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      Dosimetric validation of a magnetic resonance image gated radiotherapy system using a motion phantom and radiochromic film

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          Abstract

          Purpose

          Magnetic resonance image ( MRI) guided radiotherapy enables gating directly on the target position. We present an evaluation of an MRI‐guided radiotherapy system's gating performance using an MRI‐compatible respiratory motion phantom and radiochromic film. Our evaluation is geared toward validation of our institution's clinical gating protocol which involves planning to a target volume formed by expanding 5 mm about the gross tumor volume ( GTV) and gating based on a 3 mm window about the GTV.

          Methods

          The motion phantom consisted of a target rod containing high‐contrast target inserts which moved in the superior‐inferior direction inside a body structure containing background contrast material. The target rod was equipped with a radiochromic film insert. Treatment plans were generated for a 3 cm diameter spherical planning target volume, and delivered to the phantom at rest and in motion with and without gating. Both sinusoidal trajectories and tumor trajectories measured during MRI‐guided treatments were used. Similarity of the gated dose distribution to the planned, motion‐frozen, distribution was quantified using the gamma technique.

          Results

          Without gating, gamma pass rates using 4%/3 mm criteria were 22–59% depending on motion trajectory. Using our clinical standard of repeated breath holds and a gating window of 3 mm with 10% target allowed outside the gating boundary, the gamma pass rate was 97.8% with 3%/3 mm gamma criteria. Using a 3 mm window and 10% allowed excursion, all of the patient tumor motion trajectories at actual speed resulting in at least 95% gamma pass rate at 4%/3 mm.

          Conclusions

          Our results suggest that the device can be used to compensate respiratory motion using a 3 mm gating margin and 10% allowed excursion results in conjunction with repeated breath holds. Full clinical validation requires a comprehensive evaluation of tracking performance in actual patient images, outside the scope of this study.

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          Most cited references27

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          A technique for the quantitative evaluation of dose distributions.

          The commissioning of a three-dimensional treatment planning system requires comparisons of measured and calculated dose distributions. Techniques have been developed to facilitate quantitative comparisons, including superimposed isodoses, dose-difference, and distance-to-agreement (DTA) distributions. The criterion for acceptable calculation performance is generally defined as a tolerance of the dose and DTA in regions of low and high dose gradients, respectively. The dose difference and DTA distributions complement each other in their useful regions. A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria. Although the composite distribution identifies locations where the calculation fails the preselected criteria, no numerical quality measure is provided for display or analysis. A technique is developed to unify dose distribution comparisons using the acceptance criteria. The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria. In a space composed of dose and spatial coordinates, the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point. The minimum radial distance between the measurement point and the calculation points (expressed as a surface in the dose-distance space) is termed the gamma index. Regions where gamma > 1 correspond to locations where the calculation does not meet the acceptance criteria. The determination of gamma throughout the measured dose distribution provides a presentation that quantitatively indicates the calculation accuracy. Examples of a 6 MV beam penumbra are used to illustrate the gamma index.
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            An efficient protocol for radiochromic film dosimetry combining calibration and measurement in a single scan.

            Radiochromic film provides dose measurement at high spatial resolution, but often is not preferred for routine evaluation of patient-specific intensity modulated radiation therapy (IMRT) plans owing to ease-of-use factors. The authors have established an efficient protocol that combines calibration and measurement in a single scan and enables measurement results to be obtained in less than 30 min. This avoids complications due to postexposure changes in radiochromic film that delay the completion of a measurement, often for up to 24 h, in commonly used methods. In addition, the protocol addresses the accuracy and integrity of the measurement by eliminating environmental and interscan variability issues.
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              Physical aspects of a real-time tumor-tracking system for gated radiotherapy.

              To reduce uncertainty due to setup error and organ motion during radiotherapy of tumors in or near the lung, by means of real-time tumor tracking and gating of a linear accelerator. The real-time tumor-tracking system consists of four sets of diagnostic X-ray television systems (two of which offer an unobstructed view of the patient at any time), an image processor unit, a gating control unit, and an image display unit. The system recognizes the position of a 2.0-mm gold marker in the human body 30 times per second using two X-ray television systems. The marker is inserted in or near the tumor using image guided implantation. The linear accelerator is gated to irradiate the tumor only when the marker is within a given tolerance from its planned coordinates relative to the isocenter. The accuracy of the system and the additional dose due to the diagnostic X-ray were examined in a phantom, and the geometric performance of the system was evaluated in 4 patients. The phantom experiment demonstrated that the geometric accuracy of the tumor-tracking system is better than 1.5 mm for moving targets up to a speed of 40 mm/s. The dose due to the diagnostic X-ray monitoring ranged from 0.01% to 1% of the target dose for a 2.0-Gy irradiation of a chest phantom. In 4 patients with lung cancer, the range of the coordinates of the tumor marker during irradiation was 2.5-5.3 mm, which would have been 9.6-38.4 mm without tracking. We successfully implemented and applied a tumor-tracking and gating system. The system significantly improves the accuracy of irradiation of targets in motion at the expense of an acceptable amount of diagnostic X-ray exposure.
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                Author and article information

                Contributors
                JLamb@mednet.ucla.edu
                Journal
                J Appl Clin Med Phys
                J Appl Clin Med Phys
                10.1002/(ISSN)1526-9914
                ACM2
                Journal of Applied Clinical Medical Physics
                John Wiley and Sons Inc. (Hoboken )
                1526-9914
                24 April 2017
                May 2017
                : 18
                : 3 ( doiID: 10.1002/acm2.2017.18.issue-3 )
                : 163-169
                Affiliations
                [ 1 ] Department of Radiation Oncology David Geffen School of Medicine University of California Los Angeles CA USA
                [ 2 ] Department of Radiation Oncology University of Colorado Denver Denver CO USA
                [ 3 ] Computerized Imaging Reference Systems, Inc. Norfolk VA USA
                Author notes
                [*] [* ]Author to whom correspondence should be addressed. James M. Lamb E‐mail: JLamb@ 123456mednet.ucla.edu .
                Article
                ACM212088
                10.1002/acm2.12088
                5689863
                28436094
                4c7fa1a2-c8bb-443a-9b4e-9a030104a02b
                © 2017 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine.

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                History
                : 02 December 2016
                : 06 March 2017
                : 13 March 2017
                Page count
                Figures: 4, Tables: 3, Pages: 7, Words: 5128
                Categories
                87.55.Qr
                Radiation Oncology Physics
                Radiation Oncology Physics
                Custom metadata
                2.0
                acm212088
                May 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:16.11.2017

                motion management,mri,quality assurance,radiotherapy
                motion management, mri, quality assurance, radiotherapy

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