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Title: SU-G-JeP2-03: Automatic Quantification of MLC Positional Accuracy in An MRI Guided Radiotherapy System

Abstract

Purpose: MRI-guided-radiotherapy (MRIGRT) systems lack many features of traditional Linac based RT systems and specialized tests need to be developed to evaluate MLC performance. This work describes automatic tools for the analysis of positional accuracy of an MLC equipped MRIGRT system. Methods: This MLC analysis tool was developed for the MRIdian™ RT system which has three Co-60 equipped treatment heads each with a double focused MLC containing 30 leaf pairs, leaf thickness is 1.05cm defined at isocenter (SAD 105 cm). For MLC positional analysis a picket fence test was performed using a 25.4cm × 25.4cm Gafchromic™ RTQA2 film placed between 5cm solidwater and a 30cm × 30cm × 1cm jigwire phantom with seven embedded parallel metal strips 4cm apart. A plan was generated to deliver 2Gy per field and seven 23.1cm × 2cm fields centered over each wire in the phantom. For each leaf pair the center of the radiation profile was determined by fitting the horizontal profile with a Gaussian model and determining the center of the FWHM. This was compared with the metal strip location to determine any deviation. The following metrics were used to evaluate the deviations per gantry angle including maximum, minimum, mean, Kurtosis, and skewness.more » Results: The identified maximum/mean leaf deviations are, 1.32/0.55 mm for gantry 0°, 1.59/0.76 mm for gantry 90°, and 1.19/0.39 mm for gantry 270°. The percentage of leaf deviation less than 1mm are 90.0% at 0°, 74.6% at 90°, and 97.0% at 270°. Kurtosis/skewness of the leaf deviation are 2.41/0.14 at 0°, 2.53/0.23 at 90°, 3.33/0.83 at 270°, respectively. Conclusion: This work presents an automatic tool for evaluation of the MLC position accuracy of the MRIdian™ radiotherapy system which can be used to benchmark the performance of the MLC system for each treatment head and track the results longitudinally.« less

Authors:
; ; ; ; ;  [1]
  1. University of Miami, Miami, FL (United States)
Publication Date:
OSTI Identifier:
22649369
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; ACCURACY; COBALT 60; COLLIMATORS; LINEAR ACCELERATORS; NMR IMAGING; PERFORMANCE; RADIOTHERAPY; STATISTICS

Citation Formats

Li, X, Studenski, M, Yang, F, Dogan, N, Lamichhane, N, and Padgett, K. SU-G-JeP2-03: Automatic Quantification of MLC Positional Accuracy in An MRI Guided Radiotherapy System. United States: N. p., 2016. Web. doi:10.1118/1.4957023.
Li, X, Studenski, M, Yang, F, Dogan, N, Lamichhane, N, & Padgett, K. SU-G-JeP2-03: Automatic Quantification of MLC Positional Accuracy in An MRI Guided Radiotherapy System. United States. doi:10.1118/1.4957023.
Li, X, Studenski, M, Yang, F, Dogan, N, Lamichhane, N, and Padgett, K. 2016. "SU-G-JeP2-03: Automatic Quantification of MLC Positional Accuracy in An MRI Guided Radiotherapy System". United States. doi:10.1118/1.4957023.
@article{osti_22649369,
title = {SU-G-JeP2-03: Automatic Quantification of MLC Positional Accuracy in An MRI Guided Radiotherapy System},
author = {Li, X and Studenski, M and Yang, F and Dogan, N and Lamichhane, N and Padgett, K},
abstractNote = {Purpose: MRI-guided-radiotherapy (MRIGRT) systems lack many features of traditional Linac based RT systems and specialized tests need to be developed to evaluate MLC performance. This work describes automatic tools for the analysis of positional accuracy of an MLC equipped MRIGRT system. Methods: This MLC analysis tool was developed for the MRIdian™ RT system which has three Co-60 equipped treatment heads each with a double focused MLC containing 30 leaf pairs, leaf thickness is 1.05cm defined at isocenter (SAD 105 cm). For MLC positional analysis a picket fence test was performed using a 25.4cm × 25.4cm Gafchromic™ RTQA2 film placed between 5cm solidwater and a 30cm × 30cm × 1cm jigwire phantom with seven embedded parallel metal strips 4cm apart. A plan was generated to deliver 2Gy per field and seven 23.1cm × 2cm fields centered over each wire in the phantom. For each leaf pair the center of the radiation profile was determined by fitting the horizontal profile with a Gaussian model and determining the center of the FWHM. This was compared with the metal strip location to determine any deviation. The following metrics were used to evaluate the deviations per gantry angle including maximum, minimum, mean, Kurtosis, and skewness. Results: The identified maximum/mean leaf deviations are, 1.32/0.55 mm for gantry 0°, 1.59/0.76 mm for gantry 90°, and 1.19/0.39 mm for gantry 270°. The percentage of leaf deviation less than 1mm are 90.0% at 0°, 74.6% at 90°, and 97.0% at 270°. Kurtosis/skewness of the leaf deviation are 2.41/0.14 at 0°, 2.53/0.23 at 90°, 3.33/0.83 at 270°, respectively. Conclusion: This work presents an automatic tool for evaluation of the MLC position accuracy of the MRIdian™ radiotherapy system which can be used to benchmark the performance of the MLC system for each treatment head and track the results longitudinally.},
doi = {10.1118/1.4957023},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: Evaluate a large-field MRI phantom for assessment of geometric distortion in whole-body MRI for real-time MR guided radiation therapy. Methods: A prototype CIRS large-field MRI distortion phantom consisting of a PMMA cylinder (33 cm diameter, 30 cm length) containing a 3D-printed orthogonal grid (3 mm diameter rods, 20 mm apart), was filled with 6 mM NiCl{sub 2} and 30 mM NaCl solution. The phantom was scanned at 1.5T and 3.0T on a GE HDxt and Discovery MR750, respectively, and at 0.35T on a ViewRay system. Scans were obtained with and without 3D distortion correction to demonstrate the impact ofmore » such corrections. CT images were used as a reference standard for analysis of geometric distortion, as determined by a fully automated gradient-search method developed in Matlab. Results: 1,116 grid points distributed throughout a cylindrical volume 28 cm in diameter and 16 cm in length were identified and analyzed. With 3D distortion correction, average/maximum displacements for the 1.5, 3.0, and 0.35T systems were 0.84/2.91, 1.00/2.97, and 0.95/2.37 mm, respectively. The percentage of points with less than (1.0, 1.5, 2.0 mm) total displacement were (73%, 92%, 97%), (54%, 85%, 97%), and (55%, 90%, 99%), respectively. A reduced scan volume of 20 × 20 × 10 cm{sup 3} (representative of a head and neck scan volume) consisting of 420 points was also analyzed. In this volume, the percentage of points with less than (1.0, 1.5, 2.0 mm) total displacement were (90%, 99%, 100%), (63%, 95%, 100%), and (75%, 96%, 100%), respectively. Without 3D distortion correction, average/maximum displacements were 1.35/3.67, 1.67/4.46, and 1.51/3.89 mm, respectively. Conclusion: The prototype large-field MRI distortion phantom and developed software provide a thorough assessment of 3D spatial distortions in MRI. The distortions measured were acceptable for RT applications, both for the high field strengths and the system configuration developed by ViewRay.« less
  • Purpose: The purpose of the study is to investigate the dose effects of electron-return-effect (ERE) at air-tissue and lung-tissue interfaces under a 1.5T transverse-magnetic-field (TMF). Methods: IMRT and VMAT plans for representative pancreas, lung, breast and head & neck (H&N) cases were generated following clinical dose volume (DV) criteria. The air-cavity walls, as well as the lung wall, were delineated to examine the ERE. In each case, the original plan generated without TMF is compared with the reconstructed plan (generated by recalculating the original plan with the presence of TMF) and the optimized plan (generated by a full optimization withmore » TMF), using a variety of DV parameters, including V100%, D95% and dose heterogeneity index for PTV, Dmax, and D1cc for OARs (organs at risk) and tissue interface. Results: The dose recalculation under TMF showed the presence of the 1.5 T TMF can slightly reduce V100% and D95% for PTV, with the differences being less than 4% for all but lung case studied. The TMF results in considerable increases in Dmax and D1cc on the skin in all cases, mostly between 10-35%. The changes in Dmax and D1cc on air cavity walls are dependent upon site, geometry, and size, with changes ranging up to 15%. In general, the VMAT plans lead to much smaller dose effects from ERE compared to fixed-beam IMRT. When the TMF is considered in the plan optimization, the dose effects of the TMF at tissue interfaces are significantly reduced in most cases. Conclusion: The doses on tissue interfaces can be significantly changed by the presence of a 1.5T TMF during MR-guided RT when the TMF is not included in plan optimization. These changes can be substantially reduced or even removed during VMAT/IMRT optimization that specifically considers the TMF, without deteriorating overall plan quality.« less
  • Purpose: Pancreas is a soft-tissue organ, implanted fiducials can change positions due to migration or tissue deformation. This study quantified positional variation of fiducials in IGRT for pancreatic cancer. Methods: 20 patients had at least 3 gold fiducials implanted in pancreas under EUS guidance. Patients had 4D-CT simulation for gated treatment. Daily gated OBI kV images (Turebeam) were used for positional alignment with fiducials for total of 25 or 28 fractions. Relative distances among 3 fiducials (d{sub 1–} {sub 2}, d{sub 1–3}, d{sub 2–3}) were measured from 4D-CT end-of-expiration phase bin; and from gated kV images in first, mid, andmore » last fraction (n=180). Results: The median duration between implant and simulation was 11 (range 0–41) days. The median duration between simulation and first fraction was 17 (range 8–24) days. The median relative distance was 12 (range 4–78) mm for d{sub 1–2}, 24 (range 6–80) mm for d{sub 1–3}, and 19 (range 5–63) mm for d{sub 2–3}. The median deviation was 1 mm for d{sub 1–2}, d{sub 1–3}, d{sub 2–3} between simulation and first fraction, first and mid fraction, mid and last fraction (n=180). Two patients (10%) had deviation >= 5 mm (5, 11 mm) between simulation and first fraction. One patient (5%) had deviation >= 5 mm (11 mm) between first and mid fraction. No patient (0%) had deviation >= 5 mm between mid and last fraction. In all 3 cases with deviation >=5 mm, only one fiducial was significantly deviated. No clear evidence that deviation size was associated with time interval between implant and first fraction. Conclusion: Implanted gold fiducials were quite stable over time in their relative positions in pancreas. Our data suggested at least 3 fiducials are needed. In cases that one fiducial was significantly deviated in daily kV images, this fiducial should be excluded in image guidance.« less
  • Purpose: To evaluate localization accuracy resulting from rigid registration of locally-advanced lung cancer targets using fully automatic and semi-automatic protocols for image-guided radiation therapy. Methods: Seventeen lung cancer patients, fourteen also presenting with involved lymph nodes, received computed tomography (CT) scans once per week throughout treatment under active breathing control. A physician contoured both lung and lymph node targets for all weekly scans. Various automatic and semi-automatic rigid registration techniques were then performed for both individual and simultaneous alignments of the primary gross tumor volume (GTV{sub P}) and involved lymph nodes (GTV{sub LN}) to simulate the localization process in image-guidedmore » radiation therapy. Techniques included ''standard'' (direct registration of weekly images to a planning CT), ''seeded'' (manual prealignment of targets to guide standard registration), ''transitive-based'' (alignment of pretreatment and planning CTs through one or more intermediate images), and ''rereferenced'' (designation of a new reference image for registration). Localization error (LE) was assessed as the residual centroid and border distances between targets from planning and weekly CTs after registration. Results: Initial bony alignment resulted in centroid LE of 7.3 {+-} 5.4 mm and 5.4 {+-} 3.4 mm for the GTV{sub P} and GTV{sub LN}, respectively. Compared to bony alignment, transitive-based and seeded registrations significantly reduced GTV{sub P} centroid LE to 4.7 {+-} 3.7 mm (p = 0.011) and 4.3 {+-} 2.5 mm (p < 1 x 10{sup -3}), respectively, but the smallest GTV{sub P} LE of 2.4 {+-} 2.1 mm was provided by rereferenced registration (p < 1 x 10{sup -6}). Standard registration significantly reduced GTV{sub LN} centroid LE to 3.2 {+-} 2.5 mm (p < 1 x 10{sup -3}) compared to bony alignment, with little additional gain offered by the other registration techniques. For simultaneous target alignment, centroid LE as low as 3.9 {+-} 2.7 mm and 3.8 {+-} 2.3 mm were achieved for the GTV{sub P} and GTV{sub LN}, respectively, using rereferenced registration. Conclusions: Target shape, volume, and configuration changes during radiation therapy limited the accuracy of standard rigid registration for image-guided localization in locally-advanced lung cancer. Significant error reductions were possible using other rigid registration techniques, with LE approaching the lower limit imposed by interfraction target variability throughout treatment.« less
  • Purpose: To develop and evaluate a new video image-based QA system, including in-house software, that can display a tracking state visually and quantify the positional accuracy of dynamic tumor tracking irradiation in the Vero4DRT system. Methods: Sixteen trajectories in six patients with pulmonary cancer were obtained with the ExacTrac in the Vero4DRT system. Motion data in the cranio–caudal direction (Y direction) were used as the input for a programmable motion table (Quasar). A target phantom was placed on the motion table, which was placed on the 2D ionization chamber array (MatriXX). Then, the 4D modeling procedure was performed on themore » target phantom during a reproduction of the patient’s tumor motion. A substitute target with the patient’s tumor motion was irradiated with 6-MV x-rays under the surrogate infrared system. The 2D dose images obtained from the MatriXX (33 frames/s; 40 s) were exported to in-house video-image analyzing software. The absolute differences in the Y direction between the center of the exposed target and the center of the exposed field were calculated. Positional errors were observed. The authors’ QA results were compared to 4D modeling function errors and gimbal motion errors obtained from log analyses in the ExacTrac to verify the accuracy of their QA system. The patients’ tumor motions were evaluated in the wave forms, and the peak-to-peak distances were also measured to verify their reproducibility. Results: Thirteen of sixteen trajectories (81.3%) were successfully reproduced with Quasar. The peak-to-peak distances ranged from 2.7 to 29.0 mm. Three trajectories (18.7%) were not successfully reproduced due to the limited motions of the Quasar. Thus, 13 of 16 trajectories were summarized. The mean number of video images used for analysis was 1156. The positional errors (absolute mean difference + 2 standard deviation) ranged from 0.54 to 1.55 mm. The error values differed by less than 1 mm from 4D modeling function errors and gimbal motion errors in the ExacTrac log analyses (n = 13). Conclusions: The newly developed video image-based QA system, including in-house software, can analyze more than a thousand images (33 frames/s). Positional errors are approximately equivalent to those in ExacTrac log analyses. This system is useful for the visual illustration of the progress of the tracking state and for the quantification of positional accuracy during dynamic tumor tracking irradiation in the Vero4DRT system.« less