Using multileaf collimator interleaf leakage to extract absolute spatial information from electronic portal imaging device images

Abstract

Electronic portal imaging devices (EPIDs) are potentially valuable tools for linear accelerator quality assurance and for measuring and analyzing geometric variations in radiation treatment delivery. Geometric analysis is more robust if referenced against an absolute position such as the isocenter (collimator axis of rotation), allowing the observer to discriminate between various setup errors and jaw or multileaf collimator (MLC) calibration errors. Unfortunately, mechanical instabilities in EPIDs make such analysis difficult. In the present work, we describe how MLC interleaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocenter perpendicular to leaf travel direction. The signal from the interleaf radiation leakage is extracted to provide a precise and accurate determination of the isocenter location in the direction perpendicular to MLC leaf travel. In the direction of leaf travel, the minimization of residuals between planned and measured leaf positions is used to determine the isocenter. This method assumes that leaf positioning errors are randomly distributed. The validity of the method for determining the angular deviation between EPID image grid lines and collimator angle and for determining the known isocenter position is experimentally established.PACs numbers: 87.53.Oq, 87.53.Xd, 87.57.NK

EPID‐based verification of the MLC performance for dynamic IMRT and VMAT

“…If the lines were not vertical, the skewness of the system was determined from the slope of the lines (α). To differentiate between the leaf bank skewness relative to the collimator and the effects of collimator angle misalignment and EPID rotation in its plane, the Radon transform 47 was applied to one of the subareas between the pickets [e.g., A 3 in Fig. 1(c)] to determine the orientation of interleaf leakages (β).…”

Investigation of the sag in linac secondary collimator and MLC carriage during arc deliveries

“…In a previous study, Samant et al (2002) measured the shift in MLC leaf banks relative to the central axis using EPID images at zero gantry angle assuming their camera-based EPID had a rigid geometry (Samant et al 2002). In practice, the EPID cannot be considered as a mechanically stable structure during gantry rotations in arc treatments (Gao et al 2007, Ansbacher 2006, Rowshanfarzad et al 2012. In addition, the detection precision was restricted to the size of EPID pixels (Yang and Xing 2004).…”