Philipp Steininger

First Clinical Release of an Online, Adaptive, Aperture-Based Image-Guided Radiotherapy Strategy in Intensity-Modulated Radiotherapy to Correct for Inter- and Intrafractional Rotations of the Prostate

PURPOSE We developed and evaluated a correction strategy for prostate rotations using direct adaptation of segments in intensity-modulated radiotherapy (IMRT). METHOD AND MATERIALS Implanted fiducials (four gold markers) were used to determine interfractional translations, rotations, and dilations of the prostate. We used hybrid imaging: The markers were automatically detected in two pretreatment planar X-ray projections; their actual position in three-dimensional space was reconstructed from these images at first. The structure set comprising prostate, seminal vesicles, and adjacent rectum wall was transformed accordingly in 6 degrees of freedom. Shapes of IMRT segments were geometrically adapted in a class solution forward-planning approach, derived within seconds on-site and treated immediately. Intrafractional movements were followed in MV electronic portal images captured on the fly. RESULTS In 31 of 39 patients, for 833 of 1013 fractions (supine, flat couch, knee support, comfortably full bladder, empty rectum, no intraprostatic marker migrations >2 mm of more than one marker), the online aperture adaptation allowed safe reduction of margins clinical target volume-planning target volume (prostate) down to 5 mm when only interfractional corrections were applied: Dominant L-R rotations were found to be 5.3° (mean of means), standard deviation of means ±4.9°, maximum at 30.7°. Three-dimensional vector translations relative to skin markings were 9.3 ± 4.4 mm (maximum, 23.6 mm). Intrafractional movements in 7.7 ± 1.5 min (maximum, 15.1 min) between kV imaging and last beams electronic portal images showed further L-R rotations of 2.5° ± 2.3° (maximum, 26.9°), and three-dimensional vector translations of 3.0 ±3.7 mm (maximum, 10.2 mm). Addressing intrafractional errors could further reduce margins to 3 mm. CONCLUSION We demonstrated the clinical feasibility of an online adaptive image-guided, intensity-modulated prostate protocol on a standard linear accelerator to correct 6 degrees of freedom of internal organ motion, allowing safe and straightforward implementation of margin reduction and dose escalation.

“Augmented Reality” in Conventional Simulation by Projection of 3-D Structures into 2-D Images

Background and Purpose:In this study, a new method is introduced, which allows the overlay of three-dimensional structures, that have been delineated on transverse slices, onto the fluoroscopy from conventional simulators in real time.Patients and Methods:Setup deviations between volumetric imaging and simulation were visualized, measured and corrected for 701 patient isocenters.Results:Comparing the accuracy to mere virtual simulation lacking additional X-ray imaging, a clear benefit of the new method could be shown. On average, virtual prostate simulations had to be corrected by 0.48 cm (standard deviation [SD] 0.38), and those of the breast by 0.67 cm (SD 0.66).Conclusion:The presented method provides an easy way to determine entity-specific safety margins related to patient setup errors upon registration of bony anatomy (prostate 0.9 cm for 90% of cases, breast 1.3 cm). The important role of planar X-ray imaging was clearly demonstrated. The innovation can also be applied to adaptive image-guided radiotherapy (IGRT) protocols.Hintergrund und Ziel:Es wird ein Verfahren vorgestellt, das die Einblendung von dreidimensionalen Strukturen, die zuvor z.B. auf axialen Schnittbildern segmentiert wurden, in Durchleuchtungsaufnahmen am konventionellen Simulator in Echtzeit erlaubt.Patienten und Methodik:Mit dieser Technologie wurden an 701 Patientenisozentren Lagerungsunterschiede zwischen der Schnittbildgebung und der Simulation visualisiert, vermessen und korrigiert.Ergebnisse:Im Vergleich der Genauigkeit mit der rein virtuellen Simulation, bei der auf Röntgenbildgebung verzichtet wird, zeigte sich eine deutliche Überlegenheit der neuen Methode. Im Mittel wurden virtuell simulierte Prostatabestrahlungen um 0,48 cm (Standardabweichung [SD] 0,38) und jene der Mamma um 0,67 cm (SD 0,66) korrigiert.Schlussfolgerung:Das vorgestellte Verfahren erlaubt die einfache Bestimmung entitätsspezifischer Sicherheitsränder für Lagerungsungenauigkeiten von knöchernen Strukturen (Prostatabestrahlung 0,9 cm für 90% der Fälle, Mamma 1,3 cm; Tabelle 2). Die Bedeutung von planarer kV-Bildgebung konnte gezeigt werden. Das innovative Verfahren ist auch im Rahmen von Protokollen zur adaptiven, bildgeführten Radiotherapie (IGRT) einsetzbar.

Augmented reality in conventional simulation by projection of 3-D structures into 2-D images: a comparison with virtual methods.

Background and Purpose:In this study, a new method is introduced, which allows the overlay of three-dimensional structures, that have been delineated on transverse slices, onto the fluoroscopy from conventional simulators in real time.Patients and Methods:Setup deviations between volumetric imaging and simulation were visualized, measured and corrected for 701 patient isocenters.Results:Comparing the accuracy to mere virtual simulation lacking additional X-ray imaging, a clear benefit of the new method could be shown. On average, virtual prostate simulations had to be corrected by 0.48 cm (standard deviation [SD] 0.38), and those of the breast by 0.67 cm (SD 0.66).Conclusion:The presented method provides an easy way to determine entity-specific safety margins related to patient setup errors upon registration of bony anatomy (prostate 0.9 cm for 90% of cases, breast 1.3 cm). The important role of planar X-ray imaging was clearly demonstrated. The innovation can also be applied to adaptive image-guided radiotherapy (IGRT) protocols.Hintergrund und Ziel:Es wird ein Verfahren vorgestellt, das die Einblendung von dreidimensionalen Strukturen, die zuvor z.B. auf axialen Schnittbildern segmentiert wurden, in Durchleuchtungsaufnahmen am konventionellen Simulator in Echtzeit erlaubt.Patienten und Methodik:Mit dieser Technologie wurden an 701 Patientenisozentren Lagerungsunterschiede zwischen der Schnittbildgebung und der Simulation visualisiert, vermessen und korrigiert.Ergebnisse:Im Vergleich der Genauigkeit mit der rein virtuellen Simulation, bei der auf Röntgenbildgebung verzichtet wird, zeigte sich eine deutliche Überlegenheit der neuen Methode. Im Mittel wurden virtuell simulierte Prostatabestrahlungen um 0,48 cm (Standardabweichung [SD] 0,38) und jene der Mamma um 0,67 cm (SD 0,66) korrigiert.Schlussfolgerung:Das vorgestellte Verfahren erlaubt die einfache Bestimmung entitätsspezifischer Sicherheitsränder für Lagerungsungenauigkeiten von knöchernen Strukturen (Prostatabestrahlung 0,9 cm für 90% der Fälle, Mamma 1,3 cm; Tabelle 2). Die Bedeutung von planarer kV-Bildgebung konnte gezeigt werden. Das innovative Verfahren ist auch im Rahmen von Protokollen zur adaptiven, bildgeführten Radiotherapie (IGRT) einsetzbar.

Comparison of two different rectal spacers in prostate cancer external beam radiotherapy in terms of rectal sparing and volume consistency

BACKGROUND AND PURPOSE In external beam radiation (EBRT) of the prostate, the rectum is the dose-limiting organ at risk, and sparing of the anterior rectal wall is a prerequisite for safe delivery of doses beyond 70 Gy. Spatial sparing of the rectum can be achieved by introducing a spacer material into the retroprostatic space, thus separating the anterior rectal wall from the PTV. MATERIALS AND METHODS Two spacer technologies, Spacer OAR, a polyethylene glycol gel and ProSpace, a saline inflated balloon, were compared in terms of spacer volume, stability, and dose reduction to the anterior rectum wall in 78 patients. RESULTS Both spacer systems significantly reduced the rectum surface encompassed by the 95% isodose (gel: -35%, p<0.01; balloon -63.4%, p<0.001) compared to a control group. The balloon spacer was superior in reducing rectum dose (-27.7%, p=0.034), but exhibited an average volume loss of >50% during the full course of treatment of 37-40 fractions, while the volume of gel spacers remained fairly constant. CONCLUSIONS In choosing between the two spacer technologies, the advantageous dose reduction of the balloon needs to be weighed up against the better volume consistency of the gel spacer with respect to the duration of hypofractionated vs normofractionated regimens.

Comparison of Different Metrics for Appearance-Model-Based 2D/3D-registration with X-ray Images

The general idea of the presented work is to overcome known problems with segmentation and analysis of 2D radiographs by registering a 3D appearance-model. Therefore this paper introduces a novel method to register 2D x-rays with 3D appearance-models by optimizing the appearance and pose of the model until a virtual radiograph of the generated model-instance optimally fits the investigated x-ray. The approach was tested on a sample set of 15 human femur specimen using different metrics and optimization techniques to investigate the impact on the resulting implicit 2D-segmentation. The first promising results are presented and discussed in detail.

Filtered‐backprojection reconstruction for a cone‐beam computed tomography scanner with independent source and detector rotations

PURPOSE A new cone-beam CT scanner for image-guided radiotherapy (IGRT) can independently rotate the source and the detector along circular trajectories. Existing reconstruction algorithms are not suitable for this scanning geometry. The authors propose and evaluate a three-dimensional (3D) filtered-backprojection reconstruction for this situation. METHODS The source and the detector trajectories are tuned to image a field-of-view (FOV) that is offset with respect to the center-of-rotation. The new reconstruction formula is derived from the Feldkamp algorithm and results in a similar three-step algorithm: projection weighting, ramp filtering, and weighted backprojection. Simulations of a Shepp Logan digital phantom were used to evaluate the new algorithm with a 10 cm-offset FOV. A real cone-beam CT image with an 8.5 cm-offset FOV was also obtained from projections of an anthropomorphic head phantom. RESULTS The quality of the cone-beam CT images reconstructed using the new algorithm was similar to those using the Feldkamp algorithm which is used in conventional cone-beam CT. The real image of the head phantom exhibited comparable image quality to that of existing systems. CONCLUSIONS The authors have proposed a 3D filtered-backprojection reconstruction for scanners with independent source and detector rotations that is practical and effective. This algorithm forms the basis for exploiting the scanners unique capabilities in IGRT protocols.

Technical Note: Procedure for the calibration and validation of kilo-voltage cone-beam CT models

PURPOSE The aim of this work is to propose a general and simple procedure for the calibration and validation of kilo-voltage cone-beam CT (kV CBCT) models against experimental data. METHODS The calibration and validation of the CT model is a two-step procedure: the source model then the detector model. The source is described by the direction dependent photon energy spectrum at each voltage while the detector is described by the pixel intensity value as a function of the direction and the energy of incident photons. The measurements for the source consist of a series of dose measurements in air performed at each voltage with varying filter thicknesses and materials in front of the x-ray tube. The measurements for the detector are acquisitions of projection images using the same filters and several tube voltages. The proposed procedure has been applied to calibrate and assess the accuracy of simple models of the source and the detector of three commercial kV CBCT units. If the CBCT system models had been calibrated differently, the current procedure would have been exclusively used to validate the models. Several high-purity attenuation filters of aluminum, copper, and silver combined with a dosimeter which is sensitive to the range of voltages of interest were used. A sensitivity analysis of the model has also been conducted for each parameter of the source and the detector models. RESULTS Average deviations between experimental and theoretical dose values are below 1.5% after calibration for the three x-ray sources. The predicted energy deposited in the detector agrees with experimental data within 4% for all imaging systems. CONCLUSIONS The authors developed and applied an experimental procedure to calibrate and validate any model of the source and the detector of a CBCT unit. The present protocol has been successfully applied to three x-ray imaging systems. The minimum requirements in terms of material and equipment would make its implementation suitable in most clinical environments.

Nine‐degrees‐of‐freedom flexmap for a cone‐beam computed tomography imaging device with independently movable source and detector

Purpose: Couch‐mounted cone‐beam computed tomography (CBCT) imaging devices with independently rotatable x‐ray source and flat‐panel detector arms for acquisitions of arbitrary regions of interest (ROI) have recently been introduced in image‐guided radiotherapy (IGRT). This work analyzes mechanical limitations and gravity‐induced effects influencing the geometric accuracy of images acquired with arbitrary angular constellations of source and detector in nonisocentric trajectories, which is considered essential for IGRT. In order to compensate for geometric inaccuracies of this modality, a 9‐degrees‐of‐freedom (9‐DOF) flexmap correction approach is presented, focusing especially on the separability of the flexmap parameters of the independently movable components of the device. Methods: The 9‐DOF comprise a 3D translation of the x‐ray source focal spot, a 3D translation of the flat‐panels active area center and three Euler‐rotations of the detectors row and column vectors. The flexmap parameters are expressed with respect to the angular position of each of the devices arms. Estimation of the parameters is performed, using a CT‐based structure set of a table‐mounted, cylindrical ball‐bearing phantom. Digitally reconstructed radiograph (DRR) patches are derived from the structure set followed by local 2D in‐plane registration and subsequent 3D transform estimation by nonlinear regression with outlier detection. Results: Flexmap parameter evaluations for the factory‐calibrated system in clockwise and counter‐clockwise rotation direction have shown only minor differences for the overall set of flexmap parameters. High short‐term reproducibility of the flexmap parameters has been confirmed by experiments over 10 acquisitions for both directions, resulting in standard deviation values of ≤0.183 mm for translational components and ≤0.0219 deg for rotational components, respectively. A comparison of isocentric and nonisocentric flexmap evaluations showed that the mean differences of the parameter curves reside within their standard deviations, confirming the ability of the proposed calibration method to handle both types of trajectories equally well. Reconstructions of 0.1 mm and 0.25 mm steel wires showed similar results for the isocentric and nonisocentric cases. The full‐width at half maximum (FWHM) measure indicates an average improvement of the calibrated reconstruction of 85% over the uncalibrated reconstruction. The contrast of the point spread function (PSF) improved by 310% on average over all experiments. Moreover, a reduced amount of artifacts visible in nonisocentric reconstructions of a head phantom and a line‐pair phantom has been achieved by separate application of the 9‐DOF flexmap on the geometry described by the independently moving source arm and detector arm. Conclusions: Using a 9‐DOF flexmap approach for correcting the geometry of projections acquired with a device capable of independent movements of the source and panel arms has been shown to be essential for IGRT use cases such as CBCT reconstruction and 2D/3D registration tasks. The proposed pipeline is able to create flexmap curves which are easy to interpret, useful for mechanical description of the device and repetitive quality assurance as well as system‐level preventive maintenance. Application of the flexmap has shown improvements of image quality for planar imaging and volumetric imaging which is crucial for patient alignment accuracy.

PO-0959: Optimal dose balance between energy levels for material decomposition with dual-energy X-ray CT

Dual-energy X-ray imaging is used to extract quantitative material information, e.g., electron density and effective atomic number Z, using the differencesof X-ray attenuation coefficients of materials at different energies. A detailed study of factors that could influence the precision of extracted data is of importance. The aim of this study was to determine the optimal dose balance between the low and the high voltage acquisitions for material decomposition with dual-energy X-ray CT.

A novel class of machine-learning-driven real-time 2D/3D tracking methods: texture model registration (TMR)

We present a novel view on 2D/3D image registration by introducing a generic algorithmic framework that is based on supervised machine learning (SML). First and foremost, this class of algorithms, referred to as texture model registration (TMR), aims at making 2D/3D registration applicable for time-critical image guided medical procedures. TMR methods are two-stage. In a first offline pre-computational stage, a prediction rule is derived from a pre-interventional 3D image and according geometric constraints. This is achieved by computing digitally reconstructed radiographs, pre-processing them, extracting their texture, and applying SML methods. In a second online stage, the inferred rule is used for predicting the spatial rigid transformation of unseen intrainterventional 2D images. A first simple concrete TMR implementation, referred to as TMR-PCR, is introduced. This approach involves principal component regression (PCR) and simple intermediate pre-processing steps. Using TMR-PCR, first experimental results on five clinical IGRT 3D data sets and synthetic intra-interventional images are presented. The implementation showed an average registration rate of 48 Hz over 40000 registrations, and succeeded in the majority of cases with a mean target registration error smaller than 2 mm. Finally, the potential and characteristics of the proposed methodical framework are discussed.