Multileaf collimator

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A multileaf collimator (also known as an MLC) is device used in radiotherapy for defining the shape of a treatment field.

The goal of external beam radiotherapy is to deliver a target dose to the tumour while sparing dose to healthy tissue as much as possible. In order to achieve this goal, the radiation beam must be shaped to match the shape of the tumor from the beam's eye view. As it takes about 10cm of lead or tungsten to block radiation, this shaping is not easy to do automatically. The multileaf collimator was designed to do precisely that. A standard MLC consists of 40 or more pairs of tungsten plates called "leaves". Each leave can move independently in towards in centre of the treatment field, creating an approximation of any desired shape.

The MLC was later discovered to be ideal for intensity-modulated radiotherapy (IMRT). IMRT involves optimising the precise amount of radiation that should be applied to each part of the beam in order to deliver an ideal treatment. By layering multiple MLC deliveries with different shapes atop one another, an approximation of this ideal treatment can be achieved.