Multi-leaf Collimator
A typical multi-leaf collimator (MLC) consists of 2 sets of 20-40 leaves, each around 5mm thick and several cm in the other two dimensions. Newer MLCs now have up to 160 leaves. Each leaf in the MLC is aligned parallel to the radiation field and can be moved independently to block part of the field. This allows the dosimetrist to match the radiation field to the shape of the tumor (by adjusting the position of the leaves), thus minimizing the amount of healthy tissue being exposed to radiation. On a machine without an MLC this must be accomplished using several hand-crafted blocks.
Read more about this topic: External Beam Radiotherapy