FIGURE 1.

The relative uncertainty in the dose for 6 MV and 18 MV photon beams as a function of Δd, Δf and ΔSSD. Uncertainties outside the user's control are held fixed as listed in Table 1 (rows highlighted in grey). The Δ’s are sampled from a uniform distribution with the limits shown on the horizontal axis. For Δd < 0.5 mm there is almost no improvement in the dose uncertainty. The same is true when the Δf and the ΔSSD are less than 2 mm. The uncertainty in the dose is dominated by factors outside the user's control under these circumstances.