Leaf-positioning accuracy. One of the most impor-tant properties of an MLC with respect to IMRT is the leaf-positioning accuracy. For IMRT deliv-
ery the leaf-positioning accuracy is even more important than for most conventional treatment techniques due to the large number of small fi eld components for an average IMRT treatment. For a typical fl uence grid with a resolution of 10 mm in
both directions a leaf-positioning error of 1 mm has to be considered as a large error. This is due to the characteristic output factor curve with a very steep gradient for small fi eld sizes. An error of 1 mm for the leaf position can cause a dose error of up to approximately 10% irradiating a 10×10-mm fi eld and therefore the maximum leaf-positioning error should not be larger than 0.5 mm. It can also be concluded that the required leaf-positioning accuracy depends on the fl uence grid resolution of the fl uence maps.