MLC Tracking Research Articles

TH-AB-303-01: Benchmarking Real-Time Adaptive Radiotherapy Systems: A Multi- Platform Multi-Institutional Study

Purpose: The era of real-time adaptive radiotherapy is here: patients are being treated by CyberKnife (since 2004), Vero (2011) and MLC tracking (2013) technology, with couch tracking planned to be clinical in 2015. We have developed a common set of tools for benchmarking real-time adaptive radiotherapy systems and to test the hypothesis that, across delivery systems and institutions, real-time adaptive radiotherapy improves the dosimetric accuracy over non-adaptive radiotherapy in the presence of realistic tumor motion. Methods: Ten institutions with CyberKnife, Vero, MLC or couch tracking technology were involved in the study. Common materials were anonymized lung and prostate CT and structure sets, patient-measured motion traces (four lung, four prostate) and SBRT planning protocols (lung: RTOG1021, prostate: RTOG0938). The institutions delivered lung and prostate plans to a moving dosimeter programmed with tumor motion. For each trace the plan was delivered twice; with and without motion adaptation, each measurement was compared to the static dosimeter dose and the percentage of failed points for γ-tests recorded. Results: Eleven measurement sets were obtained for this study; two CyberKnife, two Vero, five MLC and two couch tracking sets. For all lung traces all sets show improved dose accuracy from a mean 2%/2mm γ-failrate of 1.6% with adaptation and 14.7% with no motion correction(p<0.001). For all prostate traces the mean 2%/2mm γ-failrate was 1.6% with adaptation and 17.4% with no motion correction (p<0.001). The difference between the four adaptive systems was small with an average 2%/2mm γ-failrate of <3% for all systems with adaptation for lung and prostate. Conclusion: A common set of tools has been developed for benchmarking real-time adaptive radiotherapy systems and a multi-platform multi-institutional study performed. The results show the systems all account for realistic tumor motion accurately and performed to a similar high standard, with real-time adaptation significantly outperforming non-adaptive methods.

TH-AB-303-05: Fast, Deterministic Leaf-Fitting with Explicit Underdose/overdose Constraints for Real-Time MLC Tracking

Purpose: Real-time motion management via dynamic MLC tracking requires adaptive aperture reshaping and fast leaf-fitting while still attempting to preserve the dosimetric integrity of the treatment. Due to the finite width of MLC leaves, the reshaped aperture can only be approximated by leaf refitting. A previously proposed algorithm by Ruan (2011) successfully minimized the refitting error by utilizing explicit underdose/overdose constraints to find an optimal leaf configuration for a given aperture. In its current implementation, this algorithm is computationally- and therefore time-intensive, which is a concern for real-time MLC tracking. In this work, we propose a significantly faster version of this algorithm that takes advantage of the polygonal characteristics of plan-derived apertures. Methods: Two hundred distinct leaf configurations were extracted from two conformal and three sliding-window lung IMRT plans. Twenty predetermined affine transformations were applied to each aperture to generate 4000 test shapes. Both algorithms, our piecewise approach and Ruan (2011), were then applied to each shape using the same set of ten underdose/overdose coefficients. Performance statistics, including run time and memory consumption, were gathered during testing. The resulting leaf positions as well as the overall fitting cost of each algorithm were compared for each shape. Results: Our piecewise approach reduces runtime from an average of 113ms to 0.295ms per aperture. The original algorithm showed the most consistent performance with minimum and maximum runtimes of 83.6ms and 129ms, respectively, compared to 0.104ms and 0.848ms with our approach. In both cases, memory usage was negligible, averaging 312kb for Ruan (2011) and 130kb for our approach. Conclusion: Our piecewise leaf-fitting approach showed three orders of magnitude improvement in computation time compared to the original implementation. Such improvement has a direct impact on system latency (time between target motion and MLC response) and therefore on the geometric and dosimetric accuracy of MLC tracking. Funding for this work was provided by Elekta Limited.

TH-AB-303-10: Influence of Margins and Prostate Rotations On Tracked Dynamic MLC Deliveries

Purpose: Tracked dynamic MLC treatment deliveries offer an opportunity for margin reduction and healthy tissue sparing. With our newly developed real-time online dose accumulation tool we investigate the impact of clinical target volume (CTV) to planning target volume (PTV) margins and of a prostate rotation on CTV coverage and organ-at-risk dose. Methods: We have previously developed an in-house dose accumulation tool for experimental MLC tracking deliveries on our Elekta Synergy/Agility research platform. The dose accumulation is based on pre-calculated dose influence data and updated in ≤30 ms for every MLC aperture reported by the linac. For this study, we have created a set of prostate treatment plans according to RTOG 0938 (5 Fx, 7-beams) using different CTV- to- PTV margins: 1, 3 and 5 mm isotropically; and 3 mm posteriorly + 5 mm otherwise. For each plan, the delivery was performed statically (no tracking, no motion) and dynamically (tracking, motion) with four different real-patient prostate trajectories. An additional prostate rotation of 20° about the LR axis was simulated for the dynamic deliveries. Results: For the dynamically tracked deliveries, target coverage was maintained compared to the static reference case for all CTV-to-PTV margins. Baseline drifts of the prostate posteriorly and inferiorly increased the rectum D2 dose by 0.18 Gy while decreasing the bladder D2 dose by 0.28 Gy averaged over all plans. Sudden transient motion anteriorly and superiorly increased bladder D2 by 0.14 Gy and decreased rectum D2 by 0.67 Gy. For all margins, CTV dose appeared robust against a prostate rotation when using MLC tracking. Conclusion: Residual tracking errors caused by system latency appear to be sufficiently covered by a 1 mm CTV-to-PTV margin. Initial evidence has been given that MLC tracking with smaller margins allows for uncompromised CTV coverage without overdosing the rectum or bladder compared to a conventional treatment strategy. We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.

TH-AB-303-03: Real-Time Error Estimation for Real-Time Motion Prediction

Purpose: A critical component of real-time MLC tracking is to incorporate motion prediction in order to account for system latency, the time between the occurrence of motion and MLC leaf repositioning, so as to avoid geometric and dosimetric errors. In this work, we address a major limitation of current motion prediction algorithms – estimating the inherent error in prediction in real-time. We propose using an ensemble of real-time predictors to reduce overall prediction error, and provide a quantitative metric to trigger beam holds based on a user-defined error threshold. Methods: An ensemble of eighteen kernel density estimation (KDE)-based estimators was implemented in parallel and deployed on motion traces recorded from ten lung cancer patients. For each trace, the ensemble generated a sequence of predictions for a look-ahead time of 250ms, typical for many real-time systems. Each predictor, and the ensemble as a whole, was evaluated based on their root-mean-square error (RMSE) with respect to the original data, and assigned a beam hold score between 0 (perfect) and 1 to quantify the number of false beam holds and false beam releases. Results: The average execution time of the KDE-ensemble was 16ms, well below the 250ms look-ahead. For all traces, the ensemble produced a lower RMSE and hold score than any of the individual constituent predictors. For the worst-case motion trace, the minimum RMSE(hold score) for individual predictors was 0.912cm(0.340) compared to 0.854cm(0.305) for the KDE-ensemble predictor. The hold score for individual estimators was ∼0.6 when error estimates were not used to assert beam holds. Conclusion: The KDE-ensemble approach results in improved motion prediction. More importantly, it also provides real-time estimations of prediction error, thus addressing a major gap in current motion prediction algorithms. Such error prediction can be utilized to assert beam-holds during MLC tracking, preventing serious geometric and dosimetric errors. Funding for this work was provided by Elekta Limited.

TH-AB-303-04: A Novel Leaf Sequencing Optimization Algorithm Which Considers Previous Underdose and Overdose Events for MLC Tracking Radiotherapy

Purpose: MLC-tracking radiotherapy is complex as the beam pattern needs to be modified due to the planned intensity modulation as well as the real-time target motion. The target motion cannot be planned, therefore the modified beam pattern differs from the original plan and the MLC sequence needs to be replanned online. Current MLC-tracking algorithms use a greedy heuristic in that they optimize for a given time, but ignore past errors. To overcome this problem we have developed and improved an algorithm that minimizes large underdose/overdose regions. Additionally previous underdose/overdose events are taken into account to avoid regions with high quantity of underdose/overdose. Methods: We improved the existing MLC motion control algorithm by introducing an underdose/overdose map. This map represents the actual projection of the planned tumor shape and logs occurring dose events at its specific regions. These events have an impact on the dose cost calculation and reduce recurrence of underdose/overdose.We studied the improvement of the new temporal optimization algorithm in terms of the L1-norm minimization of the sum of overdose/underdose compared to the currently used algorithm. For evaluation we simulated the delivery of 5 conformal and 14 IMRT-plans with 7 3D patient measured tumor motion traces. Results: Simulations with conformal shapes showed an improvement of L1-norm up to 8.5% after 100 MLC modification steps. The improvement in terms of L1-norm for a single step increased up to 18.2%.For IMRT, the improvement was 1.45±0.1% after 100 steps and for a single step the average improvement was 3.28±0.4%. This smaller improvement for IMRT is caused by the fact that the plans are simplified in terms of the shape. Conclusion: A novel leaf sequencing optimization algorithm which considers previous dose events for MLC-tracking radiotherapy has been developed and investigated. Reductions in underdose/overdose are observed for conformal and IMRT delivery.

TH-AB-303-06: Real-Time 4D Dose Reconstruction for Tracked Dynamic MLC Deliveries in the Presence of Respiratory Motion

Purpose: This work provides a proof-of-concept study for real-time 4D dose reconstruction on 4DCT lung data treated with MLC tracking and its impact on the size of target margins. Methods: We have implemented online dose reconstruction by connecting our research platform for dynamic dose delivery to an Agility MLC at an Elekta Synergy linac with our in-house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40ms and 33ms respectively. The dose is then accumulated in real-time by utilization of pre-calculated dose-influence matrices based on a collapsed-cone algorithm. To investigate the impact of a potentially reducible safety margin we have created and delivered treatment plans designed for a conventional internal target volume (ITV)+5mm and for a tracking scenario of a lung patient. Both were planned to meet the criteria of RTOG 1021 (3-Fx, 9-beam). For the tracking plan, a maximum target volume (MTV)+5mm was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the chosen reference phase, resulting in a unified maximum GTV to which a 5mm isotropic margin was added. The dose accumulation on a reference phase of the 4DCT (2×2×2mm3) was accomplished by the use of 3D deformation vector fields. Results: D95 for GTV on the reference phase was 98% of the RTOG prescribed dose for both the ITV+5mm plan and the MTV+5mm plan with tracking. V20 for ipsilateral lung and mean chest wall dose decreased by 10% and 12% respectively for the tracking scenario. Dose calculation and accumulation were performed in <30ms per MLC aperture. Conclusion: For the patient data and motion conditions in this study, the reconstructed dose distributions show that target dose can be maintained using dynamic MLC tracking whilst the reduced margins yield lower doses to healthy tissue. We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.

PD-0236: Performance improvement of MLC tracking investigated with an MLC tracking simulator for TrueBeam

PD-0236: Performance improvement of MLC tracking investigated with an MLC tracking simulator for TrueBeam

PO-0919: Impact of system latency on 4D SABR lung plans delivered using MLC tracking

PO-0919: Impact of system latency on 4D SABR lung plans delivered using MLC tracking

First Clinical Implementation of MLC Tracking for Prostate VMAT: MLC Tracking Improves the Agreement Between the Planned and Delivered Doses

The objective of this study is to quantify the delivered dose for 30 patients in the first prospective MLC tracking prostate VMAT clinical trial. We hypothesize that real-time adaptation with MLC tracking improves the agreement between the planned and delivered doses.

First Clinical Implementation of Electromagnetic Transponder-Guided MLC Tracking

First Clinical Implementation of Electromagnetic Transponder-Guided MLC Tracking

Performance Improvement of MLC Tracking Investigated With an Experimentally Validated Tracking Simulator

Performance Improvement of MLC Tracking Investigated With an Experimentally Validated Tracking Simulator

SU-E-T-400: Optimized 2D Motion Tracking Using Dynamic Multi-Leaf Collimator (DMLC) for a Clinical Volumetric Modulated Arc Therapy (VMAT) Plan

Purpose: This study is to develop a 2D motion tracking algorithm combined with controlled shallow breathing and collimator angle optimization to improve MLC tracking accuracy. Method and Materials: One SBRT VMAT plan for lung cancer (2 Arcs; 8 Gy/fx in 4 fx) was selected. A non-motion 3D VMAT plan was generated based on GTV from one single selected phase of the 4DCT. A motion tracking algorithm using MATLAB was developed to modify MLC positions of the 3D VMAT DICOM file based on an ideal breathing motion model. We derived the correlations between gantry angle, collimator angle and motion amplitudes in SI and AP directions that projected onto MLC aperture. All delivery constraints followed mechanical limits of Varian TrueBeam. To avoid large time interval between adjacent control points, motion amplitudes restriction and collimator angle optimization were incorporated into the algorithm. Each arc was then divided into 6 sub-arcs corresponding to the 6 breathing phases. Sub-arc doses were individually calculated and merged to acquire the 4D dose distribution. DVH comparison and 3D gamma were used for dosimetric evaluation. Results: Total delivery time of static 3D VMAT plan (collimator=45°), plan with MLC tracking under free breathing (collimator=45°, SI=10mm and AP=8mm), plan with collimator angle optimized only (collimator=77.8°, SI=10mm and AP=8mm) and collimator angle optimized under shallow breathing (collimator=80.1°, SI=8.3mm, AP=6.6mm) were 135.7s, 235.3s, 213.9s and 188.5s, respectively. Compared with the non-motion 3D VMAT, optimized MLC tracking demonstrated small changes in target dose (%ΔD90=1%, 3D gamma passing rate=97.5%). Conclusions: Total delivery time of VMAT with tracking tends to decrease as collimator angle approaches to 90°. At selected angle, plan with shorter breathing cycle period has longer delivery time when using MLC for motion compensation. MLC tracking combined with shallow breathing and optimized collimator angle cannot only reduce delivery time but increase the tracking accuracy.

WE-G-BRF-08: Robust Characterization and Patient Experience of a Time-Resolved Ultrasound System for Prostate Tracking

Purpose: Characterization of a time resolved ultrasound device (4D-US), used for localization and motion monitoring of prostate radiotherapy. Methods: The 4D-US system accuracy to reproduce prostate motion was investigated and artifacts were determined under detailed experimental and ultimately clinical conditions. Two fields of view (FOV) of 30 and 60 degrees and two corresponding acquisition speeds were evaluated under experimental conditions with gradual increased complexity. The reconstruction ability and trajectory tracking depends on the relative speed between the ultrasound probe and the moving object. Two different types of phantoms were developed; one capable of following a predetermined trajectory with variable speeds, and a second one designed to test the US ability to a track dynamically-deformed prostate. Four hypo-prostate patients (10fx) were monitored during the treatment to detect intrafraction motion. Results: Prostate speeds up to 6mm/s and 3mm/s can be successfully tracked by the system when using the small and large FOV, respectively. The system response time, calculated based on the speedinduced volumetric distortion, is dependent on the FOV, with 1.1s for the small FOV and 1.4s for the large FOV. The response time was also found dependent on the size of target, increasing from 1.4s to 1.7s for target diameters of 4cm to 2.8cm. The deformable dynamic phantom has shown that the system is able to successfully follow a highly-deformed, slowmoving prostate with an error < 0.5mm. The overall range of patient intrafraction motion was (−2.4 to 2.2)mm, (−2.6 to 2.4)mm, (−3.0 to 4.3)mm in the RL, SI and AP directions, respectively. Conclusion: The 4DUS system using a large FOV is appropriate to monitor the prostate motion as well as the adjacent organs at risk. However, infrequent abrupt motion can be problematic and the small FOV system is preferred for such patients, particularly for future applications as online prostate monitoring employed for MLC tracking.

The first clinical implementation of electromagnetic transponder-guided MLC tracking.

We report on the clinical process, quality assurance, and geometric and dosimetric results of the first clinical implementation of electromagnetic transponder-guided MLC tracking which occurred on 28 November 2013 at the Northern Sydney Cancer Centre. An electromagnetic transponder-based positioning system (Calypso) was modified to send the target position output to in-house-developed MLC tracking code, which adjusts the leaf positions to optimally align the treatment beam with the real-time target position. Clinical process and quality assurance procedures were developed and performed. The first clinical implementation of electromagnetic transponder-guided MLC tracking was for a prostate cancer patient being treated with dual-arc VMAT (RapidArc). For the first fraction of the first patient treatment of electromagnetic transponder-guided MLC tracking we recorded the in-room time and transponder positions, and performed dose reconstruction to estimate the delivered dose and also the dose received had MLC tracking not been used. The total in-room time was 21 min with 2 min of beam delivery. No additional time was needed for MLC tracking and there were no beam holds. The average prostate position from the initial setup was 1.2 mm, mostly an anterior shift. Dose reconstruction analysis of the delivered dose with MLC tracking showed similar isodose and target dose volume histograms to the planned treatment and a 4.6% increase in the fractional rectal V60. Dose reconstruction without motion compensation showed a 30% increase in the fractional rectal V60 from that planned, even for the small motion. The real-time beam-target correction method, electromagnetic transponder-guided MLC tracking, has been translated to the clinic. This achievement represents a milestone in improving geometric and dosimetric accuracy, and by inference treatment outcomes, in cancer radiotherapy.

SP-0435: Real-time dose reconstruction during volumetric modulated arc therapy with dynamic MLC tracking

SP-0435: Real-time dose reconstruction during volumetric modulated arc therapy with dynamic MLC tracking

PD-0099: Potential clinical benefit of kV image-based MLC tracking for liver SBRT with VMAT: A simulation study

PD-0099: Potential clinical benefit of kV image-based MLC tracking for liver SBRT with VMAT: A simulation study

Time‐resolved dose distributions to moving targets during volumetric modulated arc therapy with and without dynamic MLC tracking

The highly conformal doses delivered by volumetric modulated arc therapy (VMAT) may be compromised by intrafraction target motion. Although dynamic multileaf collimator (DMLC) tracking can mitigate the dosimetric impact of motion on the accumulated dose, residual errors still exist. The purpose of this study was to investigate the temporal evolution of dose errors throughout VMAT treatments delivered with and without DMLC tracking. Tracking experiments were performed on a linear accelerator connected to prototype DMLC tracking software. A three-axis motion stage reproduced representative clinical trajectories of four lung tumors and four prostates. For each trajectory, two VMAT treatment plans (low and high modulation) were delivered with and without DMLC tracking as well as to a static phantom for reference. Dose distributions were measured continuously at 72 Hz using a dosimeter with biplanar diode arrays. During tracking, the MLC leaves were continuously refitted to the 3D target position measured by an electromagnetic transponder at 30 Hz. The dosimetric errors caused in the 32 motion experiments were quantified by a time-resolved 3%/3 mm γ-test. The erroneously exposed areas in treatment beam's eye view (BEV) caused by inadequate real-time MLC adaptation were calculated and compared with the time-resolved γ failure rates. The transient γ failure rate was on average 16.8% without tracking and 5.3% with tracking. The γ failure rate correlated well with the erroneously exposed areas in BEV (mean of Pearson r = 0.83, p < 0.001). For the final accumulated doses, the mean γ failure rate was 17.9% without tracking and 1.0% with tracking. With tracking the transient dose errors tended to cancel out resulting in the low mean γ failure rate for the accumulated doses. Time-resolved measurements allow pinpointing of transient errors in dose during VMAT delivery as well as monitoring of erroneous dose evolution in key target positions. The erroneously exposed area in BEV was shown to be a good indicator of errors in the dose distribution during treatment delivery.

Experimentally Validated Simulator of Dynamic MLC Tracking Treatments: A Tool for Tracking QA

Experimentally Validated Simulator of Dynamic MLC Tracking Treatments: A Tool for Tracking QA

PD-0045: Time-resolved dose reconstruction of VMAT delivery to moving targets with and without dynamic MLC tracking

PD-0045: Time-resolved dose reconstruction of VMAT delivery to moving targets with and without dynamic MLC tracking

A Dosimetric Comparison of MLC Tracking and Gated Delivery of Arc Therapy to a Moving Target

A Dosimetric Comparison of MLC Tracking and Gated Delivery of Arc Therapy to a Moving Target