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Is Correction for the Sag of EPID Panel Necessary for Proper Evaluation of Picket Fence Test in Dynamic Mode?

R Sadagopan

R Sadagopan1*, J Kerns2, (1) UT MD Anderson Cancer Center, Houston, TX (2) UT MD Anderson Cancer Center, Houston, TX

SU-E-T-64 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Is correction for gantry and panel sag needed to properly evaluate the QA results of dynamic PF test using EPID?

Method and Materials: A dynamic MLC picket fence (PF) test was delivered and captured by the EPID panel in continuous image acquisition mode, resulting on ~100 separate images. A MATLAB © algorithm was written to analyze the results. The pixels of each image were shifted in the x and y direction for the sag correction at the gantry angle of delivery according to the correction reported by the Varian Iso-Cal©. Corrected and uncorrected images were integrated respectively and compared against each other and stationary PF images. Images were acquired with the collimator at 0° and 90º.

Results: The profiles of corrected and uncorrected dynamic and static images across the PF pattern at certain image points were plotted. The corrected image was not closer to the static image than the uncorrected image at collimator 0. There is improvement when the collimator was rotated 90º. This was expected as the Iso-cal correction map indicated larger correction factor along the in-plane direction.

Conclusions: The correction for the panel sag is ideal but not mandatory during routine monthly as there is no improvement at collimator 0. However, it is useful with 90 º collimator rotation to include all the leaves and during annual QA and following servicing of MLC as this would eliminate the need for the jig to hold film.

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