ABSTRACT
In Siemens linear accelerators, the multileaf collimator (MLC) replaces the lower (x) jaws (Figure 74.1). A variety of MLC con-gurations has been available. e earliest models (3D-MLC) had 29 leaf pairs and these were followed by models with 41 leaf pairs. e rst-generation 3D-MLC and the second-generation Optifocus™ designs both use a double-focused design as opposed to the more commonly found single-focused design. In other words, the edges of the Siemens MLC leaves are parallel to the beam divergence both parallel and perpendicular to the leaf motion direction. To accomplish this, the ends of the leaves eectively travel in an arc. is design reduces leaf end transmission penumbra and thus improves the light-to-radiation eld coincidence. Each leaf has an independent motor and drive assembly, and leaf position is detected using both a potentiometer and an encoder. e Optifocus design has increased backlash between the motor drive assembly and the pot/encoder than the 3D-MLC, potentially resulting in larger variability in leaf positions.
