SU‐E‐T‐129: An EPID‐Based Method for Testing Accuracy of MLC and Backup Jaws

Purpose: Current methods for QA of MLC leaf position are limited to measurements relative to an arbitrary reference position. There also are no reported methods for testing accuracy of backup jaw position as recommended by AAPM Task Group 142. The purpose of this project was to develop an efficient method for testing accuracy of MLC and backup jaws relative to the mechanical isocenter. Methods: A set of four EPID images is used to determine the position of backup jaws and MLC leaves relative to the mechanical isocenter. The first pair of images are of a rectangular field that is rotated 180 degrees between the two images to locate the isocenter. The third image, consisting of three field strips defined by backup jaws, allows for determination of the position of the backup jaws across its range of travel relative to a line passing through the mechanical isocenter and parallel to the jaws. The fourth image, a set of three strips defined by the MLC, allows for determination of the position of each leaf relative to the isocenter reference line determined from the backup jaw image. A software routine was developed to efficiently analyze the images. The accuracy of the test results was validated with water phantom scans. Results: The test can be completed in less than 10 minutes. The software tool reliably and automatically reports, over a 20‐cm range of travel, jaw setback for each leaf, leaf position and jaw position deviation from nominal, and provides visual displays to facilitate rapid qualitative interpretation. The test was used to guide adjustment of MLC and backup jaw calibration of two matched linacs. Conclusion: This work demonstrates a method to efficiently and accurately measure MLC leaf and backup jaw position relative to the mechanical isocenter of the collimator.