Maximum Leaf Speed Research Articles

Comparison of MLC positioning deviations using log files and establishment of specific assessment parameters for different accelerators with IMRT and VMAT

Background and purposeThe study evaluated the differences in leaf positioning deviations by the log files of three advanced accelerators with two delivery techniques, and established specific assessment parameters of leaf positioning deviations for different types of accelerators.MethodsA total of 420 treatment plans with 5 consecutive treatment log files were collected from the Trilogy, TrueBeam and Halcyon accelerators. Millennium MLC was equipped on the Trilogy and TrueBeam accelerators. A jawless design and dual-layer MLC were adopted on the Halcyon accelerator. 70 IMRT and 70 VMAT plans were selected randomly on each accelerator. The treatment sites of all plans included head and neck, chest, breast, pelvis and other sites. The parsing tasks for 2100 log files were proceeded by SunCheck software from Sun Nuclear Corporation. The maximum leaf root mean square (RMS) errors, 95th percentile errors and percentages of different leaf positioning errors were statistically analyzed. The correlations between these evaluation parameters and accelerator performance parameters (maximum leaf speed, mean leaf speed, gantry and arc angle) were analyzed.ResultsThe average maximum leaf RMS errors of the Trilogy in the IMRT and VMAT plans were 0.44 ± 0.09 mm and 0.79 ± 0.07 mm, respectively, which were higher than the TrueBeam's 0.03 ± 0.01 mm, 0.03 ± 0.01 mm and the Halcyon's 0.05 ± 0.01 mm, 0.07 ± 0.01 mm. Similar data results were shown in the 95th percentile error. The maximum leaf RMS errors were strongly correlated with the 95th percentile errors (Pearson index > 0.5). The leaf positioning deviations in VMAT were higher than those in IMRT for all accelerators. In TrueBeam and Halcyon, leaf position errors above 1 mm were not found in IMRT and VMAT plans. The main influencing factor of leaf positioning deviation was the leaf speed, which has no strong correlation with gantry and arc angles.ConclusionsCompared with the quality assurance guidelines, the MLC positioning deviations tolerances of the three accelerators should be tightened. For both IMRT and VMAT techniques, the 95th percentile error and the maximum RMS error are suggested to be tightened to 1.5 and 1 mm respectively for the Trilogy accelerator. In TrueBeam and Halcyon accelerators, the 95th percentile error and maximum RMS error of 1 and 0.5 mm, respectively, are considered appropriate.

Influence of maximum MLC leaf speed on the quality of volumetric modulated arc therapy plans.

PurposeMaximum leaf speed is a configurable parameter of MLC in a treatment‐planning system. This study investigated the influence of MLC on the quality of VMAT plans.MethodsSeven MLCs with different maximum leaf speeds (1.0, 1.5, 2.25, 3.5, 5.0, 7.5, and 10 cm/s) were configured for an accelerator in treatment‐planning system. Correspondingly, seven treatment plans, with the identical initial optimization parameter, were designed with the mdaccAutoPlan system. Six nasopharyngeal carcinoma (NPC) patients and nine rectal cancer patients were selected, representing complex and simple clinical circumstances. VMAT plan quality was evaluated with PlanIQTM software. The results were statistically analyzed with a one‐way analysis of variance (ANOVA) and pairwise comparison tests.ResultsThe relative changes of plan scores achieved by the seven configured accelerators, with specific maximum MLC leaf speed (MMLS) for each patient, were studied. Two apparent trends of MMLS influence on VMAT plan scores were observed: Plan scores increased with MMLS; Plan scores increased rapidly when MMLS increased from 1 to 3.5, thus the relative change of plan score decreased in this MMLS range. The stationary point of maximum MLC speed (MMSSP) is defined, for the specific MMLS when the relative changes of plan scores is first <5%, as MMLS increases from 1.0 to 10. For rectal plans, MMSSPs were 2.25 for six patients and 3.5 for the other three patients. For NPC plans, MMSSPs were 3.5 for five patients and 2.25 for one patient.ConclusionThis work indicates that MMLS directly influences VMAT plan quality in NPC cases and rectal cancer cases. VMAT plan quality improved conspicuously as MMLS increased from 1 to 3.5, VMAT plan quality with marginal improvement when MMLS is above 3.5.

Potential improvements of lung and prostate MLC tracking investigated by treatment simulations.

Intrafraction tumor motion during external radiotherapy is a challenge for the treatment accuracy. A novel technique to mitigate the impact of tumor motion is real-time adaptation of the multileaf collimator (MLC) aperture to the motion, also known as MLC tracking. Although MLC tracking improves the dosimetric accuracy, there are still residual errors. Here, we investigate and rank the performance of five prediction algorithms and seven improvements of an MLC tracking system by extensive tracking treatment simulations. An in-house-developed MLC tracking simulator that has been experimentally validated against an electromagnetic-guided MLC tracking system was used to test the prediction algorithms and tracking system improvements. The simulator requires a Dicom treatment plan and a motion trajectory as input and outputs all motion of the accelerator during MLC tracking treatment delivery. For lung tumors, MLC tracking treatments were simulated with a low and a high modulation VMAT plan using 99 patient-measured lung tumor trajectories. For prostate, tracking was also simulated with a low and a high modulation VMAT plan, but with 695 prostate trajectories. For each simulated treatment, the tracking error was quantified as the mean MLC exposure error, which is the sum of the overexposed area (irradiated area that should have been shielded according to the treatment plan) and the underexposed area (shielded area that should have been irradiated). First, MLC tracking was simulated with the current MLC tracking system without prediction, with perfect prediction (Perfect), and with the following five prediction algorithms: linear Kalman filter (Kalman), kernel density estimation (KDE), linear adaptive filtering (LAF), wavelet-based multiscale autoregression (wLMS), and time variant seasonal autoregression (TVSAR). Next, MLC tracking was simulated using the best prediction algorithm and seven different tracking system improvements: no localization signal latency (a), doubled maximum MLC leaf speed (b), halved MLC leaf width (c), use of Y backup jaws to track motion perpendicular to the MLC leaves (d), dynamic collimator rotation for alignment of the MLC leaves with the dominant target motion direction (e), improvements 4 and 5 combined (f), and all improvements combined (g). All results are presented as the mean residual MLC exposure error compared to no tracking. In the prediction study, the residual MLC exposure error was 47.0% (no prediction), 45.1% (Kalman), 43.8% (KDE), 43.7% (LAF), 42.1% (wLMS), 40.1% (TVSAR), and 36.5% (Perfect) for lung MLC tracking. For prostate MLC tracking, it was 66.0% (no prediction), 66.9% (Kalman), and 63.4% (Perfect). For lung with TVSAR prediction, the residual MLC exposure error for the seven tracking system improvements was 37.2%(1), 38.3%(2), 37.4%(3), 34.2%(4), 30.6%(5), 27.7%(6), and 20.7%(7). For prostate with no prediction, the residual MLC exposure error was 61.7%(1), 61.4%(2), 55.4%(3), 57.2%(4), 47.5%(5), 43.7%(6), and 38.7%(7). For prostate, MLC tracking was slightly better without prediction than with linear Kalman filter prediction. For lung, the TVSAR prediction algorithm performed best. Dynamic alignment of the collimator with the dominant motion axis was the most efficient MLC tracking improvement except for lung tracking with the low modulation VMAT plan, where jaw tracking was slightly better.

Evaluation of MLC performance in VMAT and dynamic IMRT by log file analysis

PurposeThis multi-institution study assessed the positioning accuracy of multileaf collimators (MLC) by analyzing log files. It determined the main machine parameters that affect MLC positioning errors for pre-TrueBeam (Clinac) and TrueBeam linacs. MethodsAround 30,000 dIMRT and VMAT log files belonging to 6 linacs from 4 different centers were analyzed. An in-house software was used to calculate 95th percentile and RMS error values and their correlation with certain parameters such as maximum leaf speed, mean leaf speed and gantry angle. The effect of MLC communication delay on error statistics was assessed in Clinac linacs. To that end MLC positioning error statistics were calculated with and without the delay effect. ResultsFor dIMRT treatments in Clinac linacs the mean leaf RMS error was 0.306mm with and 0.030mm without the delay effect. Leaf RMS error was closely linked to maximum and mean leaf speeds, but without the delay effect that link was weaker. No trend was observed between bank RMS error and gantry angle. Without the delay effect larger bank RMS errors were obtained for gantry angles with leaf movements against gravity. For VMAT treatments in TrueBeam linacs the mean leaf RMS error was 0.038mm. A link was also observed between leaf RMS error and maximum and mean leaf speeds. ConclusionTrueBeam MLC positioning errors are substantially lower than those of Clinac linacs. In Clinac machines the analysis of dynalogs without the delay effect allows us to study the influence of factors that are masked by the delay effect.

SU-F-T-333: Deliverability Considerations in Modulated Photon Radiotherapy (XMRT)

Purpose: Modulated Photon Radiotherapy (XMRT), which simultaneously optimizes photon beamlet energy (6 and 18 MV) and fluence, has shown dosimetric improvements for fluence map optimization (FMO) when compared to conventional single-energy intensity modulated radiotherapy. However, generating deliverable multi-leaf collimator (MLC) sequences for XMRT needs to be explored. Therefore, two problems were investigated: 1) The ability to generate MLC-sequenced fluence maps from FMO XMRT solutions for a prostate case 2) The impact of fluence smoothening constraints imposed in the FMO on the deliverability and dose distribution. Methods: XMRT FMO solutions for a clinical prostate case employing standard dosimetric constraints, prescriptions, and a seven coplanar beam arrangement were generated. Smoothening constraints in the FMO utilized a sum of positive gradients approach. Sequenced maps were generated using an in-house optimization algorithm (MLCSO). The maximum leaf speed, minimum leaf separation, and transmission through MLC leaves were set to 2.5 mm/s, 1 mm, and 1%, respectively. The resulting sequenced maps for each field were compared with the original FMO solutions through gamma analysis (0.5%/0.5 mm) and root mean square error (RMSE). This comparison was done for both the smoothed and unsmoothed XMRT solutions. Results: Average RMSE and gamma agreement of 0.44, 93%and 0.36, 95% were obtained for unsmoothed 6 and 18 MV contributions from XMRT sequenced maps. The sequenced maps with smoothening constraints had better agreement with their respective optimal fluences, with RMSEs of 0 and gamma pass rates of 100% for all comparisons. This improved smoothening led to increased dose to critical structures (rectum, bladder, and femoral heads); however solutions were still clinically acceptable. Conclusion: For a clinical prostate case, XMRT FMO fluence maps were suitable for conversion into deliverable MLC sequences. Imposing smoothening constraints during FMO resulted in improved sequenced maps, though with a slight increase in dose to the critical structures.

WE‐AB‐209‐10: Optimizing the Delivery of Sequential Fluence Maps for Efficient VMAT Delivery

Purpose:To develop an optimization model and solution approach for computing MLC leaf trajectories and dose rates for high quality matching of a set of optimized fluence maps to be delivered sequentially around a patient in a VMAT treatment.Methods:We formulate the fluence map matching problem as a nonlinear optimization problem where time is discretized but dose rates and leaf positions are continuous variables. For a given allotted time, which is allocated across the fluence maps based on the complexity of each fluence map, the optimization problem searches for the best leaf trajectories and dose rates such that the original fluence maps are closely recreated. Constraints include maximum leaf speed, maximum dose rate, and leaf collision avoidance, as well as the constraint that the ending leaf positions for one map are the starting leaf positions for the next map. The resulting model is non‐convex but smooth, and therefore we solve it by local searches from a variety of starting positions. We improve solution time by a custom decomposition approach which allows us to decouple the rows of the fluence maps and solve each leaf pair individually. This decomposition also makes the problem easily parallelized.Results:We demonstrate method on a prostate case and a head‐and‐neck case and show that one can recreate fluence maps to high degree of fidelity in modest total delivery time (minutes).Conclusion:We present a VMAT sequencing method that reproduces optimal fluence maps by searching over a vast number of possible leaf trajectories. By varying the total allotted time given, this approach is the first of its kind to allow users to produce VMAT solutions that span the range of wide‐field coarse VMAT deliveries to narrow‐field high‐MU sliding window‐like approaches.

Performance evaluation of a high-speed multileaf collimator in real-time IMRT delivery to moving targets.

Multileaf collimator (MLC) tracking can be used for motion management. However, on account of mechanical constraints, it is a crucial challenge for conventional MLCs (3-4 cm/s in leaf speed) to track fast targets, especially moving in 2D in the beam's eye view (BEV). Our group has recently developed a "high-speed" MLC (HS-MLC) prototype with a maximum leaf speed of 40 cm/s, which makes it possible to track the vast majority of moving targets without violation of mechanical constraints. The major innovation of the HS-MLC design is that it employs linear motors instead of rotary motors to drive leaves. This paper mainly aims to evaluate the performance of the HS-MLC in real-time intensity-modulated radiation therapy delivery to targets moving in 2D in the BEV. A 2D real-time tracking algorithm was proposed first based on a previous superimposing leaf sequencing method. Then, simulations were performed to evaluate the delivery performance including fluence accuracy, efficiency, delivery time, and number of monitor units under various settings of limiting coefficient and dose rate for four clinical fluence maps and two target speeds. The comparisons between the HS-MLC with a "medium-speed" MLC (MS-MLC, 10 cm/s) and a "low-speed" MLC (LS-MLC, 5 cm/s) were also made. For validation, experiments were carried out on the HS-MLC prototype in the lab environment. A camera-based measurement system was set up to detect actual leaf trajectories. Simulation results indicate that a limiting coefficient of 0.5 and a dose rate of 400 MU/min are "optimal" in the sense of getting best compromise between delivery time and number of monitor units. Under the optimal parameters, the HS-MLC achieved 100% in efficiency, 18.1 s in delivery time, and 121.2 MU in number of monitor units on average for the "fast" target speed, compared to 94%, 20.6 s, and 129.9 MU with the MS-MLC, and to 53%, 40.2 s, and 141.1 MU with the LS-MLC. The benefits of increased leaf speed were demonstrated. The experimental results agreed with the simulation ones, which further confirmed the efficacy of the HS-MLC. The HS-MLC is superior to conventional MLCs when used for tracking, benefiting from its high leaf speed. These results indicate that the novel HS-MLC is feasible for high-accuracy and high-efficiency motion management. It also offers guidance for future MLC design.

SU‐E‐T‐444: Gravity Effect On Maximum Leaf Speed in Dynamic IMRT Treatments

Purpose:A leaf sequencing algorithm has been recently developed in our department. Our purpose is to utilize this algorithm to reduce treatment time by studying the feasibility of using several maximum leaf speeds depending on gantry angle and leaf thickness (0.5 or 1 cm at isocenter). To do so, the gravity effect on MLC performance has been examined by means of analysing the dynalog files.Methods:Leaf position errors has been ascertained according to gantry angle and leaf speed in MLC Millenium120 (Varian). In order to do this, the following test has been designed: all leaves move in synchrony, with same speed and 1 cm gap between opposite leaves. This test is implemented for 18 different speeds: 0.25‐0.5‐0.75‐1‐1.25‐1.5‐1.75‐2‐2.1‐2.2‐2.3‐2.4‐2.5‐2.6‐2.7‐2.8‐2.9‐3.0 cm/s and 8 gantry angles: 0‐45‐90‐135‐180‐225‐270‐315. Collimator angle is 2 degrees in all cases since it is the most usual one in IMRT treatments in our department. Dynamic tolerance is 2 mm. Dynalogs files of 10 repetitions of the test are analysed with a Mathlab in‐house developed software and RMS error and 95th percentiles are calculated. Varian recommends 2.5 cm/s as the maximum leaf speed for its segmentation algorithm. In our case, we accept this speed in the most restrictive situation: gantry angle 270 and 1 cm leaf thickness. Maximum speeds for the rest of the cases are calculated by keeping the difference between 95th percentile and dynamic tolerance. In this way, beam hold‐off probability does not increase.Results:Maximum speeds every 45 degrees of gantry rotation have been calculated for both leaf thickness. These results are 2.9‐2.9‐2.9‐2.9‐2.7‐2.6‐2.6‐2.7 cm/s for 0.5 cm leaf thickness and 2.7‐2.7‐2.7‐2.7‐2.6‐2.5‐2.5‐2.6 cm/s for 1 cm leaf thickness.Conclusion:Gravity effect on MLC positioning has been studied. Maximum leaf speed according to leaf thickness and gantry angle have been calculated which reduces treatment time.

SU‐E‐T‐360: End‐To‐End Dosimetric Testing of a Versa HD Linear Accelerator with the Agility Head Modeled in Pinnacle3

Purpose:The Versa HD incorporates a variety of upgrades, primarily including the Agility head. The distinct dosimetric properties of the head from its predecessors combined with flattening‐filter‐free (FFF) beams require a new investigation of modeling in planning systems and verification of modeling accuracy.Methods:A model was created in Pinnacle3 v9.8 with commissioned beam data. Leaf transmission was modeled as &lt;0.5% with maximum leaf speed of 3 cm/s. Photon spectra were tuned for FFF beams, for which profiles were modeled with arbitrary profiles rather than with cones. For verification, a variety of plans with varied parameters were devised, and point dose measurements were compared to calculated values. A phantom of several plastic water and Styrofoam slabs was scanned and imported into Pinnacle3. Beams of different field sizes, SSD, wedges, and gantry angles were created. All available photon energies (6 MV, 10 MV, 18 MV, 6 FFF, 10 FFF) as well four clinical electron energies (6, 9, 12, and 15 MeV) were investigated. The plans were verified at a calculation point (8 cm deep for photons, variable for electrons) by measurement with a PTW Semiflex ionization chamber. In addition, IMRT testing was performed with three standard plans (step and shoot IMRT, small and large field VMAT plans). The plans were delivered on the Delta4 IMRT QA phantom (ScandiDos, Uppsala, Sweden).Results:Homogeneous point dose measurement agreed within 2% for all photon and electron beams. Open field photon measurements along the central axis at 100 cm SSD passed within 1%. Gamma passing rates were &gt;99.5% for all plans with a 3%/3mm tolerance criteria. The IMRT QA results for the first 23 patients yielded gamma passing rates of 97.4±2.3%.Conclusion:The end‐to‐end testing ensured confidence in the ability of Pinnacle3 to model photon and electron beams with the Agility head.

SU-E-T-610: Comparison of Treatment Times Between the MLCi and Agility Multileaf Collimators

Purpose: The Agility is a new 160-leaf MLC developed by Elekta for use in their Infinity and Versa HD linacs. As compared to the MLCi, the Agility increased the maximum leaf speed from 2 cm/s to 3.5 cm/s, and the maximum primary collimator speed from 1.5 cm/s to 9.0 cm/s. The purpose of this study was to determine if the Agility MLC resulted in improved plan quality and/or shorter treatment times. Methods: An Elekta Infinity that was originally equipped with a 80 leaf MLCi was upgraded to an 160 leaf Agility. Treatment plan quality was evaluated using the Pinnacle planning system with SmartArc. Optimization was performed once for the MLCi and once for the Agility beam models using the same optimization parameters and the same number of iterations. Patient treatment times were measured for all IMRT, VMAT, and SBRT patients treated on the Infinity with the MLCi and Agility MLCs. Treatment times were extracted from the EMR and measured from when the patient first walked into the treatment room until exiting the treatment room. Results: 11,380 delivery times were measured for patients treated with the MLCi, and 1,827 measurements have been made for the Agility MLC. The average treatment timesmore » were 19.1 minutes for the MLCi and 20.8 minutes for the Agility. Using a t-test analysis, there was no difference between the two groups (t = 0.22). The dose differences between patients planned with the MLCi and the Agility MLC were minimal. For example, the dose difference for the PTV, GTV, and cord for a head and neck patient planned using Pinnacle were effectively equivalent. However, the dose to the parotid glands was slightly worse with the Agility MLC. Conclusion: There was no statistical difference in treatment time, or any significant dosimetric difference between the Agility MLC and the MLCi.« less

Direct leaf trajectory optimization for volumetric modulated arc therapy planning with sliding window delivery

The authors propose a novel optimization model for volumetric modulated arc therapy (VMAT) planning that directly optimizes deliverable leaf trajectories in the treatment plan optimization problem, and eliminates the need for a separate arc-sequencing step. In this model, a 360° arc is divided into a given number of arc segments in which the leaves move unidirectionally. This facilitates an algorithm that determines the optimal piecewise linear leaf trajectories for each arc segment, which are deliverable in a given treatment time. Multileaf collimator constraints, including maximum leaf speed and interdigitation, are accounted for explicitly. The algorithm is customized to allow for VMAT delivery using constant gantry speed and dose rate, however, the algorithm generalizes to variable gantry speed if beneficial. The authors demonstrate the method for three different tumor sites: a head-and-neck case, a prostate case, and a paraspinal case. The authors first obtain a reference plan for intensity modulated radiotherapy (IMRT) using fluence map optimization and 20 intensity-modulated fields in equally spaced beam directions, which is beyond the standard of care. Modeling the typical clinical setup for the treatment sites considered, IMRT plans using seven or nine beams are also computed. Subsequently, VMAT plans are optimized by dividing the 360° arc into 20 corresponding arc segments. Assuming typical machine parameters (a dose rate of 600 MU/min, and a maximum leaf speed of 3 cm/s), it is demonstrated that the optimized VMAT plans with 2-3 min delivery time are of noticeably better quality than the 7-9 beam IMRT plans. The VMAT plan quality approaches the quality of the 20-beam IMRT benchmark plan for delivery times between 3 and 4 min. The results indicate that high quality treatments can be delivered in a single arc with 20 arc segments if sufficient time is allowed for modulation in each segment.

TU‐E‐108‐07: A Multi‐Institutional Evaluation of Multileaf Collimator Performance

Purpose: This study examined MLC positional accuracy via MLC logs from multiple institutions and multiple delivery techniques to evaluate typical positional accuracy and treatment and mechanical parameters that affect accuracy. Typical accuracy achieved was compared against TG‐142 recommendations for MLC performance; more appropriate recommendations are suggested. Methods: Over 85,000 Varian MLC treatment logs were collected from six institutions and analyzed with Fraction CHECK. Data were binned according to institution and treatment type to determine root mean square (RMS) and 95th percentile error values, and then to look for correlations between those errors and with mechanical and treatment parameters. Results: Results of treatment logs from six institutions found that leaf RMS error and 95th percentile leaf error were fairly consistent between institutions, but varied by treatment type. The step and shoot technique had very small errors: the mean RMS leaf error was 0.008 mm. For dynamic treatments the mean RMS leaf error was 0.32 mm, while VMAT showed the largest mean RMS leaf error at 0.46 mm. For the dynamic and VMAT techniques, the mean and maximum leaf speeds were significantly linked to the leaf RMS error. For dynamic delivery, the mean leaf error was correlated with RMS error, whereas for VMAT the average gantry speed was correlated. For all treatments, the RMS error and the 95th percentile leaf error were correlated. Conclusion: Restricting the maximum leaf speed can help improve MLC performance for dynamic and VMAT deliveries. Furthermore, the tolerances of leaf RMS and error counts for all treatment types should be tightened from the TG‐142 values to make them more appropriate for clinical performance. Values of 1 mm for the 95th percentile of leaf RMS error and 1.5 mm for the 95th percentile error are suggested as action levels for all treatment types.

Beam modeling and VMAT performance with the Agility 160‐leaf multileaf collimator

The Agility multileaf collimator (Elekta AB, Stockholm, Sweden) has 160 leaves of projected width 0.5 cm at the isocenter, with maximum leaf speed 3.5cms−1. These characteristics promise to facilitate fast and accurate delivery of radiotherapy, particularly volumetric‐modulated arc therapy (VMAT). The aim of this study is therefore to create a beam model for the Pinnacle3 treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI), and to use this beam model to explore the performance of the Agility MLC in delivery of VMAT. A 6 MV beam model was created and verified by measuring doses under irregularly shaped fields. VMAT treatment plans for five typical head‐and‐neck patients were created using the beam model and delivered using both binned and continuously variable dose rate (CVDR). Results were compared with those for an MLCi unit without CVDR. The beam model has similar parameters to those of an MLCi model, with interleaf leakage of only 0.2%. The verification of irregular fields shows a mean agreement between measured and planned dose of 1.3% (planned dose higher). The Agility VMAT head‐and‐neck plans show equivalent plan quality and delivery accuracy to those for an MLCi unit, with 95% of verification measurements within 3% and 3 mm of planned dose. Mean delivery time is 133 s with the Agility head and CVDR, 171 s without CVDR, and 282 s with an MLCi unit. Pinnacle3 has therefore been shown to model the Agility MLC accurately, and to provide accurate VMAT treatment plans which can be delivered significantly faster with Agility than with an MLCi.PACS number: 87.55kd, 87.55km, 87.56bd, 87.56jk

TU‐G‐BRC‐09: A Comparison of Step‐And‐Shoot and DMLC IMRT‐Delivery for Real‐Time Tumor Tracking

Purpose: The dose distributions of static and dynamically delivered IMRT fields for real‐time target tracking were evaluated to determine which methods work best for the compensation of breathing‐induced organ motion.Methods: A single IMRT beam of a clinical plan was applied with the step‐and‐shoot delivery technique and the sliding‐window technique with the research Siemens 160MLC in order to account for a two‐dimensional, sinusoidal target motion and for a real patient breathing‐curve. For the sliding‐window dose delivery, two different maximum leaf speeds, as well as synchronized and non‐synchronized leaf trajectories were employed. All fields were delivered without and with tracking of the target using three electromagnetic transponders of the Calypso‐System. The dose distributions were measured with radio‐chromic films in a water equivalent phantom in 5cm depth and were compared to the step‐and‐shoot delivery for a non‐moving phantom. For the dose comparison a gamma criterion of 2%/2mm was utilized.Results: For both motions tracking improved the gamma‐index by 13%–32%. For the dynamic delivery no significant advantage was observed between unsynchronized and synchronized leaf trajectories. Moreover no remarkable differences were found by comparing results obtained with different maximum leaf speeds. For both target motions the step‐and‐shoot delivery shows a better dose distribution using the tracking approach: For the 2D‐motion a gamma‐passing rate of 86.3% can be achieved using step‐and‐shoot compared to 78.2% on average for the dynamic dose delivery. For the patient breathing‐motion 83.3% compared to an average of 77.1% were obtained. When comparing the results of both motion patterns, slightly higher passing rates were found with the periodic 2D‐sinusoidal motion. Conclusions: Only small differences were observed for different IMRT dose delivery modes with organ motion tracking, except for the tracked step‐and‐shoot delivery. However, more experiments have to be performed to investigate other IMRT plans and explicit dependencies on the field modulation.Supported by Siemens Healthcare/CR

Real time 4D IMRT treatment planning based on a dynamic virtual patient model: Proof of concept

To develop a novel four-dimensional (4D) intensity modulated radiation therapy (IMRT) treatment planning methodology based on dynamic virtual patient models. The 4D model-based planning (4DMP) is a predictive tracking method which consists of two main steps: (1) predicting the 3D deformable motion of the target and critical structures as a function of time during treatment delivery; (2) adjusting the delivery beam apertures formed by the dynamic multi-leaf collimators (DMLC) to account for the motion. The key feature of 4DMP is the application of a dynamic virtual patient model in motion prediction, treatment beam adjustment, and dose calculation. A lung case was chosen to demonstrate the feasibility of the 4DMP. For the lung case, a dynamic virtual patient model (4D model) was first developed based on the patient's 4DCT images. The 4D model was capable of simulating respiratory motion of different patterns. A model-based registration method was then applied to convert the 4D model into a set of deformation maps and 4DCT images for dosimetric purposes. Based on the 4D model, 4DMP treatment plans with different respiratory motion scenarios were developed. The quality of 4DMP plans was then compared with two other commonly used 4D planning methods: maximum intensity projection (MIP) and planning on individual phases (IP). Under regular periodic motion, 4DMP offered similar target coverage as MIP with much better normal tissue sparing. At breathing amplitude of 2 cm, the lung V20 was 23.9% for a MIP plan and 16.7% for a 4DMP plan. The plan quality was comparable between 4DMP and IP: PTV V97 was 93.8% for the IP plan and 93.6% for the 4DMP plan. Lung V20 of the 4DMP plan was 2.1% lower than that of the IP plan and Dmax to cord was 2.2 Gy higher. Under a real time irregular breathing pattern, 4DMP had the best plan quality. PTV V97 was 90.4% for a MIP plan, 88.6% for an IP plan and 94.1% for a 4DMP plan. Lung V20 was 20.1% for the MIP plan, 17.8% for the IP plan and 17.5% for the 4DMP plan. The deliverability of the real time 4DMP plan was proved by calculating the maximum leaf speed of the DMLC. The 4D model-based planning, which applies dynamic virtual patient models in IMRT treatment planning, can account for the real time deformable motion of the tumor under different breathing conditions. Under regular motion, the quality of 4DMP plans was comparable with IP and superior to MIP. Under realistic motion in which breathing amplitude and period change, 4DMP gave the best plan quality of the three 4D treatment planning techniques.

Analysis of RapidArc optimization strategies using objective function values and dose‐volume histograms

RapidArc is a novel treatment planning and delivery system that has recently been made available for clinical use. Included within the Eclipse treatment planning system are a number of different optimization strategies that can be employed to improve the quality of the final treatment plan. The purpose of this study is to systematically assess three categories of strategies for four phantoms, and then apply proven strategies to clinical head and neck cases. Four phantoms were created within Eclipse with varying shapes and locations for the planning target volumes and organs at risk. A baseline optimization consisting of a single 359.8° arc with collimator at 45° was applied to all phantoms. Three categories of strategies were assessed and compared to the baseline strategy. They include changing the initialization parameters, increasing the total number of control points, and increasing the total optimization time. Optimization log files were extracted from the treatment planning system along with final dose‐volume histograms for plan assessment. Treatment plans were also generated for four head and neck patients to determine whether the results for phantom plans can be extended to clinical plans. The strategies that resulted in a significant difference from baseline were: changing the maximum leaf speed prior to optimization (p<0.05), increasing the total number of segments by adding an arc (p<0.05), and increasing the total optimization time by either continuing the optimization (p<0.01) or adding time to the optimization by pausing the optimization (p<0.01). The reductions in objective function values correlated with improvements in the dose‐volume histogram (DVH). The addition of arcs and pausing strategies were applied to head and neck cancer cases, which demonstrated similar benefits with respect to the final objective function value and DVH. Analysis of the optimization log files is a useful way to intercompare treatment plans that have the same dose‐volume objectives and importance values. The results for clinical head and neck plans were consistent with phantom plans.PACS number: 87.55.x, 87.55.D, 87.55.de 87.55.dk

Overestimation of Low-Dose Radiation in Intensity-Modulated Radiotherapy with Sliding-Window Technique

To analyze different control-system limitations on the measured dose distributions in low-dose regions of simplified intensity fields with an electronic portal imaging device to ascertain the optimal settings for the control-system limitations in the planning system. The authors created one field with an "optimal fluence" of intensity 1.0 (full dose) and one field with intensity 0.0 (no dose) in the central part of the field. The influence of different dose rates (DRs) and maximum leaf speeds (LS) on the calculated and measured dose and dose profiles were analyzed. Good agreement between calculated and measured dose in the case of a field of intensity 1.0 was found. For the field with intensity 0.0, the measured dose was 20-60% lower than the dose calculated by the "actual fluence". The results were found dependent on the DR and LS. The overestimation in regions of optimal intensity 0.0 by the planning system cannot be resolved by the user. Taking the measured dose in the region of desired intensity 1.0 and other technical limitations (like beam hold interrupts or spikes in the cross and longitudinal profiles) into consideration, the application of an LS of 2.5 cm/s and a DR of 500 MU/min is recommended in order to minimize radiation dose applied to organs at risk, which are located in regions of low intensity, like, for example, the spinal cord.

MO‐D‐BRB‐05: A Novel Dynamic MLC Leaf Sequencing Algorithm for 4D Treatment with Deformable Target Motion Correction

Purpose: To investigate a dynamic MLC (DMLC) leaf sequencing method for the 4D IMRT delivery. Materials and methods: A 4D CT was obtained and 10 phase CT data sets were created retrospectively. Individual IMRT plan was generated for each phase on Pinnacle treatment planning station, and a 4D plan was created using a deformable image registration technique. Based on the DMLC leaf sequences generated from the plan of each phase, the non‐rigid motion corrected 4D leaf sequence was created by matching the time indexes of the leaf sequences and the breathing cycles of each phase. The 4D leaf sequence was iteratively adjusted for the maximum leaf speed to avoid any beam hold‐offs. At the same time, a rigid motion corrected 4D leaf sequence was calculated by adjusting the leaf center corresponding to the center of mass position of the target in each phase. The opening density map (ODM) of the 4D plan was used to create a traditional no‐motion corrected leaf sequence. All three leaf sequences were delivered by Varian 2100 C/D with Millennium 120 leaf DMLC. The MatriXX (IBA Dosimetry) was used to measure the delivered fluence map every 40 ms. The delivered fluence maps were registered back to the patient's coordinate system. Results: Using the 4D plan ODM as reference, the fluence map from the non‐rigid motion corrected 4D leaf sequence showed a maximum dosimetric difference of 3%. However, the fluence maps from the rigid motion corrected 4D leaf sequence and the no‐motion corrected leaf sequence showed maximum dosimetric differences of 30%. Conclusion: For 4D plan DMLC delivery, the non‐rigid motion corrected leaf sequencing, the rigid motion corrected 4D leaf sequencing and the no‐motion corrected leaf sequencing were investigated. Results showed that the non‐rigid motion corrected leaf sequencing is the one that closest deliverers the 4D plan.

MO‐EE‐A1‐05: Direct Aperture Optimization of Intensity Modulated Arc Therapy

Purpose: Intensity modulated radiation therapy has been a very popular and effective treatment technique for the treatment of prostate, head &amp; neck and liver etc. Meanwhile, another innovative treatment technique, intensity modulated arc therapy, was developed to complement some drawbacks of IMRT like long treatment time and low MU efficiency. Since the IMAT completes the treatment just within one or two rotations, it is not easy to get optimized leaf sequences in a deterministic way. In this study, we tried to get optimized IMAT treatment plan by genetic algorithm. Method and Materials: First, the start/end positions of MLC leaves at each rotation angle with 10° interval were selected as optimization variables and encoded into genetic chromosomes. They experience genetic operations such as generation, selection, crossover, mutation and reproduction and the most optimized solution remains in the end of iteration. The constraint of maximum leaf speed was included in these operations. And the fitness of each population was evaluated by DVH volume constraint based objective function. IMAT dose distribution was calculated as a weighted sum of MLC shape at each angle and related Dij matrix similar to IMRT dose calculation. The algorithm was implemented in our treatment planning system and the dose distributions and DVHs of single and double gantry rotation cases were compared. Results: IMAT plan gave comparable results with conventional IMRT even with single gantry rotation and there was not significant improvement in double gantry rotations. Genetic algorithm required about 3,000 generations to reach optimized value due to its stochastic nature. Conclusion: It was possible to optimize IMAT plan with genetic algorithm and the results are optimized MLC leaf sequences readily deliverable in general linear accelerators. It can be an efficient method to solve IMAT optimization problem despite of relative slow convergence.

TU-EE-A1-03: Multileaf Collimator Performance and Quality Assurance For Volume Modulated Arc Therapy

Purpose: The purpose of this study was to determine the effects of Multi Leaf Collimator (MLC) leaf velocity on leaf positional accuracy during modulated arc delivery. Method and Materials: MLC performance was measured for intensity modulated arcs delivered on a Varian21EX equipped with a 120-leaf dynamic MLC. The performance of the MLC was tested using arc-based delivery sequences developed by the investigators for routine MLC leaf velocity testing. MLC performance was also tested using actual VMAT delivery sequences created with a commercially available inverse planning system. Errors in the MLC leaf positions were measured using the MLC controller, which recorded positions of each leaf every 50 milliseconds. To characterize the performance of MLC leaves we introduce the notion of MLC leaf performance curve, which is defined to be a dependency between the MLC leaf velocity and the expected leaf position error. Results: At leaf velocities below 2 cm/sec, the MLC leaves were within 2-mm of the planned position during 98% of the test sequences. The performance of the MLC leaves did not significantly vary with the MLC moving into or out of the carriage. However, there was a significant difference in the performance between the 5-mm and 1-cm MLC leaves. Large leaf velocities translate into large magnitudes of MLC leaf positional errors. The typical shape of the leaf performance curve is linear. Summary: The maximum allowable leaf speed for VMAT treatment delivery will depend on the leaf sequencing. If the intensity is modulated with small MLC leaf separations, then the leaf velocity will need to be limited to 1 cm/sec. Leaf position errors of greater than 1-mm can have a detrimental impact on the delivered dose distributions for small leaf pair separations. Analyzing the leaf performance curves estimated for each leaf one can easily reveal poorly performing MLC leaves.