IMRT Delivery Performance With a Varian Multileaf Collimator

The use of a multileaf collimator (MLC) for intensity-modulated radiotherapy poses unique dosimetric issues. The nature of intensity-modulated radiotherapy dosimetry, centered on leaf position accuracy, is common to all MLCs. However, the mechanical and software designs of MLCs from the different manufacturers distinguish them. This report focused on the Varian Millennium 120, although the concepts are applicable to the earlier Varian Mark series as well. The factors that affect dose delivery in clinical fields (i.e., mechanical tolerances, motor fatigue, and latency effects) have been quantified. Moreover, inadequate modeling of the MLC in the planning system can be perceived as erratic performance. Individually, some problems have been shown to be insignificant; others are correctable using software. If these problems are rectified or at least understood by the physicist, quality assurance can be simplified.