Multileaf collimator

A multileaf collimator (MLC) is a device made up of individual "leaves" of a high atomic numbered material, usually tungsten, that can move independently in and out of the path of a particle beam in order to block it.
MLCs are used on linear accelerators to provide conformal shaping of radiotherapy treatment beams. Specifically, conformal radiotherapy and Intensity Modulated Radiation Therapy (IMRT) can be delivered using MLC's.