MLC Aperture Research Articles

Importance of Adequate PTV Margin in Stereotactic Body Radiotherapy (SBRT): A Comparison of Two Prescription Methods

In conventional treatment planning, the beam's-eye-view MLC aperture is often larger than the PTV. This gap between the MLC and PTV helps to ensure that the prescription dose is relatively uniform throughout the PTV. In SBRT, however, the beam's-eye-view MLC aperture is typically fit to the PTV edge and the center of PTV is purposely prescribed to a higher dose. These two prescription methods lead to different dose fall-off characteristics at the PTV periphery. The implications of this may be profound if the PTV margin is, in reality, inadequate, resulting in a marginal miss.

Dynamic MLC tracking of moving targets with a single kV imager for 3D conformal and IMRT treatments

Background. Tumor motion during radiotherapy is a major challenge for accurate dose delivery, in particular for hypofractionation and dose painting. The motion may be compensated by dynamic multileaf collimator (DMLC) tracking. Previous work has demonstrated that a single kV imager can accurately localize moving targets for DMLC tracking during rotational delivery, however this method has not been investigated for the static gantry geometry used for conformal and IMRT treatments. In this study we investigate the accuracy of single kV-imager based DMLC tracking for static-gantry delivery. Material and methods. A 5-field treatment plan with circular field shape and 200 MU per field was delivered in 20 s per field to a moving phantom with an embedded gold marker. Fluoroscopic kV images were acquired at 5 Hz perpendicular to the treatment beam axis during a 120° pre-treatment gantry rotation, during treatment delivery, and during inter-field gantry rotations. The three-dimensional marker position was estimated from the kV images and used for MLC adaptation. Experiments included 12 thoracic/abdominal tumor trajectories and five prostate trajectories selected from databases with 160 and 548 trajectories, respectively. The tracking error was determined as the mismatch between the marker position and the MLC aperture center in portal images. Simulations extended the study to all trajectories in the databases and to treatments with prolonged duration of 60 s per field. Results. In the experiments, the mean root-mean-square (rms) tracking error was 0.9 mm (perpendicular to MLC) and 1.1 mm (parallel to MLC) for thoracic/abdominal tumor trajectories and 0.6 mm (perpendicular) and 0.5 mm (parallel) for prostate trajectories. Simulations of these experiments agreed to within 0.1 mm. Simulations of all trajectories in the databases resulted in mean rms tracking errors of 0.6 mm (perpendicular) and 0.9 mm (parallel) for thorax/abdomen tumors and 0.4 mm (perpendicular) and 0.2 mm (parallel) for prostate for both 20 s and 60 s per field. Conclusion. Single kV imager DMLC tracking, which is fully compatible with IMRT, was demonstrated for static fields. The mean tracking error was sub-2 mm for most tumor trajectories with respiratory motions and sub-1 mm for most prostate trajectories.

Detailed analysis of latencies in image‐based dynamic MLC trackinga)

Previous measurements of the accuracy of image-based real-time dynamic multileaf collimator (DMLC) tracking show that the major contributor to errors is latency, i.e., the delay between target motion and MLC response. Therefore the purpose of this work was to develop a method for detailed analysis of latency contributions during image-based DMLC tracking. A prototype DMLC tracking system integrated with a linear accelerator was used for tracking a phantom with an embedded fiducial marker during treatment delivery. The phantom performed a sinusoidal motion. Real-time target localization was based on x-ray images acquired either with a portal imager or a kV imager mounted orthogonal to the treatment beam. Each image was stored in a file on the imaging workstation. A marker segmentation program opened the image file, determined the marker position in the image, and transferred it to the DMLC tracking program. This program estimated the three-dimensional target position by a single-imager method and adjusted the MLC aperture to the target position. Imaging intervals deltaT(image) from 150 to 1000 ms were investigated for both kV and MV imaging. After the experiments, the recorded images were synchronized with MLC log files generated by the MLC controller and tracking log files generated by the tracking program. This synchronization allowed temporal analysis of the information flow for each individual image from acquisition to completed MLC adjustment. The synchronization also allowed investigation of the MLC adjustment dynamics on a considerably finer time scale than the 50 ms time resolution of the MLC log files. For deltaT(image) = 150 ms, the total time from image acquisition to completed MLC adjustment was 380 +/- 9 ms for MV and 420 +/- 12 ms for kV images. The main part of this time was from image acquisition to completed image file writing (272 ms for MV and 309 ms for kV). Image file opening (38 ms), marker segmentation (4 ms), MLC position calculation (16 ms), and MLC adjustment (52 ms) were considerably faster. For deltaT(image) = 1000 ms, the total time from image acquisition to completed MLC adjustment increased to 1030 +/- 62 ms (MV) and 1330 +/- 52 ms (kV) mainly because of delayed image file writing. The MLC adjustment duration was constant 52 ms (+/- 3 ms) for MLC adjustments below 1.1 mm and increased linearly for larger MLC adjustments. A method for detailed time analysis of each individual real-time position signal for DMLC tracking has been developed and applied to image-based tracking. The method allows identification of the major contributors to latency and therefore a focus for reducing this latency. The method could be an important tool for the reconstruction of the delivered target dose during DMLC tracking as it provides synchronization between target motion and MLC motion.

SU-GG-T-230: Performance Monitoring of VMAT Delivery

Purpose: To develop a system to measure the per-segment dose of volumetric modulated arc therapy (VMAT) beams, for delivery system and patient plan quality assurance (QA). Methods and Materials: VMAT delivery validation requires measuring the dose as a function of gantry angle in order to identify gantry angle dependent errors. A dose measurement system has been developed which uses custom software and hardware, and a commercially available inclinometer, to sample the dose detected by an ionization chamber located inside of a phantom as a function of gantry angle, enabling per-segment evaluation of VMAT delivery. For routine QA of a VMAT delivery system, fields were created in which a 2 × 2 cm MLC aperture is continuously centered on an ionization chamber positioned at 8.0 cm distance from the isocenter. The MLC leaf trajectory follows a sinusoidal path over a 360 ° gantry arc. Fields were generated to deliver both a constant MU and a constant dose per segment. The measured dose is compared with the calculated values for each beam segment. Gantry angle errors were introduced to evaluate the sensitivity of the system to potential errors. The delivery system dosimetry was assessed at various dose rates and gantry speeds. Prostate and head and neck VMAT patient plans have been evaluated similarly by comparing measured and planned per-segment dose. Ion chambers were placed both at isocenter and off-axis to provide gantry angle error sensitivity. Plans were generated on Pinnacle Ver. 8.9 treatment planning system; delivered on an Elekta Synergy accelerator. Results: The dosimetry system was able to accurately measure the per-segment dose delivery of VMAT plans. Using the fields created for routine delivery system QA, gantry angle errors of 1° were detectable. Conclusions: A method has been developed to effectively monitor the dose delivery performance of a VMAT delivery system.

TU‐C‐BRA‐08: Can We Ignore Angular Acceleration and Deceleration Caused by Gantry Angular Speed Modulation in Volumetric Modulated Radiation Therapy

Objective: VMAT optimizes both MLC apertures and MU weights simultaneously. Additionally, VMAT uses a substantially higher gantry angle sampling frequency than DAO to better model the continuously moving source. VMAT achieves a desirable intensity modulation through a synergistic combination of MLC aperture, dose rate, and gantry angular speed modulations. Unlike conventional rotational techniques, VMAT requires the gantry to accelerate and decelerate frequently to deliver a given angular dose rate (MU/degree). This could potentially become a source of dosimetric error. In this study, we investigated the potential dosimetric effects of gantry angular acceleration and deceleration in VMAT delivery. Materials and Methods: A commercial phantom, MapPHAN with MapCHECK, was scanned on a CT scanner with a slice thickness of 3 mm and a matrix size of 512×512. The acquired CT images were transferred to our in‐house VMAT TPS for treatment planning. A VMAT plan with 180 control points and 360° gantry rotation was computed. On a Varian Trilogy CLINAC, the VMAT plan was delivered to the phantom six times with six different maximum dose rates: 600, 500, 400, 300, 200, and 100 MU/min, respectively. The measured dose distribution with 600 MU/min was used as the baseline for comparison purpose. Results: We found that there were observable dose differences in the dose map between the measured dose distributions for 600 MU/min and 100 MU/min. These dose discrepancies covered an extensive area, but were mainly concentrated in the PTV, i.e., in the high dose and high dose gradient regions. The dose differences ranged from –2 ∼ 6 cGy. Conclusions: For a few fractions, this difference may be trivial. But for dose escalation cases, the accumulative dose differences could be significant and should not be ignored.

SU‐GG‐J‐10: Investigation of a Novel Algorithm for True 4D VMAT Planning and Delivery

Purpose: To quantify the ability of our 4D VMAT planning algorithm to generate deliverable plans over a range of target motions. Method and Materials: Our 4D VMAT planning algorithm is an extension of the 3D algorithm by Otto (Med Phys 35 2008, 310–317) and fully incorporates target and organ motion during optimization. Delivery of each MLC aperture is synchronized to a specific phase of the target motion. The magnitude of motion between each phase is 2.5 or 5 mm. Using a phantom consisting of a cylindrical target nested within a half‐ring avoidance structure, treatment plans for a range of uniform target motions (0.5 – 4 cm) and periods (2.5 – 5.5 s) were generated. Dose prescription was 60 Gy. DVHs from the 4D VMAT plans were compared against the 3D VMAT DVH as well as 3D motion degraded DVHs. Results: 4D VMAT plans were similar in quality or superior to the 3D plan. For motion ranges of 1.5 cm and 4 cm, the volume of the avoidance structure receiving more than 20 Gy was decreased by 10.4% and 28.6% respectively while the target volume receiving greater than 58 Gy increased by 6.5% and 16.5%. Total treatment time ranged from 141 – 181 minutes to deliver the full 60 Gy prescription or 4.7 – 6.1 minutes for a 2 Gy fraction assuming maximum dose rate is 600 MU/min. Motion of 1 cm can cause noticeable degradation of the 3D DVH. Conclusion: Our 4D VMAT planning algorithm can create plans equal to or superior to 3D plans. Future work will investigate whether these benefits can be extended to actual 4D clinical patient data.

Four‐dimensional intensity‐modulated radiation therapy planning for dynamic tracking using a direct aperture deformation (DAD) method

Planning for the delivery of intensity-modulated radiation therapy (IMRT) to a moving target, referred to as four-dimensional (4D) IMRT planning, is a crucial step for achieving the treatment objectives for sites that move during treatment delivery. The authors proposed a simplistic method that accounts for both rigid and nonrigid respiration-induced target motion based on 4D computed tomography (4DCT) data sets. A set of MLC apertures and weights was first optimized on a reference phase of a 4DCT data set. At each beam angle, the apertures were morphed from the reference phase to each of the remaining phases according to the relative shape changes in the beam's eye view of the target. Three different planning schemes were evaluated for two lung cases and one pancreas patient: (1) Individually optimizing each breathing phase; (2) optimizing the reference phase and shifting the optimized apertures to other breathing phases based on a rigid-body image registration; and (3) optimizing the reference phase and deforming the optimized apertures to the other phases based on the deformation and translation of target contours. Planning results using scheme 1 serves as the "gold standard" for plan quality assessment; scheme 2 is the method previously proposed in the literature; and scheme 3 is the method the authors proposed in this article. The optimization results were compared between the three schemes for all three cases. The proposed scheme 3 is comparable to scheme 1 in plan quality, and provides improved target coverage and conformity with similar normal tissue dose compared with scheme 2. Direct aperture deformation method for 4D IMRT planning improves upon methods that only consider rigid-body motion and achieves a plan quality close to that optimized for each of the phases.

Dynamic Multileaf Collimator Tracking of Respiratory Target Motion Based on a Single Kilovoltage Imager During Arc Radiotherapy

To demonstrate and characterize dynamic multileaf collimator (DMLC) tracking of respiratory moving targets that are spatially localized with a single kV X-ray imager during arc radiotherapy. During delivery of an arc field (358 degrees gantry rotation, 72-sec duration, circular field shape), the three-dimensional (3D) position of a fiducial marker in a phantom was estimated in real time from fluoroscopic kV X-ray images acquired orthogonally to the treatment beam axis. A prediction algorithm was applied to account for system latency (570 ms) before the estimated marker position was used for DMLC aperture adaptation. Experiments were performed with 12 patient-measured tumor trajectories that were selected from 160 trajectories (46 patients) and reproduced by a programmable phantom. Offline, the 3D deviation of the estimated phantom position from the actual position was quantified. The two-dimensional (2D) beam-target deviation was quantified as the positional difference between the MLC aperture center and the marker in portal images acquired continuously during experiments. Simulations of imaging and treatment delivery extended the study to all 160 tumor trajectories and to arc treatments of 3-min and 5-min duration. In the experiments, the mean root-mean-square deviation was 1.8 mm for the 3D target position and 1.5 mm for the 2D aperture position. Simulations agreed with this to within 0.1 mm and resulted in mean 2D root-mean-square beam-target deviations of 1.1 mm for all 160 trajectories for all treatment durations. The deviations were mainly caused by system latency (570 ms). Single-imager DMLC tracking of respiratory target motion during arc radiotherapy was implemented, providing less than 2-mm geometric uncertainty for most trajectories.

Implementation of a New Method for Dynamic Multileaf Collimator Tracking of Prostate Motion in Arc Radiotherapy Using a Single kV Imager

To implement a method for real-time prostate motion estimation with a single kV imager during arc radiotherapy and to integrate it with dynamic multileaf collimator (DMLC) target tracking. An arc field with a circular aperture and 358 degrees gantry rotation was delivered to a motion phantom with a fiducial marker under continuous kV X-ray imaging at 5 Hz, perpendicular to the treatment beam. A pretreatment gantry rotation of 120 degrees in 20 sec with continuous imaging preceded the treatment. During treatment, each kV image was first used together with all previous images to estimate the three-dimensional (3D) target probability density function and then used together with this probability density function to estimate the 3D target position. The MLC aperture was then adapted to the estimated 3D target position. Tracking was performed with five patient-measured prostate trajectories that represented characteristic prostate motion patterns. Two data sets were recorded during tracking: (1) the estimated 3D target positions, for off-line comparison with the actual phantom motion; and (2) continuous portal images, for independent off-line calculation of the 2D tracking error as the positional difference between the marker and the MLC aperture center in each portal image. All experiments were also made with 1- Hz kV imaging. The mean 3D root-mean-square error of the trajectory estimation was 0.6 mm. The mean root-mean-square tracking error was 0.7 mm, both parallel and perpendicular to the MLC. The accuracy degraded slightly for 1- Hz imaging. Single-imager DMLC prostate tracking that allows arbitrary beam modulation during arc radiotherapy was implemented. It has submillimeter accuracy for most prostate motion types.

An alternative VMAT with prior knowledge about the type of leaf motion utilizing projection method for concave targets

The authors present an alternative approach to inverse planning optimization and apply it to volumetric modulated are therapy (VMAT) in one rotation with a prior knowledge about the type of leaf motions. The optimization is based on the projection theorem in inner product spaces. MLC motion is directly considered in the optimization, thus avoiding leaf segmentation characteristic of IMRT optimization. In this work they realize the method for concave irregular targets encompassing an organ at risk leading to a repetitive MLC motion pattern. Applying the projection theorem leads to a noniterative optimization method and reduces to solving few systems of linear equations with small numbers of dimensions. The solution of the inverse problem is unique, and false minima are naturally excluded. They divided the full rotation into about 50 short arc segments and for each segment decomposed dose into separate contributions related to stages of MLC motion. This results generally in an inverse problem with just four free parameters per arc segment. Practically three degrees of freedom will be used for the purpose of a constant angular speed of the gantry. Therefore the total number of degrees of freedom for a 3D problem is about 3 x 50 x number of collimator leaf pairs for irradiating the whole target volume in one rotation. Two 2D and one 3D concave target volumes are applied for a slice by slice optimization. A 6 MV photon beam model is used, including realistic scattering and attenuation, and a maximal leaf velocity of 3 cm/s is regarded. The resulting dose distributions cover the PTVs very well and have maxima at about 108% of dose in the PTVs. The OAR is spared very strong in all cases. As a result of optimization, the MLC apertures are repetitively opening and closing and can be interpreted in an intuitive way. Applying the projection method for this knowledge-based VMAT delivery scheme for concave target volumes is an alternative technique for dose optimization. There are several properties, such as uniqueness of MLC motions and their continuous dependence on geometry and prescribed dose, that make this approach interesting to inverse planning. This method is still in an investigational stage, but promising results are presented. In future work it will be extended directly (without conceptual changes) in several directions to be more clinically applicable.

SU-FF-T-225: Verification of Geometric Parameters in Volumetric Modulated Arc Therapy (VMAT)

Purpose: VMAT delivery involves simultaneous changes of multiple variables including gantry angle and rotation speed, MLC aperture shape, and dose rate. How to reliably verify the accuracy and functionality of the VMAT delivery presents a challenging problem to medical physicists. The purpose of this work is to describe an effective geometric quality assurance (QA) method for VMAT. Method and Materials: A Varian RapidArc system was used for this study. The gantry angle and rotation speed were measured every 10 ms by a dynamic inclinometer. The positions and speeds of the MLC leaves at an arbitrary gantry angle (or time point) were obtained by using cine‐mode MV imaging and a post‐delivery image processingsoftware. The geometric data so obtained were retrospectively synchronized and compared with the MLC Dynalog files, which record the gantry angle and leaf position every 50 ms during dose delivery. Three arc plans were generated using Eclipse planning system to test the system performance at different delivery situations. Each plan was delivered multiple times for error analysis.Results: Our measurements show that the mean deviation of gantry angle over the course of delivery was 0.19°–0.46° while the mean deviation of gantry rotation speed was 0.12–0.15°/sec. The position and speed accuracy of individual MLC leaves measured by mean deviation were 0.5–1.0 mm (0.5–0.7 mm for bank A leaves; 0.8–1.0 mm for bank B) and 0.09–0.12 mm/sec, respectively. It was also noted that MLC positional accuracy had a sudden change when the gantry angle reached 180 degree, presumably due to the gravitational effect. Conclusion: A clinically useful geometric QA procedure has been developed. Application of the procedure to the RapidArc delivery reveals that the planned arc sequence can generally be realized faithfully without major geometric error. Dosimetric tests should also be designed to ensure the dosimetric accuracy of VMAT delivery.

TU-E-BRD-03: DMLC IMRT Tracking of Moving Anatomy - Issues and Solutions

DMLC tracking algorithms have emerged recently as a feasible solution of the delivery of intensity modulated radiation therapy in presence of motion of patient body anatomy. In this technique a MLC aperture, and beam dose rate, are dynamically adapted to trajectories of target and other organs in motion. There exist hierarchies of ever more sophisticated deliveries of this type that respond to increasingly realistic, and at the same time increasingly more complex, treatment conditions. The possibility of these various adaptations is rooted in multitude of leaf motions, and beam dose rate functions, that impose the same IMRT map over moving targets. Goals of tracking deliveries can evolve from finding solutions that are most time efficient to those that provide most benefit from dosimetric point of view. The presentation will discuss cases of IMRTdelivery to rigid and deforming targets, will consider situations when target motions are known and reproducible at delivery as well as situations when these motions are irregular and unpredictable. The disadvantages of unintended variation in delivery to organs at risk will be discussed for solutions that deliver planned intensities to targets during tracking, while disregarding motions of organs at risk when differential motions between targets and organs at risk persist. The possibility of using additional degrees of freedom of mutual motion between organs at risk and targets for the benefit of the therapy will then be presented. The method of using beam dose rate variation for benefiting DMLC delivery will be explained. Learning Objectives: 1. Understand the origin of the DMLC IMRT tracking. 2. Understand the issues related to DMLC IMRT tracking in presence of complex motions of body anatomy. 3. Understand the issues related to therapeutic advantage rooted in motions of body tissues at radiation treatmentdelivery.

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 beam's 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-FF-J-110: Geometrical Verification of Real-Time Tumor Tracking Using Electronic Portal Imaging Device (EPID)

Purpose: We have developed a novel method to verify the geometrical accuracy of real‐time tumor tracking. An Electronic Portal Imaging Device(EPID) is used as a virtual phantom system. Methods and Materials: This virtual phantom system uses virtual target motion in the absence of a moving phantom and fluoroscopic mode of EPID for monitoring the motion of the tracking aperture. The expected apertures are designed from real‐time detection of tumor motion, and the delivered apertures are monitored from the EPID system. These two apertures are compared for geometrical accuracy of a real‐time tracking technique. The virtual phantom system was tested with several tracking patterns with irregular motion. Target motion was designed for an oval‐shaped tumor with 40 mm and 55 mm in diameters for four cases: (1) stationary; (2) rigid‐body two‐dimensional (2‐D) displacement with a period variation from 3 to 10 s and 20‐mm peak‐to‐peak distance in the superior‐inferior direction and 10 mm in the left‐right direction; (3) 2‐D displacement with deformation, and (4) 2‐D displacement with both deformation and rotation. In the continuous acquisition mode of EPID,MLC aperture images were acquired at ∼8 Hz. Root‐mean‐square (RMS) deviation between the designed and delivered aperture motions were calculated for each case. Results: The RMS deviation is 0.005 mm for the stationary case, 0.5 mm for the 2‐D displacement, 0.7 mm for the 2‐D displacement with deformation, and 0.7 mm for the 2‐D displacement combined with both deformation and rotation. Conclusion: The EPID‐based virtual system can measure geometrical accuracy of real‐time tumor tracking without a moving phantom. Besides, it can be used even for complex target motion (i.e., 2‐D displacement combined with deformation and rotation), which is not possible with any existing moving phantom. Therefore, this EPID‐based virtual system shows great potential as a quality‐assurance process to verify real‐time tumor tracking.

SU-FF-T-166: Determination of Minimum Number of Segments for Direct Machine Parameter Optimization (DMPO) for IMRT Plans and Impact On Treatment Delivery Time

Purpose: Direct machine parameter optimization in IMRT plans offers flexibility in controlling the number of segments. The objective was to device a method to determine the DMPO optimal minimum number of segments and to evaluate its impact on treatment delivery time. Method and Materials:IMRT plans for 10 different prostate patients and 10 different head and neck patients were generated using conventional IMRT planning, where intensity maps are optimized prior to conversion into deliverable MLC apertures. Converted plans were then copied and re‐optimized with DMPO using differing numbers of segments, each time maintaining the same prescription objectives and constraints. The process was repeated until a significant difference in the isodose distributions and Dose‐Volume Histograms (DVHs) between the DMPO and the conventional IMRT plans occurred. The ratio of the minimum DMPO number of segments to the conventional IMRT number of segments was calculated. Three plans from each treatment site were performed using DMPO starting from scratch and the treatment planning time evaluated. Results: The maximum target and organ doses with DMPO were found to be about 3% higher than the conventional IMRT maximum doses, resulting into higher hot spots and slightly less dose homogeneity. DMPO reduced the number of segments by 45% for prostate cases and 40% for head and neck cases. DMPO took an average of 3 hours longer to plan from scratch. DMPO provided an average of 50% decrease in treatment delivery time. Conclusion: It is easier and faster to do a conventional IMRT plan first and then apply DMPO to the resultant plan. An acceptable DMPO would need about 55% of the total number of segments of a conventional IMRT prostate plan, and 60% of a conventional head and neck plan. We did not find DMPO to offer any benefit over conventional plans with less than 60 segments.

SU-FF-T-648: Time Analysis of Image-Based Dynamic MLC Tracking

Purpose: To develop a method for detailed temporal analysis of image‐based dynamic multileaf collimator (DMLC) tracking. Method and Materials: A prototype DMLC tracking system integrated with a linear accelerator was used for tracking of a cyclically moving phantom with a marker during delivery of a treatment beam. The real‐time target localization was based on x‐ray images acquired with a portal imager and an orthogonal kV imager. As soon as an image was stored on the imagingcomputer a computer program determined the marker position and transferred it to the DMLC tracking program. This program first estimated the three‐dimensional target position either by a single‐imager method or by triangulation (when both imagers were used) and then adjusted the MLC aperture to the target. Imaging intervals from 150ms to 1000ms were investigated for kV imaging alone, MV imaging alone, and combined kV/MV imaging. In an off‐line analysis, the recorded kV and MV images were synchronized with Dynalog files of the MLC positions and log files of the tracking process. It allowed temporal analysis of the information flow for each individual image from acquisition to MLC adjustment. Results: For 150ms imaging intervals the total time from image acquisition to completed MLC adjustment was 300ms for MV and 450ms for kV images. The main part of this time was from image acquisition to completed image file writing (220ms±10ms for MV,350ms±19ms for kV). Image file opening (38ms±8ms), marker segmentation (4ms±7ms), MLC position calculation (16ms±5ms), and MLC adjustment (26ms±5ms) were much faster. Increase of the imaging interval to 1000ms linearly increased the total time to 1010ms (MV) and 1190ms (kV), because of largely delayed image file writing. Conclusion: A method for detailed temporal analysis of each individual real‐time position signal for DMLC tracking has been developed. Conflict of Interest: Supported by NIH‐ROI93626 and Varian Medical Systems.

A volumetric-modulated arc therapy using sub-conformal dynamic arc with a monotonic dynamic multileaf collimator modulation

We present a new dose delivery scheme utilizing sub-conformal dynamic arc (sub-CD-ARC) with a dynamic multileaf collimator (dMLC) modulation in a single rotation. In sub-conformal delivery, a MLC aperture conforms not to the whole target but to its portion, simultaneously completely avoiding organs at risk (OARs). In CD-ARC therapy, the dose deposition level depends on the target width and distance to the axis of rotation, and therefore the use of multiple arcs is necessary to achieve a uniform dose within the target in 3D. In our delivery scheme, the dose deposition variations symptomatic of non-modulating sub-CD-ARC are compensated for in 3D by a quasi-periodic monotonic dMLC modulation. For this reason, we call this scheme sub-conformal dynamic modulated arc (sub-CD-MARC) therapy. The advantage of using such a modulation is that MLC leaf motion has just a few control points, and that an inverse planning problem is reduced to a linear equation problem with a few unknown parameters, which have clear physical meaning. We show the general dosimetric properties of sub-CD-ARC and sub-CD-MARC for a specific geometry of the target and OARs (rotational symmetry with varying inner and outer radii along the axis of rotation). In addition, we present numerical results of sub-CD-MARC inverse planning optimization.

SU‐GG‐T‐524: Initialization Strategy and Concurrent Imaging for Single 360 Degree Arc Intensity Modulated Treatment Delivery

Purpose: Single 360 degree arc treatment with variable MU per degree and changing MLC apertures provides an efficient means for delivering highly conformal radiation therapy. Additionally, the treatment geometry is amenable to CT image acquisition during the delivery enabling verification of patient position. The challenge with the technique is finding the optimal set of apertures over the arc. Aperture based optimization (MLC leaf positions) is the natural approach, but suffers for complex targets if the initial starting condition is simply the fields exposing the target around the arc. The problem occurs when the optimal solution would have the MLC closing off a larger portion of the target requiring large positional changes from the initial condition to the optimal position. In this case gradient based optimizations fall short in that the derivative of the dose distribution with respect to the leaf position is flat for locations further from the leaf tip. In order to remedy this problem, improved initialization of the initial MLC positions around the arc based on the target geometry are proposed. Method and Materials: A research version of the Pinnacle3 RTPS was used to plan the single arc delivery. The approach uses 36 equispaced beams over a 360 degree arc, and optimizes the plan using a single MLC aperture for each beam and the direct machine parameter optimization (DMPO) technique in the system. The Elekta Synergy linear accelerator was used to deliver the treatment and for cone beam CT acquisition. Results: Initial results show dose conformality similar to that of traditional intensity modulated radiotherapy with the 360 degree delivery using geometry based pre‐initialization of the apertures. Additionally, kilovolt image acquisition during delivery was possible. Scattered radiation and the interplay between image acquisition frequency and the pulse rate of the treatment beam influence the image quality but not significantly.

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 MLC sequence and real‐time dose‐rate modulation. We have developed an algorithm for designing the MLC sequence 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 patient's lung tumor 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 MLC sequence 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‐T‐542: Arc‐Modulated Radiation Therapy (AMRT): A Novel Method for Rotational Radiation Therapy

Purpose: To develop a prototype treatment planning system for Arc‐Modulated Radiation Therapy (AMRT), which delivers the optimal radiation distribution with only one beam rotation. Method and Materials: The AMRT planning starts with inverse planning, typically using 36 equally‐spaced (fixed) beams and producing 36 continuous intensity maps. After inverse planning, an AMRT leaf sequencing algorithm converts the optimized intensity patterns to a sequence of MLC apertures along a single arc. Finally, a homegrown Monte‐Carlo superposition‐based dose calculation engine is used to quickly and accurately evaluate the plan qualities. A big challenge to AMRT is leaf sequencing. Our algorithm is based on a key observation that if each optimized intensity pattern is delivered through its neighboring beam angles within a ±5° range, there is only negligible error on the resulting dose distribution. Thus, the sequencing problem becomes how to guarantee the interconnectedness of the MLC apertures along the single arc, while reproducing or closely approximating the optimized intensity patterns. We model this problem as computing shortest paths in directed acyclic graphs, and solve it efficiently using various algorithmic techniques. Results: We applied our AMRT approach to 18 clinical cases with a wide range of treatment sites. In terms of dose conformity, we observed that AMRT plans outperform gantry‐fixed IMRT plans and rival intensity‐modulated arc therapy (IMAT) plans. In terms of delivery efficiency, AMRT plans are much more efficient and can be delivered in less than 5 minutes for complex head‐and‐neck cases. The AMRT planning time is currently about 1–2 hours. Conclusion: We developed a prototype treatment planning system for AMRT, which delivers the optimal radiation distribution with only one beam rotation. Experimental studies demonstrated the potentials of our new approach to dynamic IMRT which warrants further investigations.