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Med Phys. 2018 Apr;45(4):e53-e83. doi: 10.1002/mp.12810. Epub 2018 Mar 23.

Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218.

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Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, CO, USA.
Department of Radiation Oncology, University of Southern California and Radiation Oncology Program, Childrens Hospital of Los Angeles, Los Angeles, CA, USA.
Department of Radiation Oncology, University of Pennsylvania, Perelman Center for Advanced Medicine, Philadelphia, PA, USA.
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, USA.
Department of Radiation Oncology, University of California San Diego, La Jolla, CA, USA.
Radiological Physics Center, UT MD Anderson Cancer Center, Houston, TX, USA.
Department of Radiation Oncology, Washington University, St. Louis, MO, USA.
Department of Radiation Oncology, University of Virginia, Charlottesville, VA, USA.
Sun Nuclear Corporation, Melbourne, FL, USA.
Department of Radiation Oncology, The Cleveland Clinic, Cleveland, OH, USA.
Department of Medical Physics, University of Texas Health Sciences Center, San Antonio, TX, USA.
Department of Radiation Oncology, University of California Los Angeles, Los Angeles, CA, USA.



Patient-specific IMRT QA measurements are important components of processes designed to identify discrepancies between calculated and delivered radiation doses. Discrepancy tolerance limits are neither well defined nor consistently applied across centers. The AAPM TG-218 report provides a comprehensive review aimed at improving the understanding and consistency of these processes as well as recommendations for methodologies and tolerance limits in patient-specific IMRT QA.


The performance of the dose difference/distance-to-agreement (DTA) and γ dose distribution comparison metrics are investigated. Measurement methods are reviewed and followed by a discussion of the pros and cons of each. Methodologies for absolute dose verification are discussed and new IMRT QA verification tools are presented. Literature on the expected or achievable agreement between measurements and calculations for different types of planning and delivery systems are reviewed and analyzed. Tests of vendor implementations of the γ verification algorithm employing benchmark cases are presented.


Operational shortcomings that can reduce the γ tool accuracy and subsequent effectiveness for IMRT QA are described. Practical considerations including spatial resolution, normalization, dose threshold, and data interpretation are discussed. Published data on IMRT QA and the clinical experience of the group members are used to develop guidelines and recommendations on tolerance and action limits for IMRT QA. Steps to check failed IMRT QA plans are outlined.


Recommendations on delivery methods, data interpretation, dose normalization, the use of γ analysis routines and choice of tolerance limits for IMRT QA are made with focus on detecting differences between calculated and measured doses via the use of robust analysis methods and an in-depth understanding of IMRT verification metrics. The recommendations are intended to improve the IMRT QA process and establish consistent, and comparable IMRT QA criteria among institutions.


DTA ; IMRT QA ; tolerance limits; γ

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