While tissue-maximum ratios (TMR) for 60cobalt treatment units have been shown to be independent of source-to-axis distance (SAD), high-energy photon beams demonstrate variations in their TMR as a function of SAD. Some authors have asserted that the distance dependence of the TMR stems from electron contamination in the beams, while others have suggested low-energy, scattered photons as the cause. Using a magnet to sweep contaminant electrons out of the photon treatment beam eliminates any variation in TMR with distance. Thus, electron contamination accounts for all of the distance dependence, and any low-energy, scattered photons behave indistinguishably like the high-energy photons.