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A novel method for sub‐arc VMAT dose delivery verification based on portal dosimetry with an EPID
Author(s) -
Cools Ruud A. M.,
Dirkx Maarten L. P.,
Heijmen Ben J. M.
Publication year - 2017
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12518
Subject(s) - arc (geometry) , dosimetry , computer science , nuclear medicine , synchronization (alternating current) , image guided radiation therapy , linear particle accelerator , physics , mathematics , medical imaging , artificial intelligence , optics , medicine , beam (structure) , telecommunications , channel (broadcasting) , geometry
Purpose The EPID ‐based sub‐arc verification of VMAT dose delivery requires synchronization of the acquired electronic portal images (EPIs) with the VMAT delivery, that is, establishment of the start‐ and stop‐ MU of the acquired images. To realize this, published synchronization methods propose the use of logging features of the linac or dedicated hardware solutions. In this study, we developed a novel, software‐based synchronization method that only uses information inherently available in the acquired images. Method The EPI s are continuously acquired during pretreatment VMAT delivery and converted into Portal Dose Images ( PDI s). Sub‐arcs of approximately 10 MU are then defined by combining groups of sequentially acquired PDI s. The start‐ and stop‐ MU s of measured sub‐arcs are established in a synchronization procedure, using only dosimetric information in measured and predicted PDI s. Sub‐arc verification of a VMAT dose delivery is based on comparison of measured sub‐arc PDI s with synchronized, predicted sub‐arc PDI s, using γ‐analyses. To assess the accuracy of this new method, measured and predicted PDI s were compared for 20 clinically applied VMAT prostate cancer plans. The sensitivity of the method for detection of delivery errors was investigated using VMAT deliveries with intentionally inserted, small perturbations (25 error scenarios; leaf gap deviations ≤ 1.5 mm, leaf motion stops during ≤ 15 MU , linac output error ≤ 2%). Results For the 20 plans, the average failed pixel rates ( FPR ) for full‐arc and sub‐arc dose QA were 0.36% ± 0.26% (1 SD ) and 0.64% ± 0.88%, based on 2%/2 mm and 3%/3 mm γ‐analyses, respectively. Small systematic perturbations of up to 1% output error and 1 mm leaf offset were detected using full‐arc QA . Sub‐arc QA was able to detect positioning errors in three leaves only during approximately 20 MU and small dose delivery errors during approximately 40 MU . In an ROC analysis, the area under the curve ( AUC ) for the combined full‐arc/sub‐arc approach was 0.90. Conclusions A novel method for sub‐arc VMAT dose delivery verification with EPID s is proposed, using only dosimetric information in acquired EPI s for synchronization. Especially in combination with full‐arc QA , the established sensitivity for detection of very small errors is high, with also a high specificity.