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The Limitation of 2D Fluence Measurement: An Extreme VMAT QA Using EPID and Detector Array

W Jiang*, D Cao , F Chen , V Mehta , D Shepard , Swedish Cancer Institute, Seattle, WA


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To explore the limitation of using 2D fluence measurement to perform the Volumetric Modulated Arc Therapy (VMAT) QA and compare the sensitivity between the EPID and Detector Array on an extreme case.

Methods: A structure of head-and-neck patient was selected to perform an original VMAT planning with a standard 4-degree gantry space but without restriction on the leaf motion between control points (CP). The original plan was then delivered as a QA plan on both EPID and MatriXX detector array. An in-house software was developed to read the trial file of the original plan to perform the interpolation among CPs with 1-degree gantry space on MLC shape, Jaw positions, collimator angles, and beam weights. The interpolated plan was imported to TPS and performed the calculation on the same structure with the same prescription. The DVH generated from the new plan was compared with the original plan.

Results: Using 3%/3mm Gamma analysis, the original plan passed the EPID based QA with passing rate of 99.26% in 2D fluence analysis and 99.97% in 3D log file, but failed the composite MatriXX QA (62%). The interpolated plan passed the MatriXX QA with a passing rate of 97.36% compared with beams delivered from original plan. The mean target dose in DVH of interpolated plan is higher than the one in original plan. The most percentage difference of mean dose and maximum dose to the organ at risk observed between interpolated and original plans are -2.41% and 4.98%, respectfully.

Conclusion: The 2D fluence based VMAT QA, even when combined with the log file verification, shows certain limitation on detecting the discrepancy of MLC from the large motion between the control points. The detector array measurement is more sensitive and robust on the extreme circumstance.

Funding Support, Disclosures, and Conflict of Interest: This study is supported by Elekta.


Not Applicable / None Entered.


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