Accuracy of the Burns equation for stopping-power ratio as a function of depth and R50
The accuracy of the Burns et al. equation [Med. Phys. 23, 489-501 (1996)] for the Spencer-Attix water to air stopping-power ratio as a function of depth in a water phantom and electron beam quality in terms of R50 is investigated by comparison to the original data on which this fit was based. It is shown that using this equation provides dose estimates on the central axis in a clinical electron beam that are accurate to within 1% of dose maximum for all 24 clinical beams investigated except very close to the surface in swept beams. In contrast, the error in the dose as a percentage of the local dose is much higher for values of the depth/R50 greater than 1.2.