Tolerance limits and methodologies forIMRTmeasurement‐based verificationQA:Recommendations ofAAPMTask Group No. 218

Abstract

Purpose: 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. Methods: The performance of the dose difference/distance-to-agreement (DTA) and c 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 c verification algorithm employing benchmark cases are presented. Results: Operational shortcomings that can reduce the c 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. Conclusion: Recommendations on delivery methods, data interpretation, dose normalization, the use of c 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.

A hybrid volumetric dose verification method for single‐isocenter multiple‐target cranial SRS

“…Pinnacle results are also solid, although for two films the passing rate slipped just below 90%. The 95% confidence intervals were 98.7–99.8% for PF and 95.2–98.4% for Pinnacle, indicating that both systems can be considered in agreement with experiment by current standards 29. It is therefore not surprising that volumetric gamma analysis comparison between the two algorithms demonstrated 100% agreement for all plans at the 3%/1 mm level.…”

“…While the recent AAPM TG‐218 report29 prescribes the error thresholds and action levels for gamma analysis comparison between measured and planned dose distributions, there is no such clear guidance for purely calculational or semi‐empirical verification. We chose to retain the 3% dose‐error threshold from TG‐218, which is also similar to the point‐dose verification recommendations for complex non‐IMRT beams 30.…”

“…It is a subject of ongoing debate,31 with the latest TG‐218 report29 acknowledging but not adjudicating the issue. We characterize the approach described in this paper as semi‐empirical or hybrid, as some of the information (MLC apertures) is derived from independent measurements, while other elements are harvested from the accelerator log files.…”

“…While the risk of a false positive findings (an error reported when there is none) is easily mitigated by measurement if necessary, a potential false negative (no error reported when there is one) is more likely to slip through. The risk of accelerator absolute calibration changes after the morning checkout has always been considered sufficiently mitigated in conventional treatments by the redundancy of the dosimetry chain, and the TG‐218 specifically recommends the IMRT QA measured dose to be normalized to the daily output29 to exclude the influence of the fluctuations, which could otherwise consume a substantial part of the error budget. Thus, the remaining weakest link in the tested PF configuration (and there are other options, not described in this work) is the lack of independent verification of the accelerator axes positions (fractional MU, gantry angle, etc.)…”

Transit and non‐transit 3D EPID dosimetry versus detector arrays for patient specific QA

“…A recent AAPM TG-218 report recommended the use of 3G2 γ-pass rate as alert indicator for patient specific IMRT QA, and proposed tolerance and action levels of 95% and 90%, respectively 33. The comparison between the measured and the planned dose distributions is typically performed by γ-analysis.…”