M Gui

Verification of MLC based real‐time tumor tracking using an electronic portal imaging device

PURPOSE The authors have developed a novel technique using an electronic portal imaging device (EPID) to verify the geometrical accuracy of delivery of dose-rate-regulated tracking (DRRT). This technique, called verification of real-time tracking with EPID (VORTE), can potentially be used for both on-line and off-line quality assurance (QA) of MLC-based dynamic tumor tracking. METHODS The shape and position of target as a function of time, which is assumed to be known, is projected onto the EPID plane. This projected sequence of apertures as a function of time (target motion) is then used as the reference. The accuracy of dynamic MLC tracking can then be assessed by how well the delivered beam follows this projected target motion without the use of a physical moving phantom. The beam apertures controlled by DRRT (aperture motion) is detected by the EPID as a function of time. The aperture motion is compared to the target motion to evaluate tracking error introduced by DRRT. The accuracy of VORTE was measured using film measurements of ten static fields. The VORTE for dynamic tumor tracking was tested with several target motions, including (1) rigid-body two-dimensional (2-D) cyclic motion in the superior-inferior direction with various period and amplitude; (2) the above 2-D cyclic motion plus cyclic deformation; and (3) 2-D cyclic motion with both deformation and rotation. For each target motion, the controlled aperture motion resulting from DRRT was acquired at ∼8Hz using EPID in the continuous-acquisition mode. Leaf positions in all captured frames were measured from the EPID and compared to their expected positions. The passing rate of 2 mm criteria for all leaves from all frames was calculated for each of the four patterns of tumor motion. Additionally, the root-mean-square (RMS) deviations of the centroid of the apertures between the designed and delivered beams were calculated for all three cases. RESULTS The accuracy of MLC-leaf position determination by VORTE is 0.5 mm (1 standard deviation) by comparison to film measurements. With DRRT, the passing rates using the 2 mm criteria for all acquired frames are 100% for the 2-D displacement, 99% for the 2-D displacement with deformation, and 88% for the 2-D displacement combined with both deformation and rotation. The RMS deviations are 0.6 mm for the 2-D displacement, 1.0 mm for the 2-D displacement with deformation, and 1.1 mm for the 2-D displacement combined with both deformation and rotation. CONCLUSIONS The VORTE can measure the accuracy of MLC-based tumor tracking without the necessity of employing a moving phantom. Moreover, it can be used for complex target motion (i.e., 2-D displacement combined with deformation and rotation) that is difficult to create with physical moving phantoms. Therefore, the VORTE and the novel QA process illustrated by this study have a great potential for verifying real-time tumor tracking.

Four-dimensional dose distributions of step-and-shoot IMRT delivered with real-time tumor tracking for patients with irregular breathing: Constant dose rate vs dose rate regulation

PURPOSE Dose-rate-regulated tracking (DRRT) is a tumor tracking strategy that programs the MLC to track the tumor under regular breathing and adapts to breathing irregularities during delivery using dose rate regulation. Constant-dose-rate tracking (CDRT) is a strategy that dynamically repositions the beam to account for intrafractional 3D target motion according to real-time information of target location obtained from an independent position monitoring system. The purpose of this study is to illustrate the differences in the effectiveness and delivery accuracy between these two tracking methods in the presence of breathing irregularities. METHODS Step-and-shoot IMRT plans optimized at a reference phase were extended to remaining phases to generate 10-phased 4D-IMRT plans using segment aperture morphing (SAM) algorithm, where both tumor displacement and deformation were considered. A SAM-based 4D plan has been demonstrated to provide better plan quality than plans not considering target deformation. However, delivering such a plan requires preprogramming of the MLC aperture sequence. Deliveries of the 4D plans using DRRT and CDRT tracking approaches were simulated assuming the breathing period is either shorter or longer than the planning day, for 4 IMRT cases: two lung and two pancreatic cases with maximum GTV centroid motion greater than 1 cm were selected. In DRRT, dose rate was regulated to speed up or slow down delivery as needed such that each planned segment is delivered at the planned breathing phase. In CDRT, MLC is separately controlled to follow the tumor motion, but dose rate was kept constant. In addition to breathing period change, effect of breathing amplitude variation on target and critical tissue dose distribution is also evaluated. RESULTS Delivery of preprogrammed 4D plans by the CDRT method resulted in an average of 5% increase in target dose and noticeable increase in organs at risk (OAR) dose when patient breathing is either 10% faster or slower than the planning day. In contrast, DRRT method showed less than 1% reduction in target dose and no noticeable change in OAR dose under the same breathing period irregularities. When ±20% variation of target motion amplitude was present as breathing irregularity, the two delivery methods show compatible plan quality if the dose distribution of CDRT delivery is renormalized. CONCLUSIONS Delivery of 4D-IMRT treatment plans, stemmed from 3D step-and-shoot IMRT and preprogrammed using SAM algorithm, is simulated for two dynamic MLC-based real-time tumor tracking strategies: with and without dose-rate regulation. Comparison of cumulative dose distribution indicates that the preprogrammed 4D plan is more accurately and efficiently conformed using the DRRT strategy, as it compensates the interplay between patient breathing irregularity and tracking delivery without compromising the segment-weight modulation.

Planning 4D intensity-modulated arc therapy for tumor tracking with a multileaf collimator

This study introduces a practical four-dimensional (4D) planning scheme of IMAT using 4D computed tomography (4D CT) for planning tumor tracking with dynamic multileaf beam collimation. We assume that patients can breathe regularly, i.e. the same way as during 4D CT with an unchanged period and amplitude, and that the start of 4D-IMAT delivery can be synchronized with a designated respiratory phase. Each control point of the IMAT-delivery process can be associated with an image set of 4D CT at a specified respiratory phase. Target is contoured at each respiratory phase without a motion-induced margin. A 3D-IMAT plan is first optimized on a reference-phase image set of 4D CT. Then, based on the projections of the planning target volume in the beams eye view at different respiratory phases, a 4D-IMAT plan is generated by transforming the segments of the optimized 3D plan by using a direct aperture deformation method. Compensation for both translational and deformable tumor motion is accomplished, and the smooth delivery of the transformed plan is ensured by forcing connectivity between adjacent angles (control points). It is envisioned that the resultant plans can be delivered accurately using the dose rate regulated tracking method which handles breathing irregularities (Yi et al 2008 Med. Phys. 35 3955-62).This planning process is straightforward and only adds a small step to current clinical 3D planning practice. Our 4D planning scheme was tested on three cases to evaluate dosimetric benefits. The created 4D-IMAT plans showed similar dose distributions as compared with the 3D-IMAT plans on a single static phase, indicating that our method is capable of eliminating the dosimetric effects of breathing induced target motion. Compared to the 3D-IMAT plans with large treatment margins encompassing respiratory motion, our 4D-IMAT plans reduced radiation doses to surrounding normal organs and tissues.

SU‐E‐J‐122: Planning Four‐Dimensional Intensity‐Modulated Arc Therapy for Tumor Tracking

Purpose: We propose a practical four‐dimensional (4D) planning scheme of intensity‐modulated arc therapy (IMAT) for tumor tracking using dynamic multileaf collimators(MLC). Methods: We contour the target and critical structures on each image set of 4DCT associated with each breathing phase. Transformation vectors of the target from a reference phase to other phases are derived. A 3D‐IMAT plan is first optimized on the reference phase without motion‐induced margin. Assuming delivery starts at a predetermined breathing phase and the patient breathes the same way as during 4DCT imaging, we know the phases at which each planned segment will be delivered. Then, we transform the optimized segments from the reference phase to the phases of their delivery based on projections of transformation vectors of the target in the beams eye‐view. The connectivity of the transformed segments between adjacent beam angles is enforced. The dose for each phase is calculated using segments delivered at that phase, and transformed to the reference phase to obtain cumulative dose. The resulting 4D plan is compared with the 3D plan without considering respiratory motion and with motion‐induced margins. Effects of enforcing connectivity on the plan quality are also evaluated. Results: Compared to the PTV dose of 3D plan without tumor tracking, the 4D PTV dose is more conformal and uniform, and it approaches the ideal situation of the patient being static. The 4D plan also showed better critical organ sparing than the 3D plan with motion‐induced margins. Adding MLC motion required for tracking to the planned motion rarely causes physical constraints to be exceeded, and enforcing connectivity thus has little effect on plan quality. Conclusions: The proposed scheme of 4D‐IMAT planning for tumor tracking is a feasible and practical approach under the current clinical planning environment. Dosimetric benefits of 4D planning have been clearly demonstrated. This stusy is supported in part by NIH R01 grant 1R01CA133539‐01A2

TH-C-BRC-03: Comparison of 4D Dose Distribution Delivered with Two Different Tumor Tracking Strategies for Patients with Irregular Breathing: DRRT vs CDRT

Purpose: Dose‐rate‐regulated tracking (DRRT) is a tumor tracking strategy that programs the MLC to track the tumor and adapts to breathing irregularities during delivery using dose rate regulation. Constant‐dose‐rate tracking (CDRT) adjusts the MLC motion based on real‐time detection of tumor displacement. The purpose of this study is to illustrate the differences in the effectiveness and delivery accuracy between these two tracking methods when dealing with breathing irregularities. Methods: Step‐and‐ shoot IMRT plans optimized at a reference phase were extended to the remaining phases to generate 10‐phased 4D‐IMRT plans using segment aperture morphing (SAM) algorithm, where both tumor displacement and deformation were considered. A SAM‐based 4D plan has been demonstrated to provide better plan quality than plans not considering target deformation. However, delivering such a plan requires pre‐programming of the MLC aperture sequence. Delivery of the 4D plans using DRRT and CDRT tracking approaches was simulated assuming the breathing period is either shorter or longer than the planning day. In DRRT, dose rate was regulated to speed‐up or slow down delivery as needed such that each planned segment is delivered at the planned breathing phase. In CDRT strategy, MLC is separately controlled to follow the tumor motion, but dose rate was kept constant. Results: Delivery of preprogrammed 4D plans by the CDRT method resulted in an average of 5% increase in target dose and noticeable increase in OAR dose when patient breathing is either 10% faster or slower than the planning day. In contrast, DRRT method showed less than 1% reduction in target dose and no noticeable change in OAR dose under the same breathing irregularities. Conclusions: In delivery of SAM‐ based 4D‐IMRT treatment plans considering tumor deformation as well as motion irregularities, the DRRT tracking method was found to be much more effective and more accurate than the CDRT strategy. This research was supported in part by NIH grant 1R01CA133539‐01A2.

SU‐FF‐T‐656: 4D IMRT Planning Using a Direct Aperture Morphing Method

Purpose: To propose a 4D IMRT planning method that accounts for both rigid and non‐rigid respiration related target motion based on the 4DCT datasets. Methods and Materials: The set of MLC aperture sequences optimized on a reference phase of 4DCT is morphed to the rest of the phases according to the anatomical changes of the target projection in the beams eye view (BEV) at each beam angle, and thus ensured the continuity of the MLC aperture between adjacent phases. This method does not need complex computation or couch motion, only simple geometric relationship of target projection between different phases are employed. Three different planning schemes were evaluated. 1) Individually optimize each breathing phase should theoretically generate the best dose distributions, although such plans cannot be delivered because the apertures in different plans are not connected geometrically. This scheme is used as a benchmark of plan quality for the other schemes. 2) Optimize treatment for a reference phase and shift the optimized apertures to other phases based on a rigid‐body image registration. 3) The proposed scheme of optimizing treatment for a reference phase and deforming the optimized apertures to other phases based on the deformation and translation of target contours. Results: Direct aperture morphing method (scheme 3) illustrated comparable plan quality compared to scheme1; and demonstrated the improved target coverage and conformity compared to the scheme2 that only considers the rigid motion and comparable dose in normal tissue. Conclusion: Direct aperture morphing method can be used for 4D IMRT planning and it has equal or better plan quality compared to the method that only considers the rigid body motion.

SU-GG-J-34: Aperture Design of Two-Dimensional MLC Motion for Dose-Rate-Regulated Tracking

Purpose: Real‐time adaptive tumor tracking called Dose‐Rate‐Regulated Tracking (DRRT) is based on a preprogrammed MLCsequence and real‐time dose‐rate modulation. We have developed an algorithm for designing the MLCsequence reflecting 2‐D MLC motion. MLC apertures were designed from 4D‐CT images, and the algorithm was tested in the 3‐D phantom. Method and Materials: The 2‐D MLC motion and the corresponding tumor shape are derived from ten‐breathing bins of 4D‐CT. The closed MLC leaves that are not participating in the beam aperture are programmed to remain in motion to minimize leaf‐end leakage. This feathering motion is intended to spread the leakage dose to the normal tissue beneath the closed MLC leaves. We performed phantom studies with a 3‐D Phantom (Washington Univ.) for two cases: (1) a circular moving target (2) a patients lungtumor in the lower lobe. The results of these two measurements were compared with those for the case of a static beam aperture and a static phantom (static‐static case). The accuracy of the 2‐D MLCsequence was determined by γ analysis with those for the static‐static case. The effectiveness of the feathering motion was quantified with the leakage dose normalized to the maximum dose measured by the film. Results and Conclusion: For both of the above cases, the γ analysis showed that 95% of the pixels are less than 1 for 3 % and 3 mm criteria. The feathering motion reduced the leaf‐end‐leakage dose from ∼15% to ∼7%. A careful design of the MLC‐leaf sequence done in advance can not only facilitate real‐time tumor tracking, but also reduce the dose to healthy tissue, and thus can lead to improved results using DRRT.

SU‐GG‐J‐199: Phase and Displacement Incorporated Model for Determination of Tumor Position with the Berathing Surrogate (RPM)

Purpose: Breathing surrogate, such as Real‐Time Position Monitor (RPM), is most commonly used to predict internal motion due to the difficulties in directly measuring internal motion. In this study, to account for both linear correlated displacement and possible phase shift between RPM signal and internal organ motion, a new model that incorporates both displacement and phase of RPM signal was proposed and its accuracy was estimated and compared with that of other models.Method and Materials: The diaphragm motion was traced from fluoroscopic lung procedures for 10 patients, with respiratory surrogate signals acquired simultaneously with RPM system which tracks motion of reflective markers mounted on the abdomen with an infrared‐sensitive camera. To estimate the diaphragm motion, a general linear function using RPM displacement as the input, a 6th order polynomial using RPM phase as the input, and a new model we proposed as the weighted sum of the previous two, using both RPM displacement and phase as parameters, were used. Least square fitting was applied for all the three models respectively to derive the corresponding parameters. Respiratory gating (30% duty cycle) was performed retrospectively based on the three diaphragm motion trajectories from the three models, and 95th percentile residual motion was evaluated for each patient. Also, deviations of the estimated trajectory from the true trajectory were calculated as 95th percentile tracking error for each model. Paired t‐test was performed and p<0.05 was determined significant. Results and Conclusion: All of the 10 patients demonstrated significantly smaller tracking error (∼1.5mm, p<0.05) and residual motion (∼1.2mm, p<0.05) when using the new model compared to the other two, indicating that this new model is more effective in estimating the diaphragm motion from breathing surrogate (RPM) signals compared to the models using either RPM displacement or phase as the only parameter.