The OBI hardware mounted on a Varian 2100 EX linear accelerator. When facing the accelerator with the MV beam pointing down, the kV tube is on the left side, the kV detector is on the right side, and the electronic portal imaging device is at the bottom.
(a) Cube phantom with four fiducial markers on the anterior and later surfaces, (b) printed circuit board, and (c) marker block with one fiducial marker at the center and four markers at known locations inside the block.
Results of OBI isocenter accuracy QA over eight months measured on a daily basis for the first four months and a weekly basis for the following four months, with a total of 60 measurements. The QA test was performed using a cube phantom. The standard deviations in the measurements were and for the longitudinal and lateral directions, respectively, demonstrating the mechanical stability of the system over the one-year measurement period. The empty diamond, , indicates the incident that the phantom was mispositioned more than . The X mark indicates the incident that the wall laser was miscalibrated by . The solid circle marker, , indicates the incident that the KVD position was off by and recalibration was performed as a result.
Results of 2D2D match accuracy measurements over an eight-month period on a daily basis for the first four months and on a weekly basis for the following four months, with a total of 60 measurements. Positive in the vertical, longitudinal, and lateral directions means that the 2D2D match shift disagrees with the expected shift in the anterior, inferior, and right directions.
Results of magnification accuracy measurements over eight months on a monthly basis with a total of eight measurements. Positive in the vertical direction means the measured distance was longer than the expected distance. Positive displacement in the longitudinal and lateral directions means that the center of the KVD or KVS was displaced to the inferior and right direction from the isocenter.
The kV images of the center BB in the cube phantom with the gantry at , , , and . The images were obtained during the QA performance.
Results of OBI isocenter accuracy QA over gantry rotation over a four-month period performed on a weekly basis with the total of 11 measurements. The vector displacement was measured between the digital graticule and the center marker in the kV image.
OBI image quality QA. Contrast resolution using fluoroscopic mode (1-a) and radiographic mode (1-b). Spatial resolution using fluoroscopic mode (2-a) and radiographic mode (2-b). Fluoroscopic images and radiographic images were taken in different institutions.
Sample images for (a) contrast resolution QA and (b) spatial resolution QA.
Sample images for (a) spatial linearity QA and (b) slice thickness QA. The thickness QA required measurements of the separation between two slices. (b) shows the top wire in the upper slice with the right end of the measure tool at the right end of the wire and in the lower slice with the left end of the measure tool at the right end of the wire.
List of test items for QA of OBI safety and functionality. All items are checked daily (see Appendix A).
The HU linearity QA using full-fan and half-fan CBCT scans. The average HU values over the four-month measurements with the standard deviation (SD).
The HU uniformity QA using full-fan and half-fan CBCT scans. The average HU values over the four-month measurements with the standard deviation (SD).
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