Multileaf collimators: modern beam shaping

One of the most important aspects of therapeutic radiation delivery is beam shaping. The most common technology currently used to shape x-ray beams is the multileaf collimator or MLC.

120 leaf multileaf collimator.

MLC's consist of dozens of independently moving slats, or leaves, which can collimate the beam into nearly arbitrary shapes. MLC's were first introduced in the 1960's in Japan as a means of replacing conventional blocks in 3D conformal radiation therapy (3DCRT). Today they are standard on most therapy linacs and enable modern techniques based on intensity modulation.

Key advantages of MLC's include:

• Dynamic movement of leaves during delivery allows for intensity modulated fluence patterns not achievable with conventional blocks.
• Finite set of possible aperture shapes results in a reasonable solution space for inverse planning optimization for IMRT, VMAT, etc.
• Significantly more convenient than cutting custom blocks for every field.

Drawbacks of MLC's include:

• Large number of leaves increases possibility of mechanical failure (i.e. reliability issues).
• Non-zero inter-leaf leakage.
• Smoothness of shaping dependent on size of leaves and speed of leaf motors.

Almost everyone can agree that MLC's have been a huge advance in radiation therapy, with the advantages far outweighing the disadvantages. I plan to discuss MLC alternatives in a future post.


Image courtesy of Varian Medical Systems, Inc. All rights reserved. (source)