Program Information
Winston-Lutz Type QA for Single Isocenter SRS Treatment of Multiple Targets
D Litzenberg*, D Roberts , K Lam , University of Michigan, Ann Arbor, MI
Presentations
WE-RAM2-GePD-TT-5 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Therapy ePoster Theater
Purpose: We attempt to extend the Winston-Lutz test to verify the accuracy of the linac delivery system, especially the accuracy of the angular alignments of the CBCT with the coordinate system of the linac, for stereotactic radiosurgery of multiple brain metastases with a single isocenter.
Methods: We utilize the metal balls on the surface of the 113 mm radius cylindrical ISOCAL phantom to represent multiple brain metastases. The 3D coordinates of the balls are determined mechanically. A multiple field 3D conformal treatment is planned with each single 2 cm by 2 cm fields shaped by the MLC centered on a ball and planned with the imager moved to center the field on the panel. The ISOCAL phantom is driven to the planned location and CBCT is acquired with no realignment. The images of the balls in the fields are acquired during plan delivery. The CBCT coordinates of a subset of the balls were determined and the six-degree-of-freedom (6D) transformation of the planned room coordinates to the CBCT coordinates is computed. The measured displacements of the balls relative to the field center on the MV image are determined similar to the Winston-Lutz test and compared with the predicted displacements when the CBCT determined transformation is applied to the ball coordinates. This comparison measures the accuracy of the delivery when the alignment is confirmed by CBCT.
Results: The average distance of the balls from the field center is 0.89 mm. After the application of the 6D transformation the average distance between the predicted and measured position is 0.69 mm.
Conclusion: The results show that the method is sensitive to deviation of the targets from planned positions and shows that the CBCT accurately determines the transformation to align the phantom to the planned room coordinates.
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