Feasibility of Time-Resolved DVH Based Treatment Delivery Verification of VMAT with EPID
P Greer1*, R David2, J Wolf1, (1) Calvary Mater Newcastle, Newcastle, (2) CCLHD, Gosford, NSW, Australia, Gosford, AU
Presentations
WE-D-TRACK 1-6 (Wednesday, 7/28/2021) 2:00 PM - 3:00 PM [Eastern Time (GMT-4)]
Purpose: EPID presents a potentially efficient and comprehensive option for verification of VMAT with no setup time. However dosimetric verification of VMAT with EPID is usually performed using integrated EPID images therefore the time-resolved dose delivery is not verified. Moreover gamma based analysis is difficult to interpret for the clinical significance of any errors. This work investigates the feasibility of performing time-resolved dose-volume histogram (DVH) based analysis of VMAT with EPID.
Methods: Feasibility was assessed with a 2-arc (200 degrees rotation) VMAT lung SBRT plan modelled as 40 subarcs of 10 degrees. Dose in flat phantom for each subarc at 10 cm depth, gantry zero, was calculated with Eclipse V15.6.05. Image frames at 13 frames per second were collected during in-air delivery by a framegrabber computer with in-house acquisition software and interpolated to form 10 degree subarc images. These were then converted to dose at 10 cm depth in flat phantom using an in-house EPID to dose conversion model and dose ratio (DR) matrices to Eclipse dose generated. These DR matrices were backprojected through the original patient CT dose using ray-tracing to give a measured-weighted dose. Dose volume histogram comparisons were then performed.
Results: The 2D dose maps (n=40) were evaluated with traditional planar gamma evaluation methods with gamma pass rates (mean±SD) of 99.9±1.1% at 2%,2 mm criteria. The DVH curves generated from measured-weighted dose were virtually indistinguishable with the original planned DVH.
Conclusion: This study successfully demonstrated feasibility of time-resolved DVH based verification of VMAT with EPID. Using EPID subarc measurements during VMAT the DVHs are reproduced.
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