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Multileaf Collimator Types

The multileaf collimator (MLC) was first used in alloy block field shaping. However, it has since been widely applied to important clinical functions such as the dose delivery of radiation therapy for cancer patients. The average MLC contains between 40 and 120 leaves that block radiation beams from reaching healthy tissue. Produced by several manufacturers, the MLC consists of three configuration types: upper jaw replacement, lower jaw replacement, and tertiary level. Each has its own benefits and drawbacks.

  1. Required Steps

    • Prior to its clinical use, multileaf collimators need to undergo a few required steps. Acceptance tests must first be conducted, for both new and upgraded multileaf collimators. This is followed by additional measurements necessary for enabling multileaf collimators to plan treatments. The establishment of a quality assurance program is required to ensure effective function of the complete system, and finally, multileaf collimators require monitoring of their effect on meter unit calculations.

    Upper Jaw Replacement

    • Collimators made by Elekta are designed with this configuration. It involves replacing the upper jaw of the MLC with a set of leaves that move in the direction parallel to the gantry axis of rotation. A second collimator is present underneath the leaves to support their reduction effects. Upper jaw replacement configurations have a smaller range of motion, providing the advantage of using shorter leaves. However, the shorter leaves lie further away from the accelerator isocenter, making leaf travel tighter compared to other configurations

    Lower Jaw Replacement

    • In this configuration used by Siemens and Scanditronix Racetrack Microtron, the lower jaws are split into multiple leaves. The straight leaves travel from the full open position at a speed of 1.5 cm per second, and are independently operated. The ends and sides of the leaf match up to the beam divergence, hence the configuration is double-focused.

    Tertiary MLC

    • The tertiary system is located slightly below the standard upper and lower adjustable jaw levels. This avoids extensive downtime, as leaves are able to move manually in the event of system failure. This design does, however, provide extra bulk to the MLC, and slightly affects clearance to the isocenter. The tertiary system is used in collimators designed by Varian.

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