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      Evaluation of Intensity Modulated Radiation Therapy Delivery System using a Volumetric Phantom on the Basis of the Task Group 119 Report of American Association of Physicists in Medicine

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          Abstract

          The current work describes the implementation of the American Association of Physicists in Medicine (AAPM)'s Task Group 119 report on a volumetric phantom (Delta 4, Scandidos, Uppsala, Sweden) following the stated dose goals, to evaluate the step-and-shoot intensity modulated radiation therapy (IMRT) system. Delta 4 consists of diode detectors, lying on two crossed planes, measuring the delivered dose, and providing two-dimensional dosimetric information. Seven plans of different goals and complexity were performed, with individual structure sets. TG199 structure sets and plans were transferred and implemented on the Delta 4 phantom taking into account its cylindrical geometry. All plans were delivered with a 6 MV linear accelerator equipped with multileaf collimator of 1 cm thickness. Plan results for each test met the recommended dose goals. The evaluation was performed in terms of dose deviation, distance to agreement, and gamma index passing rate. In all test cases, the gamma index passing rate was measured >90%. Delta 4 phantom has proven to be fast, applicable, and reliable for the step-and-shoot IMRT commissioning following TG119's recommended tests. Although AAPM's TG119 report is referring to the implementation of test plans that do not correspond to patient plans, it could be used as an evaluation tool of various IMRT systems, considering the local treatment planning system and the delivery system.

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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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            IMRT commissioning: multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119.

            AAPM Task Group 119 has produced quantitative confidence limits as baseline expectation values for IMRT commissioning. A set of test cases was developed to assess the overall accuracy of planning and delivery of IMRT treatments. Each test uses contours of targets and avoidance structures drawn within rectangular phantoms. These tests were planned, delivered, measured, and analyzed by nine facilities using a variety of IMRT planning and delivery systems. Each facility had passed the Radiological Physics Center credentialing tests for IMRT. The agreement between the planned and measured doses was determined using ion chamber dosimetry in high and low dose regions, film dosimetry on coronal planes in the phantom with all fields delivered, and planar dosimetry for each field measured perpendicular to the central axis. The planar dose distributions were assessed using gamma criteria of 3%/3 mm. The mean values and standard deviations were used to develop confidence limits for the test results using the concept confidence limit = /mean/ + 1.96sigma. Other facilities can use the test protocol and results as a basis for comparison to this group. Locally derived confidence limits that substantially exceed these baseline values may indicate the need for improved IMRT commissioning.
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              Dosimetry tools and techniques for IMRT.

              Intensity modulated radiation therapy (IMRT) poses a number of challenges for properly measuring commissioning data and quality assurance (QA) radiation dose distributions. This report provides a comprehensive overview of how dosimeters, phantoms, and dose distribution analysis techniques should be used to support the commissioning and quality assurance requirements of an IMRT program. The proper applications of each dosimeter are described along with the limitations of each system. Point detectors, arrays, film, and electronic portal imagers are discussed with respect to their proper use, along with potential applications of 3D dosimetry. Regardless of the IMRT technique utilized, some situations require the use of multiple detectors for the acquisition of accurate commissioning data. The overall goal of this task group report is to provide a document that aids the physicist in the proper selection and use of the dosimetry tools available for IMRT QA and to provide a resource for physicists that describes dosimetry measurement techniques for purposes of IMRT commissioning and measurement-based characterization or verification of IMRT treatment plans. This report is not intended to provide a comprehensive review of commissioning and QA procedures for IMRT. Instead, this report focuses on the aspects of metrology, particularly the practical aspects of measurements that are unique to IMRT. The metrology of IMRT concerns the application of measurement instruments and their suitability, calibration, and quality control of measurements. Each of the dosimetry measurement tools has limitations that need to be considered when incorporating them into a commissioning process or a comprehensive QA program. For example, routine quality assurance procedures require the use of robust field dosimetry systems. These often exhibit limitations with respect to spatial resolution or energy response and need to themselves be commissioned against more established dosimeters. A chain of dosimeters, from secondary standards to field instruments, is established to assure the quantitative nature of the tests. This report is intended to describe the characteristics of the components of these systems; dosimeters, phantoms, and dose evaluation algorithms. This work is the report of AAPM Task Group 120.
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                Author and article information

                Journal
                J Med Phys
                J Med Phys
                JMP
                Journal of Medical Physics
                Medknow Publications & Media Pvt Ltd (India )
                0971-6203
                1998-3913
                Jan-Mar 2017
                : 42
                : 1
                : 33-41
                Affiliations
                [1]Department of Radiology, Medical Physics Unit, Aretaieion Hospital, University of Athens, Greece
                [1 ]Department of Radiotherapy Physics, Nottingham University Hospital, NHS Trust, Nottingham, United Kingdom
                Author notes
                Address for correspondence: Mrs. Raphaela Avgousti, Department of Radiology, Medical Physics Unit, Aretaieion Hospital, University of Athens, Greece. E-mail: raphaela.avgousti@ 123456gmail.com
                Article
                JMP-42-33
                10.4103/0971-6203.202419
                5370336
                28405106
                ebdf342d-e9a1-455d-aa98-8f5038c60485
                Copyright: © 2017 Journal of Medical Physics

                This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

                History
                : 11 October 2016
                : 29 November 2016
                : 01 December 2016
                Categories
                Technical Note

                Medical physics
                commissioning,intensity modulated radiation therapy,quality assurance,volumetric phantom

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