Gantry Motion Research Articles

Development and clinical evaluation of automatic fiducial detection for tumor tracking in cine megavoltage images during volumetric modulated arc therapy

Real-time tracking of implanted fiducials in cine megavoltage (MV) imaging during volumetric modulated arc therapy (VMAT) delivery is complicated due to the inherent low contrast of MV images and potential blockage of dynamic leaves configurations. The purpose of this work is to develop a clinically practical autodetection algorithm for motion management during VMAT. The expected field-specific segments and the planned fiducial position from the Eclipse (Varian Medical Systems, Palo Alto, CA) treatment planning system were projected onto the MV images. The fiducials were enhanced by applying a Laplacian of Gaussian filter in the spatial domain for each image, with a blob-shaped object as the impulse response. The search of implanted fiducials was then performed on a region of interest centered on the projection of the fiducial when it was within an open field including the case when it was close to the field edge or partially occluded by the leaves. A universal template formula was proposed for template matching and normalized cross correlation was employed for its simplicity and computational efficiency. The search region for every image was adaptively updated through a prediction model that employed the 3D position of the fiducial estimated from the localized positions in previous images. This prediction model allowed the actual fiducial position to be tracked dynamically and was used to initialize the search region. The artifacts caused by electronic interference during the acquisition were effectively removed. A score map was computed by combining both morphological information and image intensity. The pixel location with the highest score was selected as the detected fiducial position. The sets of cine MV images taken during treatment were analyzed with in-house developed software written in MATLAB (The Mathworks, Inc., Natick, MA). Five prostate patients were analyzed to assess the algorithm performance by measuring their positioning accuracy during treatment. The algorithm was able to accurately localize the fiducial position on MV images with success rates of more than 90% per case. The percentage of images in which each fiducial was localized in the studied cases varied between 23% and 65%, with at least one fiducial having been localized between 40% and 95% of the images. This depended mainly on the modulation of the plan and fiducial blockage. The prostate movement in the presented cases varied between 0.8 and 3.5 mm (mean values). The maximum displacement detected among all patients was of 5.7 mm. An algorithm for automatic detection of fiducial markers in cine MV images has been developed and tested with five clinical cases. Despite the challenges posed by complex beam aperture shapes, fiducial localization close to the field edge, partial occlusion of fiducials, fast leaf and gantry movement, and inherently low MV image quality, good localization results were achieved in patient images. This work provides a technique for enabling real-time accurate fiducial detection and tumor tracking during VMAT treatments without the use of extra imaging dose.

Weight-Bearing CT Imaging of the Lower Extremity

The purpose of this article is to describe weight-bearing CT of the lower extremity joints using a novel portable imager utilizing cone-beam CT technology. Cone-beam CT technology with new design and flexible gantry movements allows both supine and weight-bearing imaging of the lower extremities, with a reasonable radiation dose and excellent image quality. Weight-bearing CT of joints can provide important new clinical information in musculoskeletal radiology.

Single Room Proton Radiation Therapy

Our proton delivery system is ready for clinical use. We describe the system, operation, and integration within our clinic. The superconducting synchrocyclotron - passive scattering accelerator contains beam line options to treat shallow or deep lesions with range adjustable by 1mm, and modulation up to 20cm. The accelerator is small and light enough to be placed on a gantry revealing a footprint slightly larger than a conventional room. The outer accelerator gantry aligns with an inner treatment gantry that houses applicators (14 or 25cm), field apertures and compensators. Gantry motion is limited to 190deg. Patient positioning and imaging are provided by a 6D robotic couch with 270 degrees of rotation and fixed x-ray and CBCT systems. The system is integrated with our medical record system and treatment planning. Dosimetric data (range, modulation, distal falloff, lateral penumbra) were generated from Monte Carlo calculations, confirmed by measurements. Neutron exposure was assessed by calculations and measurements. Dose rate and delivery efficiency was also assessed. We measured a maximum range of 32cm for small fields and 25cm for large fields. Modulation (SOBP) was found to be flat (±3% over the core portion). Distal falloff (80-20%) and lateral penumbra in air (80-20%) were no greater than 7mm and 4mm, respectively. Calculated/measured data were commissioned, from which generated plans were seamlessly transferred to patient information management system and subsequently, the MEVION console, including radiographic setup images. Neutron exposure was calculated and measured to be < 1mSv per Gy delivered, at 1m from isocenter for a 10x10 field. Dose rate ranged from 2-4 Gy/min. Usability and end-to-end testing revealed efficient treatment aspects (simple single console operation for beam generation), and inefficient aspects (frequent table movement due to the limited gantry range for treatment of > 1 field/day). Uptime is thus far extraordinarily high with rapid and stable beam-on operation. Treatment planning has demonstrated superior results compared with IMRT for established proton treatment sites (pediatric, H&N, CNS, etc.). The delivery capabilities and low neutron exposure affirm our desired use for pediatrics. Future treatment of ocular melanoma will include a proper optical alignment system; while lung treatment will utilize gating of the pulsed beam. Proton radiation therapy, till now, had been limited to facilities dedicating enormous space, resources, and of course expense. The success of this project launches the potential for proton radiation therapy at facilities starved for space (i.e., in heavily populated cities) or resources/finances (smaller academic institutions). We will continue to report on our clinical utility and solutions.

Comparison of Skin Doses in Spot Scanning and Passive Scattering Proton Therapy Treatment Plans for Pediatric Patients With Spinal Ependymomas

Purpose/Objective(s): Our proton delivery system is ready for clinical use. We describe the system, operation, and integration within our clinic. Materials/Methods: The superconducting synchrocyclotron passive scattering accelerator contains beam line options to treat shallow or deep lesions with range adjustable by 1mm, and modulation up to 20cm. The accelerator is small and light enough to be placed on a gantry revealing a footprint slightly larger than a conventional room. The outer accelerator gantry aligns with an inner treatment gantry that houses applicators (14 or 25cm), field apertures and compensators. Gantry motion is limited to 190deg. Patient positioning and imaging are provided by a 6D robotic couch with 270 degrees of rotation and fixed x-ray and CBCT systems. The system is integrated with our medical record system and treatment planning. Dosimetric data (range, modulation, distal falloff, lateral penumbra) were generated from Monte Carlo calculations, confirmed by measurements. Neutron exposure was assessed by calculations and measurements. Dose rate and delivery efficiency was also assessed. Results: We measured a maximum range of 32cm for small fields and 25cm for large fields. Modulation (SOBP) was found to be flat ( 3% over the core portion). Distal falloff (80-20%) and lateral penumbra in air (80-20%) were no greater than 7mm and 4mm, respectively. Calculated/ measured data were commissioned, from which generated plans were seamlessly transferred to patient information management system and subsequently, the MEVION console, including radiographic setup images. Neutron exposure was calculated and measured to be 1 field/day). Uptime is thus far extraordinarily high with rapid and stable beam-on operation. Treatment planning has demonstrated superior results compared with IMRT for established proton treatment sites (pediatric, HN while lung treatment will utilize gating of the pulsed beam. Conclusions: Proton radiation therapy, till now, had been limited to facilities dedicating enormous space, resources, and of course expense. The success of this project launches the potential for proton radiation therapy at facilities starved for space (i.e., in heavily populated cities) or resources/finances (smaller academic institutions). We will continue to report on our clinical utility and solutions. Author Disclosure: E.E. Klein: A. Employee; Washington University. C. Block: A. Employee; Washington University. B. Pierburg: None. J. Bradley: A. Employee; Washington University.

Impact of the number of discrete angles used during dose computation for TomoTherapy treatments

To quantify systematically the effect on accuracy of discretizing gantry rotation during the dose calculation process of TomoTherapy treatments. Up to version 4.0.x included, TomoTherapy treatment planning system (TPS) approximates gantry rotation by computing dose from 51 discrete angles corresponding to the center of the projections used to control the binary multileaf collimator. Potential effects on dose computation accuracy for off-axis targets and low modulation factors have been shown previously for a few treatment configurations. In versions 4.1.x and later, TomoTherapy oversamples the projections to better account for gantry rotation, but only during full scatter optimization and final calculation (i.e., not during optimization in "beamlet" mode). The effect on accuracy of changing the number of angles was quantified with the following framework: (1) predict the impact of the discretization of gantry rotation for various modulation factors, target sizes, and off-axis positions using a simplified analytical algorithm; (2) perform regular quality assurance using measurements with EDR2 radiographic films; (3) isolating the effect of changing the number of discretized angles only (51, 153, and 459) using a previously validated Monte Carlo model (TomoPen). The diameters of the targets were 2, 3, and 5 cm; off-axis central positions of target volumes were 5, 10 and 15, and 17 cm (when accepted by the treatment unit); planned modulation factors were 1.3 and 2.0. For extreme configurations (3 cm tumor, 1.3 modulation factor, 15 cm off-axis position), effects on dose distributions were significant with 89.3% and 95.4% of the points passing gamma tests with 2%∕2 mm and 3%∕3 mm criteria, respectively, for TPS software version 4.0.x (51 gantry angles). The passing rate was 100% for both gamma criteria for the 4.1.x version (153 gantry angles). Those differences could be attributed almost completely to gantry motion discretization using TomoPen. Using 51 gantry angles for dose computation, TomoPen reproduced within statistical uncertainties (<1% standard deviation) dose distributions computed with version 4.0.x. Using 153 and 459 gantry angles, TomoPen reproduced within statistical uncertainties measurements and dose distributions computed with version 4.1.x. When low modulation factors and significant off-axis positions are used, accounting for gantry rotation during dose computation using at least 153 gantry angles is required to ensure optimal accuracy.

Patient specific QA for RapidArc

Abstract: RapidArc is a volumetric arc therapy which was recently introduced to Ireland by St. Lukes Radiation Oncology Network. RapidArc is characterised by continuous MLC and gantry motion in combination with varying gantry speed, dose rate and MLC speed. This approach leads to significantly shortened treatment times and dose conformity which is comparable to that achieved by static IMRT. Because of the complexity of RapidArc treatment plans, it is vital to implement an efficient and accurate protocol for routine patient specific QA. We decided to use the ArcCHECK phantom for this purpose. ArcCHECK is a commercial diode array designed specifically for rotational measurements. Our ArcCHECK measurements were validated by comparison with Gafchromic film measurements as this is a standard high resolution patient verification method. Specifically, our work involved delivering a number of prostate RapidArc treatment plans to ArcCHECK. Independent validation of ArcCHECK measurements with Gafchromic film, in two orientations was performed for a subset of these plans. Gafchromic film analysis was performed using the ‘triple channel dosimetry’ approach. This is a new method of evaluating radiochromic film dosimetry data which improves the integrity of the dose information in the film by removing disturbances in scanned images. As a result of following this method, a high level dosimetric accuracy was evident in our film measurements. Dose distributions were analysed using gamma criteria of 3% dose and 3 mm distance to agreement. A pass rate of at least 95% was achieved using data from ArcCHECK and Gafchromic film respectively. This work combines the construction of an efficient protocol for using ArcCHECK for patient specific QA of RapidArc treatment plans with an implementation of the ‘triple channel dosimetry’ approach for Gafchromic film analysis. Thus we have formulated a protocol for patient specific QA of RapidArc treatment plans and incorporated Gafchromic film as a suitable backup system.

Poster - Thur Eve - 47: Evaluation of the ArcCHECK device for commissioning and patient-specific QA.

The most promising method of accurately verifying VMAT treatments is by direct dose measurement over the three dimensions of irradiated volume. ArcCHECK device (Sun Nuclear, Melbourne, FL) have the potential to detect delivery errors on the treatment machine due to mechanical problems resulting from gantry and MLC motion. The estimation of the dosimetric leaf gap (DLG) parameter for Varian MLC (Varian Medical Systems, Palo Alto, CA) was attempted using ArcCHECK. Finding the optimal DLG value for use in TPS requires a measuring device like ArcCHECK to be employed especially in highly intensity modulated fields. In addition, ArcCHECK was used to assess the effect of positional error of MLC leaf in a given VMAT plan. Patient-specific QA tests were performed using the ArcCHECK device. QA results of patient plans that failed considering portal dosimetry technique were reassessed with ArcCHECK measurements for IMRT plans. The preliminary test results and performance of the ArcCHECK device were very encouraging. VMAT plans for head and neck cases were generated and their delivery was evaluated using ArcCHECK. Results have shown a success rate greater than 90% in the quality assurance of individual plans. Optimal DLG value was detected using ArcCHECK. Also, the device showed enough sensitivity to identify failed QA plans. Moreover, MLC central leaf pair position offset in a VMAT plan of the order of 1mm was fairly distinguished by ArcCHECK measurements.

TU-G-BRB-01: Continuous Path Optimization for Non-Coplanar Variant SAD IMRT Delivery Using C-Arm Machines.

To develop and investigate a continuous path optimization methodology to traverse prescribed non-coplanar IMRT beams with variant SADs, by orchestrating the couch and gantry movement with zero-collision, minimal patient motion consequence and machine travel time. We convert the given collision zone definition and the prescribed beam location/angles to a tumor-centric coordinate, and represent the traversing path as a continuous open curve. We proceed to optimize a composite objective function consisting of (1) a strong attraction energy to ensure all prescribed beams are en-route, (2) a penalty for patient-motion inducing couch motion, and (3) a penalty for travel-time inducing overall path-length. Feasibility manifold is defined as complement to collision zone and the optimization is performed with a level set representation evolved with variational flows. The proposed method has been implemented and tested on clinically derived data. In the absence of any existing solutions for the same problem, we validate by: (1) visual inspecting the generated path rendered in the 3D tumor-centric coordinates, and (2) comparing with a traveling-salesman (TSP) solution obtained from relaxing the variant SADs and continuous collision-avoidance requirement. The proposed method has generated delivery paths that are smooth and intuitively appealing. Under relaxed settings, our results outperform the generic TSP solutions and agree with specially tuned versions. We have proposed a novel systematic approach that automatically determines the continuous path to cover non-coplanar, varying SAD IMRT beams. The proposed approach accommodates patient-specific collision zone definition and ensures its avoidance continuously. The differential penalty to couch and gantry motions allows customizable tradeoff between patient geometry stability and delivery efficiency. This development paves the path to achieve safe, accurate and efficient non-coplanar IMRT delivery with the advanced robotic controls in new-generation C-arm systems, enabling practical harvesting of the dose benefit offered by non-coplanar, variant SAD IMRT treatment.

MO-D-213CD-04: 4D X-Ray DSA and 4D Fluoroscopy.

1. To understand the application of under sampling and constrained reconstruction to 4D DSA and Fluoroscopy.

TU-C-213CD-09: Dosimetric Impact of the Interplay Effect on RapidArc versus IMRT Delivery of Lung Cancer Treatment Using SBRT

Purpose: To evaluate the discrepancy between the delivered and planned doses in volumetric-arc-therapy (RapidArc) for lung SBRT, induced by the interplay effect between tumor, gantry and MLC motion, and to compare the effect to that of plans delivered with IMRT. Methods: For 6 patients receiving lung SBRT treatment and having respiratory-induced tumor motion in the range 0.7 to 1.3 cm superior/inferiorly, 4DCT images and respiratory profiles, measured during CT simulation, were available. The ITV and PTV were contoured. Treatment plans using IMRT and RapidArc (with two arcs) were created using the free-breathing CT (FBCT). Each of the arcs was divided into sub-arcs, such that each single sub-arc was delivered to a specific phase of the breathing cycle. The sub-arcs were sorted and assigned to different phases by temporally associating gantry rotation with respiration. The doses delivered to individual phases were calculated using the sub-arcs and CT images of that phase. Doses were then mapped to the FBCT and accumulated, using Demons-based deformable registration, between the FBCT and respective 4DCT dataset. To account for the interplay effect for IMRT, the dose to the individual phase was calculated by re-computing the initial treatment plan on the dataset of the specific phase and weighting the dose according to delivery time of that phase. Results: ITV minimum and mean doses were reduced by 5.5±7.2% and 0.2±0.9% for RapidArc, and by 4.2±5.8% and 0.5±1.0% for IMRT, respectively, when the interplay effect was accounted for. Decreases in PTV minimum and mean doses were 25.1±27.9% and 2.3±2.4% for RapidArc, and 21.4±23.9% and 2.2±2.7% for IMRT, respectively. Conclusions: The interplay effect reduced the ITV and PTV minimum dose significantly in both RapidArc and IMRT plans. The impact on the ITV/PTV mean doses was minimal. RapidArc dose distributions were somewhat more degraded than those of IMRT.

SU-E-T-167: Comprehensive Evaluation of EPID Image Acquisition for Integrating and Temporal Dosimetry of Fixed-Gantry IMRT and ArcIMRT.

To evaluate EPID for dosimetry applications of arc and static-gantry IMRT with sliding window (SW) and/or step-and-shoot (SS) deliveries Methods: IMRT beams (SW & SS) were designed that generate beam hold-offs and dose rate modulation due to MLC motion under 10 × 10 cm jaw. An arcIMRT beam was designed by adding gantry movement to the SW field. A 10 cm × 10 cm open beam was also used. Despite differences in delivered dose rates/pulse characteristics, the four beams should deliver the same total dose. For each beam, various MUs with 6 MV beam at 300MU/min were irradiated on EPID which operated in image acquisition of integration mode (IM), continuous scanning mode with synchronization (CMs) and without (CMn) to beam pulses. Acquired images were evaluated in repeatability, dose linearity, and reproducibility (reproduce open beam output in IM). In IM, repeatability, dose linearity, and reproducibility were within 1% for all dose levels and beams. In CMs, they were within 1-2% if dose rate was maintained steady (1) for SW beam (needed a minimum 1.3 MU/cm MLC motion) and (2) arcIMRT beam (needed a minimum 1 MU/degree and 2.8 MU/cm MLC motion) and (3) if a minimum of 38-40 MU per shoot was used for SS beam. Nonlinearity was observed for fewer MUs. This is due to the response of EPID to pulse-length reduction for fixed-gantry therapy and pulse dropping for arc therapy. The latter produces in-planar non-uniformity making EPID unsuitable for temporal dosimetry of arcIMRT. Sacrifice in temporal resolution then became necessary such as multi-frames per image (eg. ∼1 sec/image). In CMn the results were similar to those of CMs. However, they showed artifacts, thus this mode was not preferred. We found conditions under which integrating and temporal EPID dosimetry can be used for IMRT and arcIMRT dose deliveries.

VMAT testing for an Elekta accelerator

Volumetric‐modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity‐modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/ localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.PACS numbers: 87.55.de, 87.55.Qr, 87.56.bd

Fast IMRT by increasing the beam number and reducing the number of segments

PurposeThe purpose of this work is to develop fast deliverable step and shoot IMRT technique. A reduction in the number of segments should theoretically be possible, whilst simultaneously maintaining plan quality, provided that the reduction is accompanied by an increased number of gantry angles. A benefit of this method is that the segment shaping could be performed during gantry motion, thereby reducing the delivery time. The aim was to find classes of such solutions whose plan quality can compete with conventional IMRT.Materials/MethodsA planning study was performed. Step and shoot IMRT plans were created using direct machine parameter optimization (DMPO) as a reference. DMPO plans were compared to an IMRT variant having only one segment per angle ("2-Step Fast"). 2-Step Fast is based on a geometrical analysis of the topology of the planning target volume (PTV) and the organs at risk (OAR). A prostate/rectum case, spine metastasis/spinal cord, breast/lung and an artificial PTV/OAR combination of the ESTRO-Quasimodo phantom were used for the study. The composite objective value (COV), a quality score, and plan delivery time were compared. The delivery time for the DMPO reference plan and the 2-Step Fast IMRT technique was measured and calculated for two different linacs, a twelve year old Siemens Primus™ ("old" linac) and two Elekta Synergy™ "S" linacs ("new" linacs).Results2-Step Fast had comparable or better quality than the reference DMPO plan. The number of segments was smaller than for the reference plan, the number of gantry angles was between 23 and 34. For the modern linac the delivery time was always smaller than that for the reference plan. The calculated (measured) values showed a mean delivery time reduction of 21% (21%) for the new linac, and of 7% (3%) for the old linac compared to the respective DMPO reference plans. For the old linac, the data handling time per beam was the limiting factor for the treatment time reduction.Conclusions2-Step Fast plans are suited to reduce the delivery time, especially if the data handling time per beam is short. The plan quality can be retained or even increased for fewer segments provided more gantry angles are used.

Impact of Gantry Rotation Time on Plan Quality and Dosimetric Verification – Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiotherapy (IMRT)

To compare plan quality criteria and dosimetric accuracy of step-and-shoot intensity-modulated radiotherapy (ss-IMRT) and volumetric modulated arc radiotherapy (VMAT) using two different gantry rotation times. This retrospective planning study based on 20 patients was comprised of 10 prostate cancer (PC) and 10 head and neck (HN) cancer cases. Each plan contained two target volumes: a primary planning target volume (PTV) and a boost volume. For each patient, one ss-IMRT plan and two VMAT plans at 90 s (VMAT90) and 120 s (VMAT120) per arc were generated with the Pinnacle© planning system. Two arcs were provided for the PTV plans and a single arc for boost volumes. Dosimetric verification of the plans was performed using a 2D ionization chamber array placed in a full scatter phantom. VMAT reduced delivery time and monitor units for both treatment sites compared to IMRT. VMAT120 vs. VMAT90 increased delivery time and monitor units in PC plans without improving plan quality. For HN cases, VMAT120 provided comparable organs at risk sparing and better target coverage and conformity than VMAT90. In the VMAT plan verification, an average of 97.1% of the detector points passed the 3 mm, 3% γ criterion, while in IMRT verification it was 98.8%. VMAT90, VMAT120, and IMRT achieved comparable treatment plans. Slower gantry movement in VMAT120 plans only improves dosimetric quality for highly complex targets.

Helical Volumetric Modulated Arc Therapy for Treatment of Craniospinal Axis

Volumetric modulated arc therapy (VMAT) can be used with multiple isocenters to provide an effective treatment of the craniospinal axis. Additional efficiency can be achieved by simultaneously applying linear couch motion to generate a helical arc trajectory. This study investigated the treatment planning and delivery of helical VMAT for treatment of the craniospinal axis. VMAT plans were retrospectively created for 5 patients. The first plan consisted of multiple separate arcs. A second plan consisted of a single helical arc with a pitch of 10 cm. Three additional plans consisted of multiple helical arcs with the beam rotating alternately clockwise and counterclockwise to avoid the need for the gantry to pass through 180°. The three plans had a pitch of 5, 10, and 15 cm. For 1 of the patients, three possible plans with alternate gantry motion and a pitch of 10 cm were delivered helically, and the dose was verified. Relative to the plan with separate arcs, the continuous helical plan produced a mean objective value of 104.0% ± 14.8% (standard deviation), and the alternating helical plans produced an objective value of 118.9% ± 9.8%, 102.3% ± 13.5%, and 101.5% ± 15.8% for a pitch of 5 cm, 10 cm, and 15 cm, respectively (with lower values representing better plans). For the delivered plans, taking a mean of 17 min 51 s to deliver, a mean of 97.1% of the measurements were within 4% and 4 mm of the planned dose. A continuous helical VMAT plan provides comparable dose quality to a plan with separate VMAT arcs. Comparable quality is also produced by an alternating helical plan, provided the pitch is chosen appropriately. Alternating helical plans have been delivered and verified successfully. Alternating helical delivery offers the ultimate delivery efficiency for intensity-modulated radiotherapy for the craniospinal axis.

Validation of a new control system for Elekta accelerators facilitating continuously variable dose rate

Elekta accelerators controlled by the current clinically used accelerator control system, Desktop 7.01 (D7), uses binned variable dose rate (BVDR) for volumetric modulated arc therapy (VMAT). The next version of the treatment control system (Integrity) supports continuously variable dose rate (CVDR) as well as BVDR. Using CVDR opposed to BVDR for VMAT has the potential of reducing the treatment time but may lead to lower dosimetric accuracy due to faster moving accelerator parts. Using D7 and a test version of Integrity, differences in ability to control the accelerator, treatment efficiency, and dosimetric accuracy between the two systems were investigated. Single parameter tests were designed to expose differences in the way the two systems control the movements of the accelerator. In these tests, either the jaws, multi leaf collimators (MLCs), or gantry moved at constant speed while the dose rate was changed in discrete steps. The positional errors of the moving component and dose rate were recorded using the control systems with a sampling frequency of 4 Hz. The clinical applicability of Integrity was tested using 15 clinically used VMAT plans (5 prostate, 5 H&N, and 5 lung) generated by the SmartArc algorithm in PINNACLE. The treatment time was measured from beam-on to beam-off and the accuracy of the dose delivery was assessed by comparing DELTA4 measurements and PINNACLE calculated doses using gamma evaluation. The single parameter tests showed that Integrity had an improved feedback between gantry motion and dose rate at the slight expense of MLC control compared to D7. The single parameter test did not reveal any significant differences in the control of either jaws or backup jaws between the two systems. These differences in gantry and MLC control together with the use of CVDR gives a smoother Integrity VMAT delivery compared to D7 with less abrupt changes in accelerator motion. Gamma evaluation (2% of 2 Gy and 2 mm) of the calculated doses and DELTA4 measured doses corrected for systematic errors showed an average pass rate of more than 97.8% for both D7, Integrity BVDR, and Integrity CVDR deliveries. Direct comparisons between the measured doses using strict gamma criteria of 0.5% and 0.5 mm showed excellent agreement between D7 and Integrity delivered doses with average pass rates above 95.7%. Finally, the Integrity control system resulted in a significant 35% (55 +/- 13 s) reduction in treatment time, on average. Single parameter tests showed that the two control systems differed in their feedbac loops between MLC, gantry, and dose rate. These differences made the VMAT deliveries more smooth using the new Integrity treatment control system, compared to the current Desktop 7.01. Together with the use of CVDR, which results in less abrupt changes in dose rate, this further increases the smoothness of the delivery. The use of CVDR for VMAT with the Integrity desktop results in a significant reduction in treatment time compared to BVDR with an average reduction of 35%. This decrease in delivery time was achieved without compromising the dosimetric accuracy.

SU‐E‐J‐157: Investigation of Gated Cone‐Beam CT to Reduce Respiratory Motion Blurring

Purpose: Respiration‐correlated cone‐beam CT (RC‐CBCT) using slow continuous gantry rotation is commonly used to reduce respiratory motion blurring but image quality is limited by uneven projection spacing. This study investigates the efficacy of a novel gated CBCT technique (gantry motion and imaging only within gate) in reducing motion blur with improved image quality using a new type of computer‐controlled linac (Varian TrueBeam).Methods: For gated CBCT, the linac is programmed to gate gantry rotation and kV image acquisition with approximately 25% duty cycle around end expiration. The two‐part study examines images in phantom and prospectively in patients with thoracic and abdominal tumors. A motion phantom is programmed to follow a patient respiratory trace (1.4cm mean excursion) and CBCT scans are performed in standard “free‐breathing”, RC‐CBCT, and gated CBCT modes. A standard CBCT of the stationary phantom serves as ground truth. Evaluation of phantom image quality compares contrast‐to‐noise ratios (CNR) of spheres embedded in acrylic, delineated on the static image and aligned to the other images. In patient studies, image fidelity in standard free‐breathing and gated CBCT is compared relative to a respiration‐correlated CT (RCCT) on the same day, by computing a cross‐correlation metric (CCM) in an ROI around the internal target volume following rigid registration.Results: CNR in motion phantom for gated CBCT is nearly that for static phantom (ratio gated/static 0.95) and larger than for RC‐CBCT (ratio 1.64) and standard CBCT (1.22). CCM of the tumor ROI in gated CBCT is larger than standard CBCT in all three thoracic cases thus far (ratio gated/standard mean 1.04 range 1.03–1.06), indicating higher congruence with the ground‐truth RCCT Conclusions: Gated CBCT visibly reduces image blurring caused by respiratory motion. By exploiting recent computer‐controlled linac technology, complete projection sets can be acquired within a gate, yielding improved reconstruction image quality relative to RC‐CBCT.

WE‐C‐BRB‐11: Four Dimensional Analysis of VMAT Delivery

Purpose: To develop a technique for analyzing the time dependent delivery of VMAT treatment with a 4D diode array. Method and Materials: VMAT treatment plans with two full arcs for H&amp;N patients were generated with Pinnacle SmartArc. The plan was delivered and measured with a 4D diode array (ArcCHECK). ArcCHECK records data every 50 milliseconds and saves in a movie file for post analysis. Algorithm was designed to derive gantry angle for each signal update based on the intersection of beam edges with the cylindrical ArcCHECK phantom. Angular correction factor for each diode was interpolated and applied, with intrinsic sensitivity correction factor, to the raw data to remove diode response dependence. Based on the derived gantry angles, dose between any two control points can be integrated and written into an ArcCHECK measurement file. Measured dose distribution at any instance or integrated between any two control points can be compared with the Pinnacle calculated control point dose using the MapCHECK software. Results: Five IMRT beams were used to test the accuracy of the gantry angle derivation algorithm. The maximum mean error was 0.6o. For VMAT delivery, the derived gantry angles showed a smooth 360o clockwise gantry motion starting from 180o. Seven individual control points (#1, #15, #1∼3, #1∼5, #4∼5, #5∼15 and #1∼90) were selected for dose comparison in relative dose mode. The average passing rates with 3%/3mm and 2%/2mm were 97.1% and 90.5%, respectively. Conclusion: Control point based dose comparison showed satisfactory results indicating the accurate delivery of VMAT plans. The technique will be valuable in diagnosing VMAT delivery system errors, especially those originating from MLC and gantry motion.

SU-E-T-499: Assessment of Patient Setup Accuracy at a Treatment Position after the Patient Has Been Registered at Another Location Using Robotic Positioner and a Dual KV Imaging System

Purpose: We use cardinal angle and beam eye view images with DRRs to register a patient at every treatment position. This process is time consuming; moreover, every treatment position is not amenable to imaging. This study is to assess the combined robotic positioner and gantry movement accuracy at a treatment position relative to zero yaw image registration so that additional imaging at the new location can be avoided. Methods: A Styrofoam phantom 30 cm × 30 cm × 30 cm with beekleys embedded in such a way that they could easily be distinguished on a DRR. The phantom was CT scanned with a 2.5 mm slice thickness and a 50 cm field of view. Treatment planningDRRs were created at numerous table and gantry angles. The phantom was subsequently set up in the treatment room with both table yaw and gantry at zero. The table was loaded with 200 lb weight in order to replicate a typical patient. Image registration was carried out using cardinal angle images and the DRRs. The robotic positioner and gantry were then moved to a new treatment position. The accuracy of the move was evaluated using the beam eye view image, the orthogonal images at the new position and the DRRs for this location. The process was repeated for numerous table and gantry positions as predetermined during treatment planning. Results: The accuracy was found better than a 1.0 mm for all moves except near treatment position with table yaw 90° and Gantry angle between 180° to 270°, where the accuracy was still better than 1.7 mm. Conclusions: As such, the system can be clinically used for patient registration, but we are in the process of further improving the setup accuracy for all clinically relevant robotic positioner and gantry movements without additional imaging.

SU-E-T-483: Investigations of Dose Delivery Accuracy of High Dose Rate Flattening Filter Free (FFF) Beams in RapidArc™ and IMRT Deliveries on TrueBeam™

Purpose: Medical linacs with dose rates as high as 2400 cGy/min at 10MV and 1400 cGy/min at 6MV have been clinically installed in many institutions. In reality, little is known about the accuracy of delivery at these high dose rates, particularly for complex treatment plans with simultaneous motion of gantry and multileaf‐collimator (MLC) aperture. We present measurements that examine the dosimetric accuracy of dynamically‐shaped RapidArc™ and IMRT beams at the highest available dose rates. Methods: An Eclipse™ ver. 8.9 planning system commissioned with the analytic anisotropy algorithm (AAA™) for the flattening‐filter‐free X‐ray beams of the Varian TrueBeam™ linear accelerator with a high‐definition 120‐leaf MLC (HDMLC) accessory was used to create RapidArc™ and IMRT lung‐ and abdominal‐target patient treatment plans deliverable in the FFF mode of TrueBeam™. Verification plans applied to a 22‐cm diameter cylindrical polymethylmethacrylate phantom configured as water‐ equivalent material with an electron density of 1.147 were delivered at the highest available dose rates. Dose was measured with TG‐51 cross‐ calibrated ScandiDos® Delta4™dual orthogonal‐plane 1069‐diode system for conventional 2 Gy fractions and for 15Gy–16.5 Gy fractions characteristic of stereotactic body radiotherapy(SBRT). Delivery was repeated with conventional dose rate (400 cGy/min). Measured results for the two dose rates were compared with each other, and with Eclipse treatment plans. Results: Excellent agreement was found between 2400 and 400 MU/min delivery. A sample 2‐arc RapidArc™ plan gave absolute relative dose difference ratio of 0.42%±0.21% for the 439 points measuring above 10% of prescription dose, with maximum dose difference over all 1069 measurement points being 0.76% of prescription. For a small‐field 1‐ arc plan, quantities were 0.96%±0.14% for 103 points, and maximum difference over all 1069 points was 1.61%. Gated versus ungated delivery for 200 cGy/fx showed negligible differences at both 400 and 2400 dose rates. Eclipse calculations demonstrated reasonable agreement with absolute dose measurement.