MS/s and 14 bit resolution. The pulse heights are extracted with an adjustable Moving Window Deconvolution in combination with a slow shaping IIR filter (Recursive Moving Average). To generate a trigger, we implemented a digital constant fraction discriminator (DCFD) on the fast shaped signal. This signal is derived by applying a FIR filter to the deconvoluted cathode signal. The algorithm uses a symmetric [−1, 1] coefficient structure for fast computation. Our results show that the accuracy of the DCFD can be improved by optimizing the filter length. Both algorithms for energy and timestamp calculation have been evaluated with measurements of a 22Na source and at the ELBE accelerator with a pulsed beam. The initial implementation results in an energy resolution of about 5% (FWHM at 511 keV) for a randomly selected pixel and a timing resolution better than 12 ns FWHM for all events up to 10MeV.

Radiation therapy by means of protons or heavier ions can improve the performance of radiotherapy for cancer treatment by delivering dose more locally to tumor tissue. Due to the inherent precision of this irradiation modality a dose deposition monitoring is desirable. It has been shown that positron emission tomography (PET) can be used for that by exploiting the decay of positron emitters which arise from nuclear fragmentations of projectile and target nuclei. This monitoring was implemented and successfully applied to the treatment by carbon ions for more than 440 patients at the pilot facility at GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany. However, it comes with inherent drawbacks, e.g. spatial blurring of the reconstructed dose distribution due to unpredictable metabolically driven transport of the positron emitters within the patient. Alternatively, the imaging of prompt gamma-rays -- photons emitted at time and location of the beam-tissue interaction -- is proposed to estimate dose deposition distributions more precisely and probably in real-time. Determined by their origin, these gamma-ray emission distributions have certain properties, i.e. smoothly extended in the order of a decimeter and a continuous energy spectrum up to 10 MeV. A Compton camera is a single photon imaging device measuring position and energy deposition according to incoherent scattering (Compton scattering) of a photon, and its trajectory afterwards. Since the scattering angle is related to the photon energy before and after scattering a conical surface can be spanned in patient space which covers all possible source locations of the gamma-ray. This surface of response is the Compton camera equivalent to the well known line of response in PET. Spanning this surface (which means performing a backprojection and constructing the system matrix) is the major part of the image reconstruction process of Compton camera imaging (CCI) in terms of design and implementation efforts, and computation time. As shown previously, its precision also strongly impacts the quality of the image reconstruction results. Furthermore, in CCI a realistic system matrix (SM) has to take into account the physical and technological characteristics of the camera like limited energy and spatial resolution as well as Compton camera specific features as: (i) the event and image space are of high dimensionality, i.e. due to the energy of the gamma-rays and multi attribute measurements, (ii) the backprojections are surfaces instead of lines (Anger SPECT, PET) and therefore the SM occupancy is high, and (iii) incompletely absorbed photons may lead to incorrect conclusions on the initial gamma energies and scattering angles and subsequently to misarranged backprojections. These challenges of CCI including their impacts will be discussed. Approaches to overcome them will be proposed and evaluated. This is done by means of measurements using radioactive sources as well as via in-beam tests at the proton beam facility AGOR in at KVI Groningen, Netherlands. Complementary results by means of simulations will be shown.

Radiation therapy is an important treatment modality in cancer therapy. New radiation species, like protons and light ions have the potential to increase tumor conformality of irradiation and to decrease normal tissue dose. Such high precision radiotherapy treatment requires efficient quality assurance techniques. Therefore, a dose monitoring system is highly desirable. Between 1997 and 2008, the in-beam Positron Emission Tomography (PET) method was used at the GSI Helmholtzzentrum fr Schwerionenforschung, Darmstadt, Germany, for monitoring the dose delivered by 12C beams. The spatial distribution of positron emitters generated via nuclear interactions is measured during and shortly after the irradiation. By means of a comparison between measured and simulated activity distribution conclusions on the accuracy of the dose deposition can be drawn. Different modalities of PET, i.e. measuring during the irradiation versus taking data after the treatment have been compared. Recent investigation and limits of the PET method used for in-vivo dose monitoring at ion beams will be presented and discussed. Due to inherent physical restrictions of this method, a direct quantification of the delivered dose is not feasible. Therefore, another approach based on dose monitoring by detection of prompt gamma rays is currently under investigation. In contrast to PET this method relies on the detection of prompt gamma rays between 0 and 10 MeV emitted almost instantaneously during the therapeutic irradiation. To measure these photons a Compton camera design was evaluated with respect to the special requirements and conditions that arise from this application. Different concepts were compared by means of simulation. The complete chain from simulation based on the treatment plan to the iterative reconstruction of the data was developed and is now under optimization. First measurements have been successfully performed with radioactive sources and ion beams.

Therapeutic irradiation with ions and protons is superior to a treatment with gammas with respect to tumour conformity of dose and damage of normal tissue. On the other hand, mispositioning of the patient or density changes in the treated volume may easily compromise the success of treatment. For this reason, non-invasive, in-situ dose verification is necessary. The only clinically proven method up to now is Positron Emission Tomography [Eng04]. This method does not allow direct dose quantification due to limited angle artefacts and washout in the patient. Another promising approach is in-beam SPECT: the detection of prompt gamma-rays following nuclear interaction between the ions and the atoms of the penetrated tissue. Detection systems based on Compton-scattering and pair-conversion are now under investigation. So far, Compton-cameras have been used for diagnostic imaging in nuclear medicine and astronomy. In telescopes also pair-creation cameras are established [Zog04]. In contrast to this, in-beam dose verification systems have to manage the specific energy range and low count rates due to the patient dose. A prototype of a Compton-camera is under construction at OncoRay [Kor10, Fie11, Kor11]. It consists of a scatter layer made from CZT and an absorption layer (LSO). From the energy deposited in the detector planes a cone surface of all possible directions of the incident photon can be reconstructed [Sch11]. The expected energy distribution of the prompt gammas is calculated by means of simulations based on treatment plan data [Fie11]. To optimize the setup simulations are required. Therefore, the Monte-Carlo framework GEANT4 is used for: (1) Analysis of the efficiency in dependence of the geometry; (2) Study of background caused by backscattered photons and other secondary particles; (3) Development of filters for event selection; (4) Study parameters influencing the spatial resolution of the reconstructed image (multiple Compton-scattering inside a detector layer, escape of energy, intrinsic radioactivity of the detector material, Doppler-broadening); (5) Creation of test data sets as input for the reconstruction. For photon energies above 7 MeV the pair production is the dominant electromagnetic process in CZT. To use these events the Compton-camera might be combined with a pair-conversion camera by adding thin silicon layers in between to track the path of the electron and the positron produced during pair conversion. Exploiting this information the direction of the incident photon can be deduced. Unfortunately, the angular resolution is heavily degraded especially for photons with rather low incident energies by small-angle scattering of the secondary particles and by the recoil of the nuclei when a pair production takes place [Gol11]. Both effects were studied whereas the latter is not included in the GEANT4 code version 9.3. This simulation serves as basis for the development of an iterative reconstruction algorithm dedicated to in-beam SPECT with a pair production camera. Apart from the analysis of efficiency, angular resolution and noise, the combination of a pair production and a Compton-camera is a challenging task. Results on simulations for optimizing the setup and the refinement of the reconstruction algorithm for a clinical dose verification system will be presented. [Eng04] W. Enghardt et al., Nucl. Instr. and Meth. A 525 (2004) [Fie11] F. Fiedler et al., Nucl. Sci. Symp. IEEE (2011), accepted [Gol11] C. Golnik et al., Nucl. Sci. Symp. IEEE (2011), accepted [Kor10] T. Kormoll et al., Nucl. Instr. and Meth. A (2010) [Kor11] T. Kormoll et al., Nucl. Sci. Symp. IEEE (2011), accepted [Ric10] M.-H. Richard et al., IEEE Transactions on Nuclear Science (2010) [Sch11] S. Schöne et al., Proc. Fully 3D Meeting (2011) [Zog04] A. Zoglauer et al., Nucl. Sci. Symp. Conf. Rec. IEEE (2004)

Ion beams offer advantages over conventional treatment modalities, such as photons. Because of the way ions deposit their energy on their path through tissue they allow for an increased dose deposition in the tumor volume and reduce the collateral damage to the surrounding healthy tissue. However, small changes in the irradiated volume will lead to a misalignment of the deposited dose maximum and the tumor. Therefore, a dose monitoring system is highly desirable. Positron Emission Tomography (PET) was clinically applied for example between 1997 and 2008 at the GSI Helmholtzzentrum fr Schwerionenforschung, Darmstadt, Germany, for monitoring the dose delivered by 12C beams. Due to inherent limitations of this method, a direct quantification of the delivered dose is not feasible. Therefore, another approach currently under investigation monitors the dose by means of the detection of prompt gamma-rays. A Compton camera may be a feasible technical solution for such a monitoring system. A first prototype for imaging radioactive sources has been developed. Results of simulations used for the optimization of the detector configuration will be presented as well as an approach for the reconstruction.

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