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Graphical User Interface for Digital Winston Lutz Test-Based Localization of Linac Radiation Isocenter and IGRT Image Centers

W Du

W Du*, University of Texas MD Anderson Cancer Center, Houston, TX


PO-BPC-Exhibit Hall-2 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: Digital Winston-Lutz (DWL) tests have been increasingly used in radiation therapy quality assurance (QA) to replace the traditional film-based WL tests. The radiation isocenter of a linear accelerator can be determined precisely with DWL using the digital images of a ball bearing (BB) phantom and square or circular radiation fields. The DWL tests have also been used to check the spatial accuracy for on-board IGRT systems. There is a growing need of a software package for these DWL-based QA procedures.

Methods: A MATLAB graphics user interface (GUI) was developed to process the DWL images acquired on multiple linacs equipped with a variety of IGRT systems. The centers of the BB and the radiation fields were determined with subpixel accuracy using Hough transform algorithms. The radiation isocenter was located with respect to the center of the BB. The IGRT image centers were also determined relative to the center of the BB. Finally, the congruence of the image centers and the radiation isocenter was calculated for each IGRT system including OBI, EPID, CBCT and ExacTrac.

Results: The GUI computed the locations of the radiation isocenter and the image centers under one minute. Repeated DWL tests showed reproducible results of the congruence of IGRT image centers and the radiation isocenter. The reproducibility of the results indicated (1) the mechanical stability of the linacs, (2) the independence of the image centers vs. radiation isocenter congruence on the phantom position, and (3) the accuracy of data analysis. The GUI was able to process images from a Varian 2100 linac and an Elekta Versa linac.

Conclusion: The DWL-based GUI provided a platform on which the radiation isocenter of a linac was located quickly and accurately and the IGRT image centers were checked against the radiation isocenter.

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