Figure 1.

A demonstration of the challenge of using a multileaf collimator to track motion perpendicular to the leaf direction for conformal radiotherapy (left) and IMRT (right). If a target being tracked at time t moves perpendicular to the leaf motion (in this case ∼1.5 leaf widths for the conformal case and 1 leaf for the IMRT case) at time t+Δ, then the target motion can result in a much larger motion of some of the individual leaves. The leaves with the largest motion are light shaded, and the magnitude of motion of these leaves is shown with arrows. This problem can be further exaggerated by IMRT delivery (right), where the leaf sequencing process can result in adjacent leaves having positions several centimeters apart.