S Mutic

Two-and-a-half-year clinical experience with the world's first magnetic resonance image guided radiation therapy system

Purpose Magnetic resonance image guided radiation therapy (MR-IGRT) has been used at our institution since 2014. We report on more than 2 years of clinical experience in treating patients with the worlds first MR-IGRT system. Methods and materials A clinical service was opened for MR-IGRT in January 2014 with an MR-IGRT system consisting of a split 0.35T magnetic resonance scanner that straddles a ring gantry with 3 multileaf collimator-equipped 60Co heads. The service was expanded to include online adaptive radiation therapy (ART) MR-IGRT and cine gating after 6 and 9 months, respectively. Patients selected for MR-IGRT were enrolled in a prospective registry between January 2014 and June 2016. Patients were treated with a variety of radiation therapy techniques including intensity modulated radiation therapy and stereotactic body radiation therapy (SBRT). When applicable, online ART was performed and gating on sagittal 2-dimensional cine MR was used. The charts of patients treated with MR-IGRT were reviewed to report on the clinical and treatment characteristics of the initial patients who were treated with this novel technique. Results A total of 316 patients have been treated with the MR-IGRT system, which has been integrated into a high-volume clinic. The cases were most commonly selected for improved soft tissue visualization, ART, and cine gating. Seventy-six patients were treated with 3-dimensional conformal radiation therapy, 146 patients with intensity modulated radiation therapy, and 94 patients with SBRT. The most commonly treated disease sites were the abdomen (28%), breast (26%), pelvis (22%), thorax (19%), and head and neck (5%). Sixty-seven patients were treated with online ART over a total of 244 adapted fractions. Cine treatment gating was used for a total of 81 patients. Conclusions MR-IGRT has been successfully implemented in a high-volume radiation clinic and provides unique advantages in the treatment of a variety of malignancies. Additional clinical trials are in development to formally evaluate MR-IGRT in the treatment of multiple disease sites with techniques such as SBRT and ART.

WE‐C‐214‐04: ADQ — A Software Tool That Automatically, Autonomously, Intelligently and Instantly Verify Patient Radiation Therapy Beam Deliveries

Purpose: To effectively mitigate errors in IMRTradiation therapydelivery, all beams, all fractions, for all patients should be checked in vivo, immediately, automatically, autonomously and intelligently for integrity, quality and safety. For this purpose, ADQ or Automatic Dynalog QA, is implemented for instant and automatic patient delivery beam verification. Methods: ADQ contains multiple functional modules. DICOM receiver program is developed in C++ to receive DICOM‐RT Plans from treatment planning systems, process data and save to database. The verification tool is implemented in MATLAB that automatically validates beam parameters (gantry angle, collimator angle and positions, MLC positions, fluence maps, etc.) between the treatment plans and recorded dynamic MLC log files, generate reports for each treatment session, and send out alert emails for detected urgent problems. Report reviewer is implemented in C++ that enables physicists to review, comment and confirm reports. ADQ programs use own database and Mosaiq R&V database. Simple, automatic and no human intervention is needed unless an error is detected. Results: ADQ is running to generate near real‐time QA reports for every treatment date. DICOM receiver is running 24 hours to collect plans. Report reviewer deployed through network facilitates easy access to reports. All IMRT beams delivered to the patients are checked for a period of four months to study the reliability, MLC performance, false positive rate and importantly identify true positive. More than 80000 thousand beams from 4 different Linacs were analyzed up‐to‐date. Conclusions: We developed new software tools to improve the RT treatment QA by automatic checking patient treatment beam delivery records for each patient and each treatment session. Report data achieved in database can be easily used for further studies, for example, analysis of MLC leaf failures.

SU-C-304-04: A Compact Modular Computational Platform for Automated On-Board Imager Quality Assurance

Purpose: Traditionally, the assessment of X-ray tube output and detector positioning accuracy of on-board imagers (OBI) has been performed manually and subjectively with rulers and dosimeters, and typically takes hours to complete. In this study, we have designed a compact modular computational platform to automatically analyze OBI images acquired with in-house designed phantoms as an efficient and robust surrogate. Methods: The platform was developed as an integrated and automated image analysis-based platform using MATLAB for easy modification and maintenance. Given a set of images acquired with the in-house designed phantoms, the X-ray output accuracy was examined via cross-validation of the uniqueness and integration minimization of important image quality assessment metrics, while machine geometric and positioning accuracy were validated by utilizing pattern-recognition based image analysis techniques. Results: The platform input was a set of images of an in-house designed phantom. The total processing time is about 1–2 minutes. Based on the data acquired from three Varian Truebeam machines over the course of 3 months, the designed test validation strategy achieved higher accuracy than traditional methods. The kVp output accuracy can be verified within +/−2 kVp, the exposure accuracy within 2%, and exposure linearity with a coefficient of variation (CV) of 0.1.morexa0» Sub-millimeter position accuracy was achieved for the lateral and longitudinal positioning tests, while vertical positioning accuracy within +/−2 mm was achieved. Conclusion: This new platform delivers to the radiotherapy field an automated, efficient, and stable image analysis-based procedure, for the first time, acting as a surrogate for traditional tests for LINAC OBI systems. It has great potential to facilitate OBI quality assurance (QA) with the assistance of advanced image processing techniques. In addition, it provides flexible integration of additional tests for expediting other OBI quality assurance tests, such as 2D/3D image quality, making completely automated QA possible. Research Funding from Varian Medical Systems Inc. . Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.«xa0less

SU-E-T-269: Differential Hazard Analysis For Conventional And New Linac Acceptance Testing Procedures

Purpose: New techniques and materials have recently been developed to expedite the conventional Linac Acceptance Testing Procedure (ATP). The new ATP method uses the Electronic Portal Imaging Device (EPID) for data collection and is presented separately. This new procedure is meant to be more efficient then conventional methods. While not clinically implemented yet, a prospective risk assessment is warranted for any new techniques. The purpose of this work is to investigate the risks and establish the pros and cons between the conventional approach and the new ATP method. Methods: ATP tests that were modified and performed with the EPID were analyzed. Five domain experts (Medical Physicists) comprised the core analysis team. Ranking scales were adopted from previous publications related to TG 100. The number of failure pathways for each ATP test procedure were compared as well as the number of risk priority numbers (RPN’s) greater than 100 were compared. Results: There were fewer failure pathways with the new ATP compared to the conventional, 262 and 556, respectively. There were fewer RPN’s > 100 in the new ATP compared to the conventional, 41 and 115. Failure pathways and RPN’s > 100 for individual ATP tests on average were 2 and 3.5 times higher in the conventional ATP compared to the new, respectively. The pixel sensitivity map of the EPID was identified as a key hazard to the new ATP procedure with an RPN of 288 for verifying beam parameters. Conclusion: The significant decrease in failure pathways and RPN’s >100 for the new ATP mitigates the possibilities of a catastrophic error occurring. The Pixel Sensitivity Map determining the response and inherent characteristics of the EPID is crucial as all data and hence results are dependent on that process. Grant from Varian Medical Systems Inc.

SU-E-T-775: Use of Electronic Portal Imaging Device (EPID) for Quality Assurance (QA) of Electron Beams On Varian Truebeam System

Purpose: In a previous study we have demonstrated the feasibility of using EPID to QA electron beam parameters on a single Varian TrueBeam LINAC. This study aims to provide further investigation on (1) reproducibility of using EPID to detect electron beam energy changes on multiple machines and (2) evaluation of appropriate calibration methods to compare results from different EPIDs. Methods: Ad-hoc mode electron beam images were acquired in developer mode with XML code. Electron beam data were collected on a total of six machines from four institutions. A custom-designed double-wedge phantom was placed on the EPID detector. Two calibration methods - Pixel Sensitivity Map (PSM) and Large Source-to-Imager Distance Flood Field (LSID-FF) - were used. To test the sensitivity of EPID in detecting energy drifts, Bending Magnet Current (BMC) was detuned to invoke energy changes corresponding to ∼±1.5 mm change in R50% of PDD on two machines from two institutions. Percent depth ionization (PDI) curves were then analyzed and compared with the respective baseline images using LSID-FF calibration. For reproducibility testing, open field EPID images and images with a standard testing phantom were collected on multiple machines. Images with and without PSM correction for same energies on different machines were overlaid and compared. Results: Two pixel shifts were observed in PDI curve when energy changes exceeded the TG142 tolerance. PSM showed the potential to correct the differences in pixel response of different imagers. With PSM correction, the histogram of images differences obtained from different machines showed narrower distributions than those images without PSM correction. Conclusion: EPID is sensitive for electron energy changes and the results are reproducible on different machines. When overlaying images from different machines, PSM showed the ability to partially eliminate the intrinsic variation of various imagers. Research Funding from Varian Medical Systems Inc.Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

WE‐E‐BRB‐07: Direct 3D Fluence Calculation from Machine Beam Parameters for VMAT Delivery Verification

PURPOSEnMLC dynamic log files have been clinically used for quality assurance for years. The logged machine parameters and the derived beam 2D fluence maps can be compared to the ones obtained in the treatment plans in order to evaluate the accuracy and consistency of IMRT beam deliveries. In this study, we propose a computationally efficient method, called Direct 3D Fluence Calculation or D3DFC, to extend 2D fluence map derivation to 3D fluence volume computation. The aim is to extend dynalog-based QA from fixed-gantry IMRT to rotational-gantry VMAT.nnnMETHODSnD3DFC calculates the 3D volume of photon fluence distribution directly from the machine parameters (gantry angles, jaw positions, MLC positions, collimator rotation angle, MU) contained in the dynamic log files or DICOM plans, without converting to 2D fluence maps per gantry angle in order to allow higher computation speed and accuracy. For testing, results were verified with film-in-air measurements. 3D fluence volumes computed from VMAT delivery records (with artificially introduced delivery errors) are compared to ones computed from the treatment plans to determine if these delivery errors can be identified.nnnRESULTSnD3DFC is implemented in MATLAB and supports the DICOM plans, Varian MLC dynamic logs and Varian Truebeam machine logs. Computation takes 10 to 20 seconds for a single-arc VMAT plan or delivery records. The results showed that 1 mm MLC errors can be clearly detected using delivery-to-plan fluence volume comparison.nnnCONCLUSIONSnDirect computation from machine parameters allows higher computation speed and accuracy. These advantages are useful for beam delivery verification purposes for which (slower) full patient CT based dose computation is less necessary. The calculated 3D photon fluence volume is useful to detect and visually present the VMAT delivery discrepancies.

TH-CD-304-07: Chamber Volume Effect On Absolute Dosimetry in A Magnetic Field

Purpose: To evaluate the effect of ionization chamber volume size and orientation on dosimetric accuracy in the presence of a magnetic field. Methods: The magnetic resonance image-guided radiotherapy (MR-IGRT) system (ViewRay, Inc., Oakwood, OH) was placed into clinical use after extensive calibration measurements with ionization chambers and verification tests using thermo-and opti-luminescent dosimeters. To further investigate the effect of the magnetic field on the ionization chamber results, measurements were made using four orientations and two different chambers with significantly different volumes: the Standard Imaging (Middleton, WI) A18 and the PTW N30013 Farmer chamber, with collecting volumes of 0.123 and 0.6 cc, respectively. High-resolution computed tomography scans were obtained of the chambers to ensure accurate modeling in the Monte Carlo (MC) calculations with EGSnrc. The four different orientations of the chambers relative to the magnetic field direction were achieved by having the chambers parallel to the magnetic field (pointing into the bore) and perpendicular to the magnetic field (pointing down), using a 10.5×10.5 cm 2 open field incident on the chambers from either 90 or 270 degrees (IEC). Results: MC calculations and measured data showed dependence on both orientation and volume in the presence of magnetic field. The greatest differences for both chambers were at 270 degrees pointing down, with A18 at 3.2% and Farmer at 4.4% as compared to the same setup with no magnetic field. The difference to zero magnetic field is minimized when the chamber axis is oriented parallel to the field. The measured and simulated chamber doses showed reasonable agreement. The MC simulations demonstrated a strong dependence of results when small air gaps between the chamber and build-up cup are present in the perpendicular orientation. Conclusion: Accurate absolute dosimetry is possible in presence of low-strength magnetic fields, but care must be taken in chamber selection and orientation. Drs. Green & Mutic served as non-compensated consultants for ViewRay, Inc., and have received honoraria and travel reimbursement.

SU-F-303-11: Implementation and Applications of Rapid, SIFT-Based Cine MR Image Binning and Region Tracking

Purpose: To develop a novel and rapid, SIFT-based algorithm for assessing feature motion on cine MR images acquired during MRI-guided radiotherapy treatments. In particular, we apply SIFT descriptors toward both partitioning cine images into respiratory states and tracking regions across frames. Methods: Among a training set of images acquired during a fraction, we densely assign SIFT descriptors to pixels within the images. We cluster these descriptors across all frames in order to produce a dictionary of trackable features. Associating the best-matching descriptors at every frame among the training images to these features, we construct motion traces for the features. We use these traces to define respiratory bins for sorting images in order to facilitate robust pixel-by-pixel tracking. Instead of applying conventional methods for identifying pixel correspondences across frames we utilize a recently-developed algorithm that derives correspondences via a matching objective for SIFT descriptors. Results: We apply these methods to a collection of lung, abdominal, and breast patients. We evaluate the procedure for respiratory binning using target sites exhibiting high-amplitude motion among 20 lung and abdominal patients. In particular, we investigate whether these methods yield minimal variation between images within a bin by perturbing the resulting image distributions among bins. Moreover, we compare the motion between averaged images across respiratory states to 4DCT data for these patients. We evaluate the algorithm for obtaining pixel correspondences between frames by tracking contours among a set of breast patients. As an initial case, we track easily-identifiable edges of lumpectomy cavities that show minimal motion over treatment. Conclusions: These SIFT-based methods reliably extract motion information from cine MR images acquired during patient treatments. While we performed our analysis retrospectively, the algorithm lends itself to prospective motion assessment. Applications of these methods include motion assessment, identifying treatment windows for gating, and determining optimal margins for treatment.

WE-G-BRD-04: BEST IN PHYSICS (JOINT IMAGING-THERAPY): An Integrated Model-Based Intrafractional Organ Motion Tracking Approach with Dynamic MRI in Head and Neck Radiotherapy

Purpose: In-treatment dynamic cine images, provided by the first commercially available MRI-guided radiotherapy system, allow physicians to observe intrafractional motion of head and neck (H&N) internal structures. Nevertheless, high anatomical complexity and relatively poor cine image contrast/resolution have complicated automatic intrafractional motion evaluation. We proposed an integrated model-based approach to automatically delineate and analyze moving structures from on-board cine images. Methods: The H&N upper airway, a complex and highly deformable region wherein severe internal motion often occurs, was selected as the target-to-be-tracked. To reliably capture its motion, a hierarchical structure model containing three statistical shapes (face, face-jaw, and face-jaw-palate) was first built from a set of manually delineated shapes using principal component analysis. An integrated model-fitting algorithm was then employed to align the statistical shapes to the first to-be-detected cine frame, and multi-feature level-set contour propagation was performed to identify the airway shape change in the remaining frames. Ninety sagittal cine MR image sets, acquired from three H&N cancer patients, were utilized to demonstrate this approach. Results: The tracking accuracy was validated by comparing the results to the average of two manual delineations in 20 randomly selected images from each patient. The resulting dice similarity coefficient (93.28+/−1.46 %) and margin error (0.49+/−0.12 mm) showed good agreement with the manual results. Intrafractional displacements of anterior, posterior, inferior, and superior airway boundaries were observed, with values of 2.62+/−2.92, 1.78+/−1.43, 3.51+/−3.99, and 0.68+/−0.89 mm, respectively. The H&N airway motion was found to vary across directions, fractions, and patients, and highly correlated with patients’ respiratory frequency. Conclusion: We proposed the integrated computational approach, which for the first time allows to automatically identify the H&N upper airway and quantify in-treatment H&N internal motion in real-time. This approach can be applied to track other structures’ motion, and provide guidance on patient-specific prediction of intra-/inter-fractional structure displacements.

SU-E-T-781: Using An Electronic Portal Imaging Device (EPID) for Correlating Linac Photon Beam Energies

Purpose: Electronic portal imaging devices (EPIDs) have proven to be useful for measuring several parameters of interest in linear accelerator (linac) quality assurance (QA). The purpose of this project was to evaluate the feasibility of using EPIDs for determining linac photon beam energies. Methods: Two non-clinical Varian TrueBeam linacs (Varian Medical Systems, Palo Alto, CA) with 6MV and 10MV photon beams were used to perform the measurements. The linacs were equipped with an amorphous silicon based EPIDs (aSi1000) that were used for the measurements. We compared the use of flatness versus percent depth dose (PDD) for predicting changes in linac photon beam energy. PDD was measured in 1D water tank (Sun Nuclear Corporation, Melbourne FL) and the profiles were measured using 2D ion-chamber array (IC-Profiler, Sun Nuclear) and the EPID. Energy changes were accomplished by varying the bending magnet current (BMC). The evaluated energies conformed with the AAPM TG142 tolerance of ±1% change in PDD. Results: BMC changes correlating with a ±1% change in PDD corresponded with a change in flatness of ∼1% to 2% from baseline values on the EPID. IC Profiler flatness values had the same correlation. We observed a similar trend for the 10MV beam energy changes. Our measurements indicated a strong correlation between changes in linac photon beam energy and changes in flatness. For all machines and energies, beam energy changes produced change in the uniformity (AAPM TG-142), varying from ∼1% to 2.5%. Conclusions: EPID image analysis of beam profiles can be used to determine linac photon beam energy changes. Flatness-based metrics or uniformity as defined by AAPM TG-142 were found to be more sensitive to linac photon beam energy changes than PDD. Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.