What Is a Multileaf Collimator?

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  • Written By: Andrew Kirmayer
  • Edited By: Shereen Skola
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  • Last Modified Date: 18 January 2020
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A multileaf collimator (MLC) is a medical machine used to adjust a beam of radiation into the shape of a tumor or other abnormality. Connected to a linear accelerator, the machine consists of collimators, or movable jaws, that shape the radiation field. Sets of shields called leaves, manufactured from tungsten metal, act as blocks to focus the beam into the desired shape. The device also comes in the form of a micro multileaf collimator (mMLC), suited for treating irregularities that are about three inches (about eight centimeters) across. It also has a slightly smaller leaf diameter than the full sized system; computers are used in both types to detect and control the position of the leaves.

Parts of the multileaf collimator include a control system for the leaves, the mechanics that position the leaves, and position sensors. A video-optical system may be installed to visually show where each leaf is located on a computer screen. Position detection is also aided by limit switches and linear encoders, while linear motion systems are used in conjunction with the motors for precise positioning of leaves. Calibration is necessary for accurate readings between the leaf positions and the signals generated by the positioning equipment.


Multileaf collimators can be especially beneficial because they help focus radiation on tumors, at the exact dosage needed. They shape a radiation beam and control the dose so that healthy tissue, such as that in the lungs, spinal cord, or eyes, is not affected. In targeting a specific area, a multileaf collimator can send multiple beams of radiation to a site so that the intensity does not cause damage to healthy parts of the body. Radiation detectors are also included in the machine; in the case of the mMLC, a specific type of film is used to measure the dose, how the radiation is transmitted, and whether it leaks.

The mechanical operation of a multileaf collimator is complex, with motion of the different components controlled on different axes. An operator uses computer software to monitor the position of each part and to control the precise movements that are necessary. The software also provides a graphical representation of the radiation field, so the operator knows exactly where the radiation is and can adhere to safety guidelines from the machine’s manufacturer. All of the functions of the machine, and common configurations, can be controlled according to the exact shape of the area that needs to be treated.


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