The maximum difference between the X-ray dose delivered to any two points that are equidistant and symmetrical about the central axis and within the central 80% FWHM (full width at half measure) region, measured at 100 cm TSD (target-to-skin distance) at a depth of 10 cm.  This specification applies to the radial and transverse axes of all square field sizes from 10 x 10 cm2 to 40 x 40 cm2.