Tolerance levels and methodologies for IMRT verification QA

Abstract

Purpose Patient-specific IMRT QA measurement is a process designed to identify discrepancies between calculated and delivered doses. Error tolerance limits are not well-defined or consistently applied across centers. To improve the understanding and consistency of this process, the AAPM TG-218 report provides recommendations for tolerance limits and methodologies in patient-specific IMRT QA. Methods The performance of the dose difference/distance-to-agreement (DTA) and gamma verification 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 was reviewed and analyzed. Different vendor implementations of verification algorithms were tested using benchmark cases. Results Operational shortcomings that can reduce the tool effectiveness and accuracy 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 were used to develop guidelines and recommendations on tolerance and action limits for IMRT QA. Conclusion Recommendations on delivery methods, data interpretation, normalization, the use of gamma analysis routines and choice of tolerance limits for IMRT QA are made. The focus is on detecting differences between calculated and measured doses using 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 criteria among institutions for IMRT QA.