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A High Sensitivity Dosimetric Auditing Method

P Greer1*, J Lehmann1 , K Bobrowski2 , (1) Calvary Mater Newcastle, Newcastle, (2) University of Wollongong, Wollongong,


(Wednesday, 7/17/2019) 7:30 AM - 9:30 AM

Room: 303

Purpose: Dosimetric audits are an important component of clinical trial quality assurance and provide independent assessment of a centers treatment quality. However existing methods using loose assessment criteria do not provide sufficient sensitivity to detect clinically significant quality issues. The aim of this work is to develop a novel method for dosimetric auditing of treatment centers with improved sensitivity over existing methods.

Methods: The auditing methodology known as VESPA+ uses EPID images acquired at the treatment center and an image to dose-in-phantom conversion. Images were acquired for two trial IMRT plans (post-prostatectomy (PP) and head-and-neck (HN)) developed separately at 10 treatment centers to demonstrate the method. EPID converted doses were compared to the center’s treatment planning system calculation using conventional gamma criteria (3%,3mm, 3%,2mm, 2%,2mm, 2%,1mm) with 10% low dose threshold and global maximum dose criteria.

Results: The mean (standard deviation) of the gamma results for the PP plan (n=10) at above criteria for the coronal mid-phantom plane were (%) 99.9 (0.13), 99.8 (0.15), 99.0 (1.1), and 96.9 (2.7) respectively. 3D gamma results were similar with 97.3% at 2%,2mm. For the HN plan (n=7) the gamma results were lower with more variability at (%) 99.2 (1.3), 98.8 (1.9), 94.1 (11.0), 92.1 (9.4) respectively with 3D 97.2% at 2%,2mm. All centers achieved > 90% pass-rate at 2%,1mm for the PP and all but one center for the HN plan. This center showed significantly lower results than all others.

Conclusion: With audit results for 10 centers the new VESPA+ method has been demonstrated to be applicable to the use of much tighter gamma assessment criteria than previous audit methodologies. This will allow better identification of poor quality IMRT and VMAT delivery as well as future studies investigating sensitivity of in-house QA to artificially induced errors.


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