Using MLC Inter-Leaf Leakage to Extract Absolute Spatial Information from EPID Images

Abstract

Abstract: Electronic portal imaging devices are valuable for measuring and analyzing geometric variations in radiation treatment delivery. The analysis is more robust if referenced against an absolute position such as the isocentre (collimator axis of rotation), allowing discrimination between setup errors and Jaw/MLC calibration errors. Unfortunately mechanical instabilities in EPIDs make this difficult. Conventionally, investigators have extracted field edges through global thresholding, and compared treated to planned field placement using techniques such as the method of moments and normalized cross-correlation. The accuracy of these methods depends on the accuracy and consistency of the MLC and Jaw calibration as well as on various aspects of EPID performance including geometric stability and response uniformity. In this work we describe how the MLC inter-leaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocentre. The peak locations of inter-leaf radiation leakage in a portal image are extracted providing a very precise and accurate determination of the isocentre location in the direction perpendicular to the MLC leaf travel. Orthogonal (collimator rotation) portal images can provide a very accurate determination of beam isocentre in both directions. The orientation of the inter-leaf leakage can also be used to quantify mechanical inconsistencies in the EPID structure as a function of gantry angle. The image analysis of the inter-leaf radiation leakage for determining imager translation and orientation is described.