Arc Delivery Research Articles

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

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

SU‐E‐T‐508: End to End Testing of a Prototype Eclipse Module for Planning Modulated Arc Therapy On the Siemens Platform

Purpose:The latest clinical implementation of the Siemens Artiste linac allows for delivery of modulated arcs (mARC) using full‐field flattening filter free (FFF) photon beams. The maximum doserate of 2000 MU/min is well suited for high dose treatments such as SBRT. We tested and report on the performance of a prototype Eclipse TPS module supporting mARC capability on the Artiste platform.Method:our spine SBRT patients originally treated with 12/13 field static‐gantry IMRT (SGIMRT) were chosen for this study. These plans were designed to satisfy RTOG0631 guidelines with a prescription of 16Gy in a single fraction. The cases were re‐planned as mARC plans in the prototype Eclipse module using the 7MV FFF beam and required to satisfy RTOG0631 requirements. All plans were transferred from Eclipse, delivered on a Siemens Artiste linac and dose‐validated using the Delta4 system.Results:All treatment plans were straightforwardly developed, in timely fashion, without challenge or inefficiency using the prototype module. Due to the limited number of segments in a single arc, mARC plans required 2‐3 full arcs to yield plan quality comparable to SGIMRT plans containing over 250 total segments. The average (3%/3mm) gamma pass‐rate for all arcs was 98.5±1.1%, thus demonstrating both excellent dose prediction by the AAA dose algorithm and excellent delivery fidelity. Mean delivery times for the mARC plans(10.5±1.7min) were 50‐70% lower than the SGIMRT plans(26±2min), with both delivered at 2000 MU/min.Conclusion:A prototype Eclipse module capable of planning for Burst Mode modulated arc delivery on the Artiste platform has been tested and found to perform efficiently and accurately for treatment plan development and delivered‐dose prediction. Further investigation of more treatment sites is being carried out and data will be presented.

TU‐D‐9A‐01: TG176: Dosimetric Effects of Couch Tops and Immobilization Devices

The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices are often overlooked. These devices also increase surface dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased surface dose compared to the ‘tennis racket’ style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations we make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of skin dose and attenuation, how to report these values, and for the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.Learning Objectives: What are the dosimetric effects of couch tops What are the dosimetric effects of immobilization devices How can one model couch tops in the treatment planning system How can one measure attenuation and surface dose changes due to external devices

SU-E-T-624: Quantitative Evaluation of 2D Versus 3D Dosimetry for Stereotactic Volumetric Modulated Arc Delivery Using COMPASS

Purpose: The purpose of this study was to evaluate quantitatively 2D versus 3D dosimetry for stereotactic volumetric modulated arc delivery using COMPASS with 2D array. Methods: Twenty-five patients CT images and RT structures of different sites like brain, head & neck, thorax, abdomen and spine were taken from Multiplan planning system for this study. All these patients underwent radical stereotactic treatment in Cyberknife. For each patient, linac based VMAT stereotactic plans were generated in Monaco TPS v 3.1 using Elekta Beam Modulator MLC. Dose prescription was in the range of 5-20Gy/fraction.TPS calculated VMAT plan delivery accuracy was quantitatively evaluated with COMPASS measured dose and calculated dose based on DVH metrics. In order to ascertain the potential of COMPASS 3D dosimetry for stereotactic plan delivery, 2D fluence verification was performed with MatriXX using Multicube. Results: For each site, D9 5 was achieved with 100% of prescription dose with maximum 0.05SD. Conformity index (CI) was observed closer to 1.15 in all cases. Maximum deviation of 2.62 % was observed for D9 5 when compared TPS versus COMPASS measured. Considerable deviations were observed in head and neck cases compare to other sites. The maximum mean and standard deviation for D9 5, average target dose and average gamma were -0.78±1.72, -1.10±1.373 and 0.39±0.086 respectively. Numbers of pixels passing 2D fluence verification were observed as a mean of 99.36% ±0.455 SD with 3% dose difference and 3mm DTA. For critical organs in head & neck cases, significant dose differences were observed in 3D dosimetry while the target doses were matched well within limit in both 2D and 3D dosimetry. Conclusion: The quantitative evaluations of 2D versus 3D dosimetry for stereotactic volumetric modulated plans showed the potential of highlighting the delivery errors. This study reveals that COMPASS 3D dosimetry is an effective tool for patient specific quality assurance compared to 2D fluence verification

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, e.g., accounting for temperature and pressure, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices is often overlooked. These devices also increase skin dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased skin dose compared to the "tennis racket" style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations the authors make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of surface dose and attenuation and how to report these values. For the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.

Inverse planning in the age of digital LINACs: station parameter optimized radiation therapy (SPORT)

The last few years have seen a number of technical and clinical advances which give rise to a need for innovations in dose optimization and delivery strategies. Technically, a new generation of digital linac has become available which offers features such as programmable motion between station parameters and high dose-rate Flattening Filter Free (FFF) beams. Current inverse planning methods are designed for traditional machines and cannot accommodate these features of new generation linacs without compromising either dose conformality and/or delivery efficiency. Furthermore, SBRT is becoming increasingly important, which elevates the need for more efficient delivery, improved dose distribution. Here we will give an overview of our recent work in SPORT designed to harness the digital linacs and highlight the essential components of SPORT. We will summarize the pros and cons of traditional beamlet-based optimization (BBO) and direct aperture optimization (DAO) and introduce a new type of algorithm, compressed sensing (CS)-based inverse planning, that is capable of automatically removing the redundant segments during optimization and providing a plan with high deliverability in the presence of a large number of station control points (potentially non-coplanar, non-isocentric, and even multi-isocenters). We show that CS-approach takes the interplay between planning and delivery into account and allows us to balance the dose optimality and delivery efficiency in a controlled way and, providing a viable framework to address various unmet demands of the new generation linacs. A few specific implementation strategies of SPORT in the forms of fixed-gantry and rotational arc delivery are also presented.

Improving radiotherapy planning, delivery accuracy, and normal tissue sparing using cutting edge technologies.

In the United States, more than half of all new invasive cancers diagnosed are non-small cell lung cancer, with a significant number of these cases presenting at locally advanced stages, resulting in about one-third of all cancer deaths. While the advent of stereotactic ablative radiation therapy (SABR, also known as stereotactic body radiotherapy, or SBRT) for early-staged patients has improved local tumor control to >90%, survival results for locally advanced stage lung cancer remain grim. Significant challenges exist in lung cancer radiation therapy including tumor motion, accurate dose calculation in low density media, limiting dose to nearby organs at risk, and changing anatomy over the treatment course. However, many recent technological advancements have been introduced that can meet these challenges, including four-dimensional computed tomography (4DCT) and volumetric cone-beam computed tomography (CBCT) to enable more accurate target definition and precise tumor localization during radiation, respectively. In addition, advances in dose calculation algorithms have allowed for more accurate dosimetry in heterogeneous media, and intensity modulated and arc delivery techniques can help spare organs at risk. New delivery approaches, such as tumor tracking and gating, offer additional potential for further reducing target margins. Image-guided adaptive radiation therapy (IGART) introduces the potential for individualized plan adaptation based on imaging feedback, including bulky residual disease, tumor progression, and physiological changes that occur during the treatment course. This review provides an overview of the current state of the art technology for lung cancer volume definition, treatment planning, localization, and treatment plan adaptation.

Simultaneous cone beam CT scanning during prostate radiotherapy to produce clinically useful images.

220 Background: Cone beam CT (CBCT) images can be acquired during delivery of rotational radiotherapy. We assessed the quality of simultaneous CBCTs captured during prostate radiotherapy and studied the effects of arc delivery time on image quality. Methods: Fifty patients with localised prostate cancer were treated with radical rotational radiotherapy. Treatment was delivered using a single eight or 10 MV arc on an Elekta Synergy linear accelerator. Standard (stCBCT) and simultaneous CBCTs (siCBCT) were captured on fractions 1, 6, 11, and 16. The siCBCT image was reconstructed and optimised using in-house software. The image quality was assessed by four observers using a validated scoring tool. The scoring clinician was blinded to whether the scan was stCBCT or siCBCT. Results: Seventy-four siCBCT scans were performed with a mean arc delivery time of 120 seconds during which 688 CBCT frames were acquired. Following a software upgrade the subsequent 122 siCBCTs were obtained with a mean arc delivery time of 83 seconds during which 502 CBCT frames were acquired. There was moderate agreement between the four observers regarding the quality of the CBCTs (Fleiss’ kappa statistic 0.51). StCBCTs were of good quality and the four observers agreed that 98 to 100% of these scans were of sufficient quality to be used to make a clinical decision. The four observers assessed that 35 to 79% of siCBCTs were of sufficient quality to be used to make clinical decisions. Before the software upgrade 65 to 93% of siCBCTs were judged to be of sufficient quality to make a clinical decision ("slow" simultaneous CBCT). Following the software upgrade this proportion fell to 16 to 69% ("fast" simultaneous CBCT). Conclusions: SiCBCT can produce images which are clinically useful. Image quality is affected by bowel gas, patient habitus and scatter from the MV beam. Reducing arc delivery time and the number of CBCT frames acquired by approximately one-third reduces the quality of siCBCTs. Further development and refinement of siCBCT scanning is warranted. Future reductions in arc delivery times may have to be limited if the use of siCBCT scanning becomes standard practice.

Late radiologic changes after stereotactic ablative radiotherapy for early stage lung cancer: A comparison of fixed-beam versus arc delivery techniques

Background and purposeTo characterize the radiologic changes occurring following arc stereotactic ablative radiotherapy (SABR) for early-stage non-small cell lung cancer relative to those following fixed-beam SABR. MethodsTwenty-nine patients treated with arc SABR without local recurrence and more than two years follow-up were retrospectively evaluated using a published scoring system. The late morphologic patterns, timing and severity of radiologic change were assessed and compared to 54 patients treated with fixed-beam SABR that we previously assessed using the same system. ResultsThe baseline characteristics and follow-up of both cohorts were well matched and SABR technique was not associated with morphologic differences before 6months (p=0.23). Thereafter the predicted probabilities of a modified-conventional pattern following arc and fixed-beam SABR were 96.3% vs. 68.9%, respectively (OR 11.7, 95% CI 3.38–40.8, p<0.001). In addition, at 1year follow-up the predicted probabilities of arc and fixed-beam SABR patients having expected or pronounced radiologic changes were 64.9% and 22.1%, respectively (OR=6.56, 95% CI: 3.13–13,7, p<0.001). ConclusionsPost-SABR radiologic changes differ with delivery technique, which has important implications during follow-up. Confirmation in larger studies is required and etiologic factors remain to be determined.

Negative margin technique — a novel planning strategy to improve dose conformation in SBRT using dynamic conformal arc delivery

The purpose of this study was to introduce a planning strategy for dynamic conformal arc therapy (DCAT), named negative margin technique (NMT), and evaluate its dosimetric gain in lung stereotactic body radiation therapy (SBRT). In DCAT, the field aperture is continuously conformed to the planning target volume (PTV) with an aperture margin (AM) to compensate for the penumbra effect with gantry rotation. It is a common belief the AM should be positive (or at least ‘zero'). However, the radial penumbra width becomes significantly wider because of continuously overlapped beams in arc delivery. Therefore, we hypothesize if the ‘negative margin’ is applied in the radial direction, it would improve the PTV dose conformation while reducing normal tissue dose. For verification, trial plans were made using the NMT and compared with ‘zero margin (ZM)’ plans for five lung SBRT cases representing different situations depending on the location of the PTV and organs at risk. All plans met 95% PTV coverage with the prescription dose and spared the spinal cord below the tolerance. Two conventional conformation indices (the ratio of prescription isodose volume to the PTV (CI100) and the ratio of 50% prescription isodose volume to the PTV (CI50)) and a modified conformation index were investigated. The maximum dose at 2 cm from the PTV (Dmax‐2cm) and the percent of lung volume receiving 20 Gy (V20) were also evaluated. Another planning simulation was performed with a total of ten randomly selected lung SBRT cases to mimic actual practice. In this simulation, optimization with ZM was first performed and further optimization using the NMT was processed for cases that could not meet a goal of CI100 = 1.2 with the ZM optimization. In all cases, both the CI100 and CI50 values were significantly reduced (overall, 9.4% ± 4.1% and 5.9% ± 3.1% for CI100 and CI50, respectively). The modified conformation index values also showed similar improvement (overall, 10.1% ± 5.7% increase). Reduction of Dmax‐2cm was also observed in all cases (4.5% ± 2.2%). V20 values decreased in all cases but one (5.7% ± 3.9%, excluding the increased case). In the random group simulation, it was possible to achieve the goal with just one NMT trial for five out of six cases that did not meet the goal in the ZM optimization. Interestingly, however, one case needed as many as six iterations to get the CI100 = 1.2 goal. The NMT turned out to be an effective planning strategy that could bring significant improvement of dose conformation. The NMT can be easily implemented in most clinics with no prerequisite.PACS number: 87.55.D‐

WE‐E‐141‐07: Automating Linac QA for Delivery and Analysis

Purpose: To assess the quantitative performance and reproducibility of a new generation linear accelerator using images and trajectory log files across institutions. Methods: A test suite was created to include tests recommended by TG142 (e.g, picket fence at cardinal static gantry angles and during VMAT) and TG179 (e.g. image quality). The test suite, distributed to a consortium of 7 institutions, consisted of DICOM‐RT files and a phantom with BBs at known locations. During each delivery, EPID images were acquired along with trajectory log files. Baseline for each irradiation was set with a flood field without the table or phantom. The phantom was then placed in position and the remaining tests were performed. An analysis program, created in Matlab, assessed the accuracy of leaf, jaw, and collimator positions utilizing the EPID images. Trajectory log files were analyzed as well to assess dynamic parameters such as the reproducibility of gantry motion during arc delivery. Results: Fifteen irradiations were performed on 5 accelerators. Leaf position reproducibility was 0.095 mm for a standard MLC and 0.110 mm for an HDMLC, with maximum standard deviations of 0.019, 0.053, and 0.002 mm for static, IMRT, and arc fields over all linacs. Trajectory logs were consistent with measurements. The maximum gantry deviation was 0.247 ± 0.0160 degrees. Using two different materials, the contrast‐to‐noise ratio was 1.43 ±0.740 and 7.41 ±0.24 for kV and MV images with kV CNR varying by more than a factor of 2 between different machines. Conclusion: EPID and trajectory logs demonstrated thresholds for detection of leaf position errors that were an order of magnitude less than TG142 requirements for different delivery types across institutions. Trajectory log files provided more detailed information regarding stability of gantry position. When tracked over time, these data can be used to reassess the frequency of different test types. This work is supported by Varian Medical Systems.

SU‐E‐J‐26: Intra‐Fraction Motion Management for Radiosurgical Treatments of Trigeminal Neuralgia: Clinical Experience and Motion Analysis

Purpose: To evaluate the patient positioning and intra‐fraction motion management performance of an image‐guidance protocol established for radiosurgical treatments of trigeminal neuralgia patients. Specifically, to analyze patient data for the evaluation of current motion tolerance levels utilized for repositioning. Methods: NovalisTx equipped with Exactrac is used for stereoscopically positioning and treatments. Treatments are delivered with 4mm collimator using 7 equally spaced arcs that are 20 degrees apart and span 100 arc degrees each. Following initial Exactrac positioning, CBCT is obtained for independent confirmation of patient position. Patient is then stereoscopically imaged prior to the delivery of each arc and repositioned when 0.5mm translational tolerance in any direction is exceeded. Data from 48 patients with 607 image pairs has been analyzed. Results: The mean magnitude of 3D deviation was 0.64mm±0.12mm (range 0.07–2.74mm). With the current 0.50mm tolerance level for repositioning, patients exceeded the tolerance 51.4% of the time considering only images following an arc delivery. For those instances, patients were repositioned with a mean magnitude of 0.85mm±0.15mm. With 0.75mm and 1.00mm tolerance limits, the tolerance would have been exceeded only 21.5% and 6.6% of the time, with a mean magnitude of 1.08mm±0.21mm and 1.34mm±0.24mm respectively. Conclusion: Current 0.50mm tolerance level results in a significant number of patient repositions that Result from actual patient motion convolved with the accuracy and precision of the image analysis. Relaxing the re‐positioning tolerance could more selectively correct for actual patient motion at the expense of more variations in patient position. A 0.75mm would decrease the repositioning rate by approximately a factor of two; however, the permissible magnitude of motion will increase, leading to possible dosimetric consequences. Once treatment begins, there was no trend as to when patients exceeded the tolerance. Future analysis will include quantitative dosimetric consequences due to intra‐fraction motion.

SU‐E‐T‐477: Study of MLC Apertures and Their Significance for Volumetric Arc Therapy in the Management of Non‐Small Cell Lung Cancer Using Stereotactic Body Radiotherapy

Purpose: Electronic disequilibrium concerns limit the minimum field sizes to 3.5 × 3.5 cm2 for NSCLC patients per various RTOG protocols which is easily achievable using 3DCRT. However, the inverse planning nature of VMAT does not provide any control on the field‐size or MLC shapes and may often violate it. This study aims to analyze the difference between VMAT and 3DCRT planning techniques by evaluating aperture size &amp; hence conformity with RTOG protocols. Methods: 25 patients with NSCLC [5 tumors per lobe] who were treated using 3DCRT were retrospectively inverse planned for VMAT. Dosimetrically, VMAT plans were almost identical with that of clinical 3DCRT plans. DICOM plans were imported into MATLAB to determine the actual MLC area of the active leaves per control point and its relative weight in the overall dose delivery of that VMAT arc. The effective‐weight of all the MLC areas which were less than 3.5 × 3.5 cm2 was summated (VMAT3.5fs). We also studied the conditions where the total MLC area did satisfy the minimum requirement of 3.5 × 3.5 cm2 but where any two opposite MLC leaf positions were less than 3.5 cm apart (VMAT3.5MLCwt). These evaluations are contained in VMAT3.5MLCwt which gives the weighted sum of all small MLC segments in the entire VMAT arc. A zero value of both ensures that the VMAT arc satisfies the RTOG requirement of minimal jaws size in its entirety. Results: Respective values of VMAT3.5fs &amp; VMAT3.5MLCwt were found to be 0.88 ± 0.23 &amp; 0.9 ± 0.21 for 63 VMAT arcs. Only 2 patients had a value of 0 for both indices implying compliance to RTOG field size limits. Conclusion: Proposed indices provide an objective method to evaluate the suitability of VMAT plans in satisfying RTOG constraints for NSCLC treatment using SBRT.

SU‐E‐T‐440: Analysis of Plan Quality with Different Penalties On Segment Width for Volumetric Modulated Arc Delivery of Stereotactic Body Radiotherapy

Purpose: To investigate the plan quality and delivery efficiency with different penalty on minimum segment width in volumetric modulated arc delivery for SBRT cases. Methods: Total of eighteen cases with tumors in lung, liver and abdomen were taken for the study. For each case, three VMAT SBRT plans were generated with different penalties on minimum segment width of 0.5, 1.0 and 1.5cm using CMS Monaco v3.1 TPS. Dose prescriptions were in the range of 7 to 20Gy per fraction. Each plan with different penalty was developed in a way to achieve same dose distribution and evaluated with dose indices(DIs), Conformity Index, Conformity Number and Gradient Index. Plan efficiency was analyzed in terms of monitor units(MUs) and delivery accuracy. The plan with 0.5cm of minimum segment width was used as a reference plan. All these plans were delivered using Elekta Beam Modulator having 4mm leaf width. The delivery accuracy was verified by measured fluence and point dose using Matrixx 2D detector array. Results: The difference between the reference and compared plan DIs were achieved within 2%. When compared with reference plan the total MUs was increased on average of 15.04±8.0% and 22.14±10.7% for 1.0cm and 1.5cm of minimum segment width plans respectively. Analysis of measured dose with TPS calculated showed a deviation of 3.1±1.9%, 2.9±1.2% and 2.7±0.8% for point dose verification and 95.9±2.8%, 96.5±2.6% and 97.8±1.6% for planar dose measurements with pass criteria of 2%/2mm for 0.5, 1.0 and 1.5cm of minimum segment width plans respectively. Conclusion: VMAT SBRT plans with increased value of minimum segment width shows a clear merit of better plan quality and delivery efficiency. Reducing the MUs by controlling redundant modulation for VMAT SBRT plans with optimal values of minimum segment width will increase the comfort for the patients with motion management system.

SU-E-T-384: Combining Multiple Arcs Into a Single Arc for Efficient Delivery

Purpose: Linear accelerators have introduced delivery control systems with the ability to deliver Volumetric Modulated Arc therapy (VMAT) either by a single arc or by multiple-arc. VMAT delivery for complex treatments such as head and neck requires providing more number of segments for a given plan. The primary goal of this study is to combine these multiple arcs into a single arc and compare the dose distribution and the treatment time for the two plans. Methods: Five Head and Neck plans and five prostate plans were considered for this study. VMAT plans were generated using Varian SmartArc treatment planning system version 8.5. A combined DICOM plan with single arc is created which has half the spacing as the original multiple arc plan and have all the segments of the multiple arc plan. For the combined arc plan and the multiple arc plans, the delivery time for optimal delivery under delivery constraints is determined using the delivery algorithm. A fluence map comparison between the combined arc and multiple arcs is made to prove the dosimetric similarity of the arc plans. Results: The delivery time for the Head & Neck plans for the combined arc is improved by 35% and for the prostate plan is improved by 37% as compared to the multiple arc plans. Difference fluence is calculated from the multiple arc fluence and the combined arc fluence for all the plans. There is less than 5% dosimetric difference between the multiple arc plans and the combined arc plans for both prostate and head and neck plans. Conclusion: Combined arc delivery provides comparable plan quality and improved treatment delivery time. Hence, for more complex sites combined arc delivery is preferable due to the improvement in the treatment time while maintaining similar dose distribution with that of the multiple arc plans.

Motion as a perturbation: Measurement-guided dose estimates to moving patient voxels during modulated arc deliveries

To present a framework for measurement-guided VMAT dose reconstruction to moving patient voxels from a known motion kernel and the static phantom data, and to validate this perturbation-based approach with the proof-of-principle experiments. As described previously, the VMAT 3D dose to a static patient can be estimated by applying a phantom measurement-guided perturbation to the treatment planning system (TPS)-calculated dose grid. The fraction dose to any voxel in the presence of motion, assuming the motion kernel is known, can be derived in a similar fashion by applying a measurement-guided motion perturbation. The dose to the diodes in a helical phantom is recorded at 50 ms intervals and is transformed into a series of time-resolved high-density volumetric dose grids. A moving voxel is propagated through this 4D dose space and the fraction dose to that voxel in the phantom is accumulated. The ratio of this motion-perturbed, reconstructed dose to the TPS dose in the phantom serves as a perturbation factor, applied to the TPS fraction dose to the similarly situated voxel in the patient. This approach was validated by the ion chamber and film measurements on four phantoms of different shape and structure: homogeneous and inhomogeneous cylinders, a homogeneous cube, and an anthropomorphic thoracic phantom. A 2D motion stage was used to simulate the motion. The stage position was synchronized with the beam start time with the respiratory gating simulator. The motion patterns were designed such that the motion speed was in the upper range of the expected tumor motion (1-1.4 cm∕s) and the range exceeded the normally observed limits (up to 5.7 cm). The conformal arc plans for X or Y motion (in the IEC 61217 coordinate system) consisted of manually created narrow (3 cm) rectangular strips moving in-phase (tracking) or phase-shifted by 90° (crossing) with respect to the phantom motion. The XY motion was tested with the computer-derived VMAT MLC sequences. For all phantoms and plans, time-resolved (10 Hz) ion chamber dose was collected. In addition, coronal (XY) films were exposed in the cube phantom to a VMAT beam with two different starting phases, and compared to the reconstructed motion-perturbed dose planes. For the X or Y motions with the moving strip and geometrical phantoms, the maximum difference between perturbation-reconstructed and ion chamber doses did not exceed 1.9%, and the average for any motion pattern∕starting phase did not exceed 1.3%. For the VMAT plans on the cubic and thoracic phantoms, one point exhibited a 3.5% error, while the remaining five were all within 1.1%. Across all the measurements (N = 22), the average disagreement was 0.5 ± 1.3% (1 SD). The films exhibited γ(3%∕3 mm) passing rates ≥90%. The dose to an arbitrary moving voxel in a patient can be estimated with acceptable accuracy for a VMAT delivery, by performing a single QA measurement with a cylindrical phantom and applying two consecutive perturbations to the TPS-calculated patient dose. The first one accounts for the differences between the planned and delivered static doses, while the second one corrects for the motion.

DASSIM-RT is likely to become the method of choice over conventional IMRT and VMAT for delivery of highly conformal radiotherapy

Lei Xing, Mark H. Phillips, and Colin G. Orton Citation: Medical Physics 40, 020601 (2013); doi: 10.1118/1.4773025 View online: http://dx.doi.org/10.1118/1.4773025 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/40/2?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Lung sparing and dose escalation in a robust-inspired IMRT planning method for lung radiotherapy that accountsfor intrafraction motion Med. Phys. 40, 061705 (2013); 10.1118/1.4805101 Comparison of Elekta VMAT with helical tomotherapy and fixed field IMRT: Plan quality, delivery efficiency andaccuracy Med. Phys. 37, 1350 (2010); 10.1118/1.3326965 Frontiers of Radiation Therapy and Oncology, Vol. 40: IMRT, IGRT, SBRT: Advances in the Treatment Planningand Delivery of Radiotherapy Med. Phys. 35, 3822 (2008); 10.1118/1.2957612 Quantifying the interplay effect in prostate IMRT delivery using a convolution-based method Med. Phys. 35, 1703 (2008); 10.1118/1.2897972 Segmentation of IMRT plans for radical lung radiotherapy delivery with the step-and-shoot technique Med. Phys. 31, 892 (2004); 10.1118/1.1668372

Hybrid MV-kV 3D respiratory motion tracking during radiation therapy with low imaging dose

A novel real-time adaptive MV-kV imaging framework for image-guided radiation therapy is developed to reduce the thoracic and abdominal tumor targeting uncertainty caused by respiration-induced intrafraction motion with ultra-low patient imaging dose. In our method, continuous stereoscopic MV-kV imaging is used at the beginning of a radiation therapy delivery for several seconds to measure the implanted marker positions. After this stereoscopic imaging period, the kV imager is switched off except for the times when no fiducial marker is detected in the cine-MV images. The 3D time-varying marker positions are estimated by combining the MV 2D projection data and the motion correlations between directional components of marker motion established from the stereoscopic imaging period and updated afterwards; in particular, the most likely position is assumed to be the position on the projection line that has the shortest distance to the first principal component line segment constructed from previous trajectory points. An adaptive windowed auto-regressive prediction is utilized to predict the marker position a short time later (310 ms and 460 ms in this study) to allow for tracking system latency. To demonstrate the feasibility and evaluate the accuracy of the proposed method, computer simulations were performed for both arc and fixed-gantry deliveries using 66 h of retrospective tumor motion data from 42 patients treated for thoracic or abdominal cancers. The simulations reveal that using our hybrid approach, a smaller than 1.2 mm or 1.5 mm root-mean-square tracking error can be achieved at a system latency of 310 ms or 460 ms, respectively. Because the kV imaging is only used for a short period of time in our method, extra patient imaging dose can be reduced by an order of magnitude compared to continuous MV-kV imaging, while the clinical tumor targeting accuracy for thoracic or abdominal cancers is maintained. Furthermore, no additional hardware is required with the proposed method.

Can Flattening Filter-free (FFF) Beams Be Used for Off-Axis Targets in Conformal Arc Delivery Of SBRT?

Dynamic conformal arc therapy (DCAT) in full co-planar rotation is commonly adopted for efficient conformal dose delivery in SBRT. FFF beams can further increase delivery efficiency. For laterally located targets which are often seen in lung/liver SBRT, off-axis geometry (OAG) is inevitable to obtain full rotation without collision. However, it is questionable whether FFF beams can provide acceptable dose distributions in OAG due to their significantly asymmetric incident fluence profile. This issue was systematically investigated.

Experimental investigation of a general real-time 3D target localization method using sequential kV imaging combined with respiratory monitoring

The goal of this work was to experimentally quantify the geometric accuracy of a novel real-time 3D target localization method using sequential kV imaging combined with respiratory monitoring for clinically realistic arc and static field treatment delivery and target motion conditions. A general method for real-time target localization using kV imaging and respiratory monitoring was developed. Each dimension of internal target motion T(x, y, z; t) was estimated from the external respiratory signal R(t) through the correlation between R(ti) and the projected marker positions p(xp, yp; ti) on kV images by a state-augmented linear model: T(x, y, z; t) = aR(t) + bR(t − τ) + c. The model parameters, a, b, c, were determined by minimizing the squared fitting error ∑‖p(xp, yp; ti) − P(θi) · (aR(ti) + bR(ti − τ) + c)‖2 with the projection operator P(θi). The model parameters were first initialized based on acquired kV arc images prior to MV beam delivery. This method was implemented on a trilogy linear accelerator consisting of an OBI x-ray imager (operating at 1 Hz) and real-time position monitoring (RPM) system (30 Hz). Arc and static field plans were delivered to a moving phantom programmed with measured lung tumour motion from ten patients. During delivery, the localization method determined the target position and the beam was adjusted in real time via dynamic multileaf collimator (DMLC) adaptation. The beam-target alignment error was quantified by segmenting the beam aperture and a phantom-embedded fiducial marker on MV images and analysing their relative position. With the localization method, the root-mean-squared errors of the ten lung tumour traces ranged from 0.7–1.3 mm and 0.8–1.4 mm during the single arc and five-field static beam delivery, respectively. Without the localization method, these errors ranged from 3.1–7.3 mm. In summary, a general method for real-time target localization using kV imaging and respiratory monitoring has been experimentally investigated for arc and static field delivery. The average beam-target error was 1 mm.