The maximum variation from the mean dose delivered 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. The mean is the average of the maximum and minimum points within the central 80% FWHM region. The specification of +2. 5% applied to both the radial and transverse axes of all square field sizes from 20 x 20 cm2 to 40 x 40 cm2, inclusive. A specification of +3. 0% applies to all square field sizes between 10 x 10 cm2 and 20 x 20 cm2, and for 20 MV for all square field sizes larger than 30 x 30 cm2.