This test measures the dose in CU in the center of the image. The measurement is obtained by defining a 5 mm (configurable as CenterDoseRadius_mm) radius circle in the isoplane and calculating the average CU of the pixels within that circle, as shown below:
error = measured - expected
The center of the circle is defined with the coordinates obtained by Collimator Centering . Because a circle is being mapped onto a rectangular grid of pixels, sub-pixel variations in collimator centering can change which pixels are considered within the circle.
A pixel is considered to be inside the circle if the center of the pixel is equal to or less than the configured radius.
