Investigating Slit-Collimator-Produced Carbon Ion Minibeams with High-Resolution CMOS Sensors

Particle minibeam therapy has demonstrated the potential for better healthy tissue sparing due to spatial fractionation of the delivered dose. Especially for heavy ions, the spatial fractionation could enhance the already favorable differential biological effectiveness at the target and the entrance region. Moreover, spatial fractionation could even be a viable option for bringing ions heavier than carbon back into patient application. To understand the effect of minibeam therapy, however, requires careful conduction of pre-clinical experiments, for which precise knowledge of the minibeam characteristics is crucial. This work introduces the use of high-spatial-resolution CMOS sensors to characterize collimator-produced carbon ion minibeams in terms of lateral fluence distribution, secondary fragments, track-averaged linear energy transfer distribution, and collimator alignment. Additional simulations were performed to further analyze the parameter space of the carbon ion minibeams in terms of beam characteristics, collimator positioning, and collimator manufacturing accuracy. Finally, a new concept for reducing the neutron dose to the patient by means of an additional neutron shield added to the collimator setup is proposed and validated in simulation. The carbon ion minibeam collimator characterized in this work is used in ongoing pre-clinical experiments on heavy ion minibeam therapy at the GSI

Experimental Comparison of Fiducial Markers Used in Proton Therapy: Study of Different Imaging Modalities and Proton Fluence Perturbations Measured With CMOS Pixel Sensors

Fiducial markers are used for image guidance to verify the correct positioning of the target for the case of tumors that can suffer interfractional motion during proton therapy. The markers should be visible on daily imaging, but at the same time, they should produce minimal streak artifacts in the CT scans for treatment planning and induce only slight dose perturbations during particle therapy. In this work, these three criteria were experimentally investigated at the Heidelberg Ion Beam Therapy Center. Several small fiducial markers with different geometries and materials (gold, platinum, and carbon-coated ZrO₂) were evaluated. The streak artifacts on treatment planning CT were measured with and without iMAR correction, showing significantly smaller artifacts from markers lighter than 6 mg and a clear improvement with iMAR correction. Daily imaging as X-ray projections and in-room mobile CT were also performed. Markers heavier than 6 mg showed a better contrast in the X-ray projections, whereas on the images from the in-room mobile CT, all markers were clearly visible. In the other part of this work, fluence perturbations of proton beams were measured for the same markers by using a tracker system of several high spatial resolution CMOS pixel sensors. The measurements were performed for single-energy beams, as well as for a spread-out Bragg peak. Three-dimensional fluence distributions were computed after reconstructing all particle trajectories. These measurements clearly showed that the ZrO₂ markers and the low-mass gold/platinum markers (0.35mm diameter) induce perturbations being 2–3 times lower than the heavier gold or platinum markers of 0.5mm diameter. Monte Carlo simulations, using the FLUKA code, were used to compute dose distributions and showed good agreement with the experimental data after adjusting the phase space of the simulated proton beam compared to the experimental beam

Applications pour les capteurs à pixels CMOS en hadronthérapie

In ion-beam therapy, high precision measurements are essential for having robust basic data to deliver the prescribed treatment to the patient. In this study, MIMOSA-28 pixel sensors were used as a tracker system for different medical applications. Several hardware and software improvements were implemented leading to a spatial track resolution < 10 μm. The experiments were conducted with success in different medical and research facilities. In this work, beam profiles were measured along the beam axis and the width of the beam along the axis could be calculated with a transportation code based on multiple Coulomb scattering. Moreover, an online beam monitoring was developed in order to have fast information about the beam profile. In another study, the fluence perturbation of 12C ion beams due to small fiducial markers was investigated. After reconstruction and extrapolation of single track, a 3D fluence distribution could be performed and the maximum perturbation and its position along the beam axis could be quantified. In this work, the measured cold spot varied between less than 3% up to 9.2% for a defined marker and a defined primary energy beam.En hadronthérapie, des mesures de haute précision sont essentielles pour avoir une base de données robuste et délivrer le traitement prescrit au patient. Dans ce travail, un système de trajectométrie, composé de capteurs à pixels MIMOSA-28, a été utilisé pour différentes applications cliniques. Plusieurs améliorations ont été implémentées au niveau matériel et logiciel résultant à une résolution spatiale de trace < 10 μm. Les expériences ont été menées avec succès dans différents centres médicaux et de recherche. Les profils de faisceaux ont été mesurés et la largeur du faisceau le long de l'axe a pu être calculée grâce à un code de transport basé sur la diffusion. Un outil en ligne de suivi de faisceau a été développé pour avoir une information rapide de son profil. D'autre part, les perturbations de la fluence dues à des marqueurs de repères pour un faisceau 12C ont été évaluées. Après reconstruction et extrapolation de chaque trace, une distribution 3D de la fluence a pu être établie et la perturbation maximale de la fluence et sa position ont pu être quantifiées. Les points froids mesurés varient entre moins de 3% à 9.2% pour un marqueur et une énergie de faisceau définis

Investigating Slit-Collimator-Produced Carbon Ion Minibeams with High-Resolution CMOS Sensors

Particle minibeam therapy has demonstrated the potential for better healthy tissue sparing due to spatial fractionation of the delivered dose. Especially for heavy ions, the spatial fractionation could enhance the already favorable differential biological effectiveness at the target and the entrance region. Moreover, spatial fractionation could even be a viable option for bringing ions heavier than carbon back into patient application. To understand the effect of minibeam therapy, however, requires careful conduction of pre-clinical experiments, for which precise knowledge of the minibeam characteristics is crucial. This work introduces the use of high-spatial-resolution CMOS sensors to characterize collimator-produced carbon ion minibeams in terms of lateral fluence distribution, secondary fragments, track-averaged linear energy transfer distribution, and collimator alignment. Additional simulations were performed to further analyze the parameter space of the carbon ion minibeams in terms of beam characteristics, collimator positioning, and collimator manufacturing accuracy. Finally, a new concept for reducing the neutron dose to the patient by means of an additional neutron shield added to the collimator setup is proposed and validated in simulation. The carbon ion minibeam collimator characterized in this work is used in ongoing pre-clinical experiments on heavy ion minibeam therapy at the GSI

Fluence perturbation from fiducial markers due to edge-scattering measured with pixel sensors for ¹²C ion beams

International audienceFiducial markers are nowadays a common tool for patient positioning verification before radiotherapy treatment. These markers should be visible on X-ray projection imaging, produce low streak artifacts on CTs and induce small dose perturbations due to edge-scattering effects during the ion-beam therapy treatment. In this study, the latter effect was investigated and the perturbations created by the markers were evaluated with a new measurement method using a tracker system composed of six CMOS pixel sensors. The present method enables the determination of the particle trajectory before and after the target. The experiments have been conducted at the Marburg Ion Therapy Center with carbon ion beams and the measurement concept was validated by comparison with radiochromic films. This work shows that the new method is very efficient and precise to measure the perturbations due to fiducial markers with a tracker system. Three dimensional fluence distributions of all particle trajectories were reconstructed and the maximum cold spots due to the markers and their position along the beam axis were quantified. In this study, four small commercial markers with different geometries and materials (gold or carbon-coated ZrO2) were evaluated. The gold markers showed stronger perturbations than the lower density ones. However, it is important to consider that low density and low atomic number fiducial markers are not always visible on X-ray projections

Beam Monitor Calibration for Radiobiological Experiments With Scanned High Energy Heavy Ion Beams at FAIR

Precise and reliable monitoring of the particle rate is of great importance at accelerator facilities worldwide. In this article we describe the standard beam monitor calibration currently employed at the multi-purpose experimental sites Cave A and Cave M at GSI, where intense highly energetic ion beams are routinely used for a wide variety of experiments. An absolute dose-to-water measurement is performed with an air-filled ionization chamber and transferred into a calibration per primary particle. This is necessary for the raster scanning system used to enable the irradiation of extended fields, required for biophysical experiments in the research fields of particle therapy or space radiation protection. The main focus of this work is to understand through Monte Carlo simulations whether the currently used dosimetry procedure is valid for all the ion species and energies that are provided at GSI Cave A and Cave M by the SIS18 synchrotron and that will be provided by the SIS100 at FAIR. With this aim the detailed geometry of the PTW 30013 Farmer ionization chamber currently used at GSI was implemented in the transport code FLUKA and the beam quality correction factor kQ for different energies and ion species was calculated. Further details about the robustness of the calibration are investigated as well, e.g., appropriate irradiation depth of biological samples. Evidence is presented that for ions above 1 GeV/u the kQ factor decreases due to the density effect, which modifies the water-to-air stopping power ratio at relativistic energies. These findings are of particular importance for future biophysics experiments with ion beams from the SIS100 in the framework of the FAIR project. For energies in the regime of several GeV/u the constant kQ value as used in common practice should be replaced with the energy-dependent correction factor provided in this work