Vero SBRT System Research Articles

Performance of gimbal-based dynamic tumor tracking for treating liver carcinoma

BackgroundSince the introduction of tumor tracking in radiotherapy, it is possible to ensure a precise irradiation of moving targets. To follow the tumor movement, most systems rely on the detection of implanted markers and correlation models between the internal and external patient movement. This study reports the clinical workflow and first results of the dynamic tumor tracking (DTT) performance for patients with liver carcinoma at the Vero SBRT system of the University Hospital Erlangen regarding the detection of the internal marker and the changes of the determined correlation models.MethodsSo far 13 liver patients were treated with DTT. For each patient, two fiducial markers (FM), which are monitored with X-rays during treatment, were implanted in the vicinity of the tumor. All patients received a fraction dose of 4–6 Gy with 8 to 12 fractions. Treatment and patient data is evaluated by processing the acquired log-files of the DTT treatment. Based on this, the marker detection and the changes of the correlation model between the internal and external movement is investigated.ResultsThe median treatment time was 19:42 min. During treatment a median of 173 X-ray stereoscopic images were acquired. The marker detection was successful in 64.6% of the images. The FM detection is independent of the relative angle between the marker and the imager, but shows a dependency on the average intensity surrounding the FM position within the kV images. The number of correlation models needed during treatment increases in the presence of baseline shifts. The comparison of the correlation models shows large differences in the internal-external correlation between the different models acquired for one patient.ConclusionThirteen liver patients were treated with DTT at the Vero SBRT system and the marker detection was analyzed. Furthermore, the importance of regularly monitoring the internal target motion could be shown, since the correlation between the internal and external motion changes considerably over the course of the treatment.

Motion management during SBRT for oligometastatic cancer: Results of a prospective phase II trial

PurposeTo optimize the local control of stereotactic body radiotherapy (SBRT) using the Vero-SBRT system and respiratory motion management in patients with oligometastatic cancer. Materials and methodsPatients with five or less metastases were eligible. In metastases with significant motion, a fiducial was implanted for Vero dynamic tracking. For other metastases an internal target volume (ITV) was defined to encompass the respiratory tumor trajectory. A dose of 50Gy in 10 fractions was prescribed on the 80% isodose line. ResultsWe treated 87 metastases in 44 patients, with colorectal cancer as the most common primary origin (65.9%). Metastatic sites were mainly lung (n=62) and liver (n=17). Twenty-seven metastases were treated with dynamic tracking, the remaining 60 using the ITV-concept. Three patients (7%) experienced grade ⩾3 toxicity. After a median follow-up of 12months, the overall one-year local control (LC) amounted to 89% (95% CI 77–95%), with corresponding values of 90% and 88% for the metastases irradiated with the ITV-approach and dynamic tracking, respectively. Median progression-free survival reached 6.5months, one-year overall survival 95%. ConclusionsSBRT with proper respiratory motion management resulted in a high LC and an acceptable toxicity profile in oligometastatic cancer patients.

Initial characterization, dosimetric benchmark and performance validation of Dynamic Wave Arc.

BackgroundDynamic Wave Arc (DWA) is a clinical approach designed to maximize the versatility of Vero SBRT system by synchronizing the gantry-ring noncoplanar movement with D-MLC optimization. The purpose of this study was to verify the delivery accuracy of DWA approach and to evaluate the potential dosimetric benefits.MethodsDWA is an extended form of VMAT with a continuous varying ring position. The main difference in the optimization modules of VMAT and DWA is during the angular spacing, where the DWA algorithm does not consider the gantry spacing, but only the Euclidian norm of the ring and gantry angle. A preclinical version of RayStation v4.6 (RaySearch Laboratories, Sweden) was used to create patient specific wave arc trajectories for 31 patients with various anatomical tumor regions (prostate, oligometatstatic cases, centrally-located non-small cell lung cancer (NSCLC) and locally advanced pancreatic cancer-LAPC). DWA was benchmarked against the current clinical approaches and coplanar VMAT. Each plan was evaluated with regards to dose distribution, modulation complexity (MCS), monitor units and treatment time efficiency. The delivery accuracy was evaluated using a 2D diode array that takes in consideration the multi-dimensionality of DWA during dose reconstruction.ResultsIn centrally-located NSCLC cases, DWA improved the low dose spillage with 20 %, while the target coverage was increased with 17 % compared to 3D CRT. The structures that significantly benefited from using DWA were proximal bronchus and esophagus, with the maximal dose being reduced by 17 % and 24 %, respectively. For prostate and LAPC, neither technique seemed clearly superior to the other; however, DWA reduced with more than 65 % of the delivery time over IMRT. A steeper dose gradient outside the target was observed for all treatment sites (p < 0.01) with DWA. Except the oligometastatic cases, where the DWA-MCSs indicate a higher modulation, both DWA and VMAT modalities provide plans of similar complexity. The average ɣ (3 % /3 mm) passing rate for DWA plans was 99.2 ± 1 % (range from 96.8 to 100 %).ConclusionsDWA proven to be a fully functional treatment technique, allowing additional flexibility in dose shaping, while preserving dosimetrically robust delivery and treatment times comparable with coplanar VMAT.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-016-0633-7) contains supplementary material, which is available to authorized users.

SU-E-J-59: Feasibility of Markerless Tumor Tracking by Sequential Dual-Energy Fluoroscopy On a Clinical Tumor Tracking System

Purpose: To evaluate the feasibility of markerless tumor tracking through the implementation of a novel dual-energy imaging approach into the clinical dynamic tracking (DT) workflow of the Vero SBRT system. Methods: Two sequential 20 s (11 Hz) fluoroscopy sequences were acquired at the start of one fraction for 7 patients treated for primary and metastatic lung cancer with DT on the Vero system. Sequences were acquired using 2 on-board kV imaging systems located at ±45° from the MV beam axis, at respectively 60 kVp (3.2 mAs) and 120 kVp (2.0 mAs). Offline, a normalized cross-correlation algorithm was applied to match the high (HE) and low energy (LE) images. Per breathing phase (inhale, exhale, maximum inhale and maximum exhale), the 5 best-matching HE and LE couples were extracted for DE subtraction. A contrast analysis according to gross tumor volume was conducted based on contrast-to-noise ratio (CNR). Improved tumor visibility was quantified using an improvement ratio. Results: Using the implanted fiducial as a benchmark, HE-LE sequence matching was effective for 13 out of 14 imaging angles. Overlying bony anatomy was removed on all DE images. With the exception of two imaging angles, the DE images showed no significantly improved tumor visibility comparedmore » to HE images, with an improvement ratio averaged over all patients of 1.46 ± 1.64. Qualitatively, it was observed that for those imaging angles that showed no significantly improved CNR, the tumor tissue could not be reliably visualized on neither HE nor DE images due to a total or partial overlap with other soft tissue. Conclusion: Dual-energy subtraction imaging by sequential orthogonal fluoroscopy was shown feasible by implementing an additional LE fluoroscopy sequence. However, for most imaging angles, DE images did not provide improved tumor visibility over single-energy images. Optimizing imaging angles is likely to improve tumor visibility and the efficacy of dual-energy imaging. This work was in part sponsored by corporate funding from BrainLAB AG.(BrainLAB AG, Feldkirchen, Germany)« less

SU-E-T-140: Dynamic Wave Arc Trajectory Verification Using KV X-Ray Fluoroscopy

Purpose: Purpose: This study investigates the geometric accuracy of simultaneous Gantry/Ring rotation during Dynamic Wave Arc (DWA) delivery. Methods: The Vero SBRT system consists of a 6MV LINAC mounted on an O-ring gantry that can rotate around the vertical axis (±60°), similar to couch rotation on C-arm gantries. To provide CBCT and fluoroscopy imaging functionalities, two orthogonal kV imaging units are attached to the O-ring at −45°/+45° from the beam axis.Dynamic Wave Arc maximizes Vero's motion capabilities by employing synchronized gantry and ring motion on a complex non-coplanar trajectory in combination with aperture based optimized MLC segments.Four wave arc trajectories (T1-4) were delivered using a cubic phantom with a configuration of five lead beads. O-ring gantry position information was retrieved through continuous dual-source kV X-ray image acquisition during DWA. An in-house algorithm read in the image set, extracted the projected marker positions and determined the angulation through reconstruction of the beam source position. The geometric error was quantified as the distance between the independently detected positions from kV-images and reference trajectory derived from the treatment plan in the Ring-Gantry space. Results: The average displacement between the 3D gantry/ring positions reconstructed from the fluoroscopy images and the reference trajectory was 0.346 mm (SD 0,171) for T1. A mean offset of 0.348 mm (SD 0,182) and 0.357 mm (SD 0.194) was observed for trajectory T2(2segmens) and T3(4segments), respectively. The saw shape T4 presented a mean geometric error of 0.363 (SD 0.156). The overall systematic error of 0.350 was caused by the difference between planned reference trajectory created by linear interpolation between CP, and the machine delivery following a spline curve. Conclusion: An independent geometric QA approach has been developed for DWA delivery verification, successfully applied on diverse trajectories and disclosed that the Vero system is capable of accurately follow complex trajectories.

Feasibility of using the Vero SBRT system for intracranial SRS

The Vero SBRT system was benchmarked in a planning study against the Novalis SRS system for quality of delivered dose distributions to intracranial lesions and assessing the Vero system's capacity for SRS. A total of 27 patients with one brain lesion treated on the Novalis system, with 3 mm leaf width MLC and C‐arm gantry, were replanned for Vero, with a 5 mm leaf width MLC mounted on an O‐ring gantry allowing rotations around both the horizontal and vertical axis. The Novalis dynamic conformal arc (DCA) planning included vertex arcs, using 90° couch rotation. These vertex arcs cannot be reproduced with Vero due to the mechanical limitations of the O‐ring gantry. Alternative class solutions were investigated for the Vero. Additionally, to distinguish between the effect of MLC leaf width and different beam arrangements on dose distributions, the Vero class solutions were also applied for Novalis. In addition, the added value of noncoplanar IMRT was investigated in this study. Quality of the achieved dose distributions was expressed in the conformity index (CI) and gradient index (GI), and compared using a paired Student's t‐test with statistical significance for p‐values ≤0.05. For lesions larger than 5 cm3, no statistical significant difference in conformity was observed between Vero and Novalis, but for smaller lesions, the dose distributions showed a significantly better conformity for the Novalis (ΔCI=13.74%, p=0.0002) mainly due to the smaller MLC leaf width. Using IMRT on Vero reduces this conformity difference to nonsignificant levels. The cutoff for achieving a GI around 3, characterizing a sharp dose falloff outside the target volume was 4 cm3 for Novalis and 7 cm3 for Vero using DCA technique. Using noncoplanar IMRT, this threshold was reduced to 3 cm3 for the Vero system. The smaller MLC and the presence of the vertex fields allow the Novalis system to better conform the dose around the lesion and to obtain steeper dose falloff outside the lesion. Comparable dosimetric characteristics can be achieved with Vero for lesions larger than 3 cm3 and using IMRT.PACS number: 87.55.D

TH-A-137-11: First Clinical Experience Treating Patients with the Gimbaled Linac Tumor Tracking of the Vero SBRT System

Purpose: Reporting on the first group of patients treated with real‐time tumor tracking (RTTT) radiation therapy using the Vero gimbaled linac system and a single implanted fiducial marker. Methods: In total 8 patients diagnosed with NSCLC or oligo‐metastatic disease in lung or liver, and showing substantial breathing related tumor motion were included in this study. One single Visicoil fiducial marker was implanted inside or close to the tumor. SBRT schemes were applied with doses/fraction ranging from 5Gy up to 20Gy. Treatment planning was done using both the classic ITV approach as a reference using 5mm ITV‐PTV margin, and the RTTT planning applying a 5mm isotropic CTV‐PTV margin. RTTT was driven by a correlation model of external‐to‐internal motion and IR skin markers on the upper abdomen. The correlation model was rebuild multiple times per fraction. Data on the delivery process in terms of performance and time efficiency was accumulated. Results: The observed average peak‐to‐peak tumor motion reached up to 25mm, the average CTV volume was 9.0cc. An average PTV volume reduction was accomplished of 38%, ranging up to 52%. For this patient group, the average mean lung dose and mean liver dose were reduced from 2.6Gy to 2.0Gy, and from 11.7Gy to 9.4Gy respectively. Maximum average treatment time, including patient setup, ranged up to 35min. The correlation models were used for 6min 24 sec on average of which 2min 50sec was building time. The probability of the beams‐eye‐view tracking error to be less than 5mm averaged over all patients was exceeding 98%. Conclusion: A novel approach for real‐time tumor tracking using a gimbaled linac was successfully applied on a first group of patients. An initial evaluation was made of the possibilities of tracking to reduce the PTV volume and reestablish acceptable treatment fraction duration for respiratory correlated treatments. This collaborative work was supported by the Flemish government through the Hercules foundation, the Fonds voor Wetenschappelijk Onderzoek Vlaanderen grants G.0486.06 and G.0412.08, and corporate funding from BrainLab AG.

EP-1205: Feasibility of using the novel Vero SBRT system for intracranial SRS

EP-1205: Feasibility of using the novel Vero SBRT system for intracranial SRS

Geometric accuracy of real-time tumor tracking with the gimbal Linac system of the novel VERO SBRT system

Geometric accuracy of real-time tumor tracking with the gimbal Linac system of the novel VERO SBRT system

WE-G-213CD-03: A Dual Complementary Verification Method for Dynamic Tumor Tracking on Vero SBRT.

to use complementary cine EPID and gimbals log file analysis for in-vivo tracking accuracy monitoring. A clinical prototype of dynamic tracking (DT) was installed on the Vero SBRT system. This prototype version allowed tumor tracking by gimballed linac rotations using an internal-external correspondence model. The DT prototype software allowed the detailed logging of all applied gimbals rotations during tracking. The integration of an EPID on the vero system allowed the acquisition of cine EPID images during DT. We quantified the tracking error on cine EPID (E-EPID) by subtracting the target center (fiducial marker detection) and the field centroid. Dynamic gimbals log file information was combined with orthogonal x-ray verification images to calculate the in-vivo tracking error (E-kVLog). The correlation between E-kVLog and E-EPID was calculated for validation of the gimbals log file. Further, we investigated the sensitivity of the log file tracking error by introducing predefined systematic tracking errors. As an application we calculate gimbals log file tracking error for dynamic hidden target tests to investigate gravity effects and decoupled gimbals rotation from gantry rotation. Finally, calculating complementary cine EPID and log file tracking errors evaluated the clinical accuracy of dynamic tracking. A strong correlation was found between log file and cine EPID tracking error distribution during concurrent measurements (R=0.98). We found sensitivity in the gimbals log files to detect a systematic tracking error up to 0.5 mm. Dynamic hidden target tests showed no gravity influence on tracking performance and high degree of decoupled gimbals and gantry rotation during dynamic arc dynamic tracking. A submillimetric agreement between clinical complementary tracking error measurements was found. Redundancy of the internal gimbals log file with x-ray verification images with complementary independent cine EPID images was implemented to monitor the accuracy of gimballed tumor tracking on Vero SBRT. Research was financially supported by the Flemish government (FWO), Hercules Foundation and BrainLAB AG.

SU‐GG‐J‐13: Geometric Accuracy of Real‐Time Tumor Tracking with the Gimbaled Linac System of the Novel VERO SBRT System

Purpose: The VERO system, a novel platform for image guided stereotactic body radiotherapy, is a joint product of BrainLAB and MHI. A new type of 6MV linac with attached MLC is mounted on an O‐ring gantry. Orthogonal gimbals hold the linac‐MLC assembly, which allows pan and tilt motions of the therapeutic beam. This offers the possibility to perform tracking of moving tumors. A prototype is currently installed in our hospital which can actively track an IR marker with the beam using the gimbals system. The purpose is to determine the geometric accuracy of the tumor tracking. Methods and Materials: A video camera based detection unit was developed to simultaneously acquire the position of the tracked object and of the tracking beam using the light field of the linac system, at a 30fps frame rate. An assessment was made of the tracking capabilities in terms of tracking errors, system lag and the equivalence of pan and tilt motion performance. To determine the tracking error, an IR marker was placed on a 1D moving phantom. A sinusoidal motion was produced by the phantom with different frequencies from 0.085Hz to 0.5Hz, and a fixed amplitude of 20mm. The tracking error was characterized in terms of systematic error, root mean square error (RMSE) and 90% percentile of the absolute tracking error (E90%). Results: Systematic tracking errors were below 0.21mm. The system lag was 47.7ms (2.3ms) and 47.6ms (2.0ms) for the pan and tilt respectively. The system can track the IR marker sinusoidal motion accurately, with an E90% &lt; 0.82mm, up to frequencies of 0.5Hz with similar performance for pan and tilt. Additionally, the tracking is demonstrated for representative patient signals. Conclusion: The accuracy is considered adequate for real‐time tracking of tumors. The performance for pan‐tilt is equivalent. Conflict of Interest: Research sponsored by BrainLAB.