WE-D-351-06: Automated Quality Assurance Test for MLC-Based Stereotactic Radiosurgery

Abstract

Purpose: We present an automated method of Linac mechanical quality assurance (QA) for MLC‐based Stereotactic Radiosurgery. The test is designed to determine 3D deviation of the Linac beam central axis defined by the center of the collimator aperture defined by the MLC leaves from the machine isocenter defined by the small tungsten sphere target. Method and Materials: The automation is achieved using Portal Imager as a 2D detector. Based on the approach of Winston‐Lutz Alignment Test, we monitor the alignment of the isocentrically‐placed tungsten sphere target, mounted on a metal stem, with the center of the MLC‐defined small square field. Multiple projection images are acquired for a specified set of gantry angles. After acquisition, the EPIDimages are saved in DICOM format by the imager's software for subsequent automated analysis on a Windows‐based PC running MATLAB®. Results: The center of the sphere deviated by 0.75 ±0.17 mm (1SD) from the MLC defined field center. The lateral, longitudinal and vertical sphere displacements were [0.66±0.17, 0.20±0.18, 0.217±0.13] mm, respectively. The average extent of the MLC motion in the direction parallel to the leaf travel was within 0.2 mm maximum span, while the carriage motion in the perpendicular direction ranged from 0.2 mm for 5×5cm2 field to 1 mm motion for 2×2cm2 field. All results were automatically processed using GUI. Conclusion: Our test is capable of sub‐millimeter sensitivity to the MLC leaf position and carriage position, as well as gantry sag and carriage shift due to the effect gravity at multiple gantry angles. The system employs a graphic interface, which is intended to facilitate the frequent clinical use of the presented QA technique. Conflict of Interest: This work is partially supported by Varian Medical Systems (PaloAlto, CA).