Katja Roemer

Range assessment in particle therapy based on prompt γ-ray timing measurements

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 patients 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, KVI-CART, 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.

A technique for measuring the energy resolution of low-Z scintillators

Scintillator-based Compton cameras for remote localization and identification of radio nuclides require scatter detectors made of low-Z materials. The energy resolution of such detectors in a range dominated by Compton scattering is a crucial parameter. It has to be known for performance estimates, and it must be quantified and optimized for detector designs to be used in real systems, but it is hard to measure. The Compton Coincidence Technique (CCT) appears to be the best method for reliable and direct measurements, but appropriate facilities are expensive. This paper suggests and investigates a modified CCT which provides less expensive means for qualifying of scatter detectors in a reasonable time frame. The assembly consists of a single HPGe detector, the scatter detector to be investigated, and one or more common gamma sources in close geometry. Pulse height and timing information from both detectors is gathered by multi-parameter data acquisition. Coincidences of both detectors are due to a plurality of Compton scattering angles and corresponding energy transfers. A thorough data analysis then allows extracting the detector resolution as well as the non-linearity as a function of energy from data sets measured within hours. Results obtained for NaI and plastic scatter detectors will be presented and discussed.

Characterization of the microbunch time structure of proton pencil beams at a clinical treatment facility.

Proton therapy is an advantageous treatment modality compared to conventional radiotherapy. In contrast to photons, charged particles have a finite range and can thus spare organs at risk. Additionally, the increased ionization density in the so-called Bragg peak close to the particle range can be utilized for maximum dose deposition in the tumour volume. Unfortunately, the accuracy of the therapy can be affected by range uncertainties, which have to be covered by additional safety margins around the treatment volume. A real-time range and dose verification is therefore highly desired and would be key to exploit the major advantages of proton therapy. Prompt gamma rays, produced in nuclear reactions between projectile and target nuclei, can be used to measure the protons range. The prompt gamma-ray timing (PGT) method aims at obtaining this information by determining the gamma-ray emission time along the proton path using a conventional time-of-flight detector setup. First tests at a clinical accelerator have shown the feasibility to observe range shifts of about 5 mm at clinically relevant doses. However, PGT spectra are smeared out by the bunch time spread. Additionally, accelerator related proton bunch drifts against the radio frequency have been detected, preventing a potential range verification. At OncoRay, first experiments using a proton bunch monitor (PBM) at a clinical pencil beam have been conducted. Elastic proton scattering at a hydrogen-containing foil could be utilized to create a coincident proton-proton signal in two identical PBMs. The selection of coincident events helped to suppress uncorrelated background. The PBM setup was used as time reference for a PGT detector to correct for potential bunch drifts. Furthermore, the corrected PGT data were used to image an inhomogeneous phantom. In a further systematic measurement campaign, the bunch time spread and the proton transmission rate were measured for several beam energies between 69 and 225 MeV as well as for variable momentum limiting slit openings. We conclude that the usage of a PBM increases the robustness of the PGT method in clinical conditions and that the obtained data will help to create reliable range verification procedures in clinical routine.

Simulation and experimental verification of prompt gamma-ray emissions during proton irradiation

Irradiation with protons and light ions offers new possibilities for tumor therapy but has a strong need for novel imaging modalities for treatment verification. The development of new detector systems, which can provide an in vivo range assessment or dosimetry, requires an accurate knowledge of the secondary radiation field and reliable Monte Carlo simulations. This paper presents multiple measurements to characterize the prompt γ-ray emissions during proton irradiation and benchmarks the latest Geant4 code against the experimental findings. Within the scope of this work, the total photon yield for different target materials, the energy spectra as well as the γ-ray depth profile were assessed. Experiments were performed at the superconducting AGOR cyclotron at KVI-CART, University of Groningen. Properties of the γ-ray emissions were experimentally determined. The prompt γ-ray emissions were measured utilizing a conventional HPGe detector system (Clover) and quantitatively compared to simulations. With the selected physics list QGSP_BIC_HP, Geant4 strongly overestimates the photon yield in most cases, sometimes up to 50%. The shape of the spectrum and qualitative occurrence of discrete γ lines is reproduced accurately. A sliced phantom was designed to determine the depth profile of the photons. The position of the distal fall-off in the simulations agrees with the measurements, albeit the peak height is also overestimated. Hence, Geant4 simulations of prompt γ-ray emissions from irradiation with protons are currently far less reliable as compared to simulations of the electromagnetic processes. Deviations from experimental findings were observed and quantified. Although there has been a constant improvement of Geant4 in the hadronic sector, there is still a gap to close.

Electron response of some low-Z scintillators in wide energy range

Light yield nonproportionality and the intrinsic resolution of some low atomic number scintillators were studied by means of the Wide Angle Compton Coincidence (WACC) technique. The plastic and liquid scintillator response to Compton electrons was measured in the energy range of 10 keV up to 4 MeV, whereas a CaF2:Eu sample was scanned from 3 keV up to 1 MeV. The nonproportionality of the CaF2:Eu light yield has characteristics typical for inorganic scintillators of the multivalent halides group, whereas tested organic scintillators show steeply increasing nonproportionality without saturation point. This is in contrast to the behavior of all known inorganic scintillators having their nonproportionality curves at saturation above energies between tens and several hundred keV.

Simulation of Template Spectra for Scintillator Based Radionuclide Identification Devices Using GEANT4

The performance of radioisotope identification devices (RID) for homeland security applications is strongly influenced by nuclide identification algorithms. The identiFINDERtrade exploits template matching - a technique correlating measured gamma ray energy spectra with a library of reference spectra (templates) stored in the device. Of course the template quality impacts the identification performance. Templates can be measured, but such a purely experimental approach is cumbersome and might be expensive, particularly for nuclides which are not always accessible. Therefore Monte Carlo (MC) simulations have been utilized for template generation. The GEANT4 toolkit provides the benefits of an open source program with easy access to appropriate material and radioactive decay data, considering all relevant physical processes. However, dedicated computing techniques had to be applied in order to obtain the necessary (statistical) accuracy at a justifiable computing time. The implemented models have also been used to study the influence of detector size and material, source mantling or matrix, and detector-source geometry on the measured gamma ray spectrum.

Characterization of scintillator crystals for usage as prompt gamma monitors in particle therapy

Particle therapy in oncology is advantageous compared to classical radiotherapy due to its well-defined penetration depth. In the so-called Bragg peak, the highest dose is deposited; the tissue behind the cancerous area is not exposed. Different factors influence the range of the particle and thus the target area, e.g. organ motion, mispositioning of the patient or anatomical changes. In order to avoid over-exposure of healthy tissue and under-dosage of cancerous regions, the penetration depth of the particle has to be monitored, preferably already during the ongoing therapy session. The verification of the ion range can be performed using prompt gamma emissions, which are produced by interactions between projectile and tissue, and originate from the same location and time of the nuclear reaction. The prompt gamma emission profile and the clinically relevant penetration depth are correlated. Various imaging concepts based on the detection of prompt gamma rays are currently discussed: collimated systems with counting detectors, Compton cameras with (at least) two detector planes, or the prompt gamma timing method, utilizing the particle time-of-flight within the body. For each concept, the detection system must meet special requirements regarding energy, time, and spatial resolution. Nonetheless, the prerequisites remain the same: the gamma energy region (2 to 10 MeV), high counting rates and the stability in strong background radiation fields. The aim of this work is the comparison of different scintillation crystals regarding energy and time resolution for optimized prompt gamma detection.

Comparison of LSO and BGO block detectors for prompt gamma imaging in ion beam therapy

A major weakness of ion beam therapy is the lack of tools for verifying the particle range in clinical routine. The application of the Compton camera concept for the imaging of prompt gamma rays, a by-product of the irradiation correlated to the dose distribution, is a promising approach for range assessment and even three-dimensional in vivo dosimetry. Multiple position sensitive gamma ray detectors arranged in scatter and absorber planes, together with an imaging algorithm, are required to reconstruct the prompt gamma emission density map. Conventional block detectors deployed in Positron Emission Tomography (PET), which are based on Lu2SiO5:Ce (LSO) and Bi4Ge3O12 (BGO) scintillators, are suitable candidates for the absorber of a Compton camera due to their high density and absorption efficiency with respect to the prompt gamma energy range (several MeV). We compare experimentally LSO and BGO block detectors in clinical-like radiation fields in terms of energy, spatial and time resolution. The high energy range compensates for the low light yield of the BGO material and boosts significantly its performance compared to the PET scenario. Notwithstanding the overall superiority of LSO, BGO catches up in the field of prompt gamma imaging and can be considered as a competitive alternative to LSO for the absorber plane due to its lower price and the lack of intrinsic radioactivity.

Energy resolution and nonlinearity of NaI(Tl), CaF 2 (Eu), and plastic scintillators measured with the wide-angle Compton-coincidence technique

Compton cameras are of general interest in various fields of operation. Because of the ability to locate and identify remote sources, homeland security supports the development of such devices in a rugged and reliable form. The decisions upon appropriate materials for the scatter- and absorber plane depend on performance and economical trade-offs. In order to estimate the expected performance of the Compton camera, simulations are necessary. Certain experimentally determined parameters have to be fed into simulations, such as the energy resolution of the detector. Two materials with low effective atomic number (Zeff), CaF2 and plastic, promise to be good candidates for the scattering plane. Those scintillators are known for quite some time, but not very well characterized with respect of energy resolution and nonlinearity. A modified Compton coincidence technique using a high purity Germanium (HPGe) detector in coincidence with the investigated scintillator is discussed in this paper: The wide-angle Compton-coincidence (WACC) setup provides a fast and reliable means for characterization of low-Z scintillators. For quality control purposes, the actual scatter detector can be monitored in-house using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.

Application of LaBr 3 (Ce 3+ ) scintillators in radio-isotope identification devices

The advantage of LaBr3(Ce3+) over NaI(TI) scintillators with respect to gamma spectroscopy and radioisotope identification has been discussed in many papers. However, implementing such detectors in field instruments for routine use in homeland security applications differs from demonstrating their capabilities in a laboratory. This paper reviews several issues solved in the development phase of a commercial hand-held radio-isotope identification device (RIID), the identiFINDERreg ultra L. Aspects as the detector stabilization, selection and optimization of an appropriate photomultiplier tube, as well as calibration, spectrum linearization, and quality assurance procedures in the production process are addressed.