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Program Information

VMAT QA With 2D Radiation Measuring Equipment Attached to Gantry

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A Fung

A Fung*, University Medical Center, SWCC, Lubbock, TX


SU-F-T-264 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To introduce a method of VMAT QA by 2D measuring device. The 2D device is attached to the gantry throughout measurement duration. This eliminates error caused by the angular dependence of the radiation detectors.
Methods: A 2D radiation measuring device was attached to the gantry of linear accelerator. The center of the detector plane was at the isocenter. For each patient plan, two verification plans were created for QA purpose. One was like an ordinary VMAT plan, to be used for radiation delivery. The other is a plan with gantry angle fixed at zero, so the dose distribution as seen by the rotating 2D device. Points above 10% dose threshold were analyzed. Data is in tolerance if it fits within the 3 mm or 3% dose gamma criteria. For each patient, the plan was passed when 95% of all the points in the 2D matrix fit the gamma criteria. The following statistics were calculated: number of patient plans passed, percentage of all points passed, average percentage difference of all points.
Results: VMAT QA was performed for patients during one year in our department, and the results were analyzed. All irradiation was with 6 MV photon beam. Each plan has calculated and measured doses compared. After collecting one year’s result, with 81 patient plans analyzed, all (100%) of the plans passed the gamma criteria. Of the points analyzed from all plans, 98.8% of all points passed.
Conclusion: This method of attaching a 2D measuring device on the linac gantry proves to be an accurate way for VMAT QA. It is simple to use and low cost, and it eliminates the problem of directional dependence.

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