Requirements for leaf position accuracy for dynamic multileaf collimation

Intensity modulated radiation therapy can be achieved by driving the leaves of a multileaf collimator (MLC) across an x-ray therapy beam. Algorithms t o generate the required leaf trajectories assume that the leaf positions ar e exactly known to the MLC controller. In practice, leaf positions depend u pon calibration accuracy and stability and may vary within set tolerances. The purpose of this study was to determine the effects of potential leaf po sition inaccuracies on intensity modulated beams. Equations are derived which quantify the absolute error in delivered monito r units given a known error in leaf position. The equations have been verif ied by ionization chamber measurements in dynamically delivered flat fields , comparing deliveries in which known displacements have been applied to th e defined leaf positions with deliveries without displacements;applied. The equations are then applied to two clinical intensity modulations: an in verse planned prostate field and a breast compensating field. It is shown t hat leaf position accuracy is more critical for a highly modulated low-dose intensity profile than a moderately modulated high-dose intensity profile. Suggestions are given regarding the implications for quality control of dy namic MLC treatments.