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Error Detection Comparison of EPID and MLC Log File Based IMRT QA Systems

D Defoor

D Defoor1*, M Obeidat1 , P Linden1 , N Kirby1 , P Mavroidis2 , N Papanikolaou1 , S Stathakis1 , (1) University of Texas Health Science Center at San Antonio, San Antonio, TX, (2) University of North Carolina, Chapel Hill, NC


SU-E-T-363 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In this study we will compare the ability of three QA methods (Delta4, MU-EPID, Dynalog QA) to detect specific errors.

Methods: A Varian Novalis Tx with a HD120 MLC and aS1000 Electronic Portal Imaging Device (EPID) was used in our study. Multi-leaf collimator (MLC) errors, gantry angle and dose errors were introduced into 5 volumetric arc therapy (VMAT) plans. 3D dose distributions calculated with data from the EPID and Dynalog QA methods were compared with the planned dose distribution. The gamma passing percentages as well as percentage error of planning target volume (PTV) dose were used for passing determination. Baselines for gamma passing percentages and PTV dose were established by measuring the original plan 5 times consecutively. Standard passing thresholds as well as thresholds derived from receiver operator characteristic (ROC) analysis and 2 standard deviation (SD) criteria were used.

Results: When applying the standard 95% pass rate at 3%/3mm gamma analysis 14, 21 and 8 of 30 errors were detected by the Delta4, MU-EPID and Dynalog QA methods respectively. Thresholds set at 2 SD from our base line measurements resulted in the detection of 18, 9 and 14 of 30 errors for the Delta4, MU-EPID and Dynalog QA methods respectively. When using D2 of the PTV as a metric the Dynalog QA detected 20 of 30 errors while the EPID method detected 14 of 30 errors. Using D98 of the PTV, Dynalog QA detected 13 of 30 while the EPID detected 3 of 30 errors.

Conclusion: Although MU-EPID detected the most errors at the standard 95% cutoff it also produced the most false detections in the baseline data. The Dynalog QA was the most effective when the ROC adjusted passing threshold was used. D2 was more effective as a metric for detecting errors than D98.

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