The experimental setup for EPID-based isocenter measurements using W-L phantom. The radiation beam is collimated by a drill hole collimator mounted on the gantry head.
A sample EPID image of the W-L phantom: the first and last points with pixel value equal to or larger than 50% of the maximum gray level in YE direction are specified as points (a) and (b). The average of these points has been used to identify where to plot the profile in XE direction.
Sample profile of an EPID image plotted in XE direction (crossplane): XC represents the central point of the field identified by 50% of the maximum pixel value on each side. The center of the ball is determined by a similar method, using the central part of the EPID image (upper part of the figure) which contains the ball shadow.
A sample output sheet for isocenter verification algorithm showing the deviations in each direction.The angles exceeding the acceptance limits for SRS/SRT are marked, warning messages have appeared along with the maximum offset angle, and variation ranges are specified in each direction. The date of the experiment is extracted from the image header.
Results of the isocenter motions as a function of gantry angle using the algorithm for continuous imaging mode in clockwise and counter-clockwise directions compared with gantry static images and the mechanical pointer measurements in (a) X direction, and (b) Y direction.
Results of the reproducibility test for cine images acquired during a 360° arc in both directions.
Analysis of radial isocenter deviations from the reference locus for three experiments carried out on: (a) the same day, and (b) three successive weeks. The lines indicate the acceptance criteria.
The effect of couch rotation on the position of isocenter relative to the reference point (gantry and couch at zero angle) for one of the tested linacs. Most error bars are smaller than the symbols used for plotting, with maximum standard deviations of 19 and 17 microns in X and Y directions, respectively.
The effect of collimator rotation on isocenter displacement for one of the tested linacs. The error bars are smaller than the symbols used for plotting with maximum standard deviations of 6 and 9 microns in X and Y directions, respectively.
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