Multileaf collimator

 
The multileaf collimator was developed to replace the traditional blocks. 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.The MLC has movable leaves, which can block some fraction of the radiation beam, typical MLCs have 52-160 leaves, arranged in pairs.

 

 

The MLC has several advantages compared with conventional field shaping:
1. Time for shaping and inserting of MLC is not required, reshaping and modulation of beam intensity in IMRT is an automatic process. The patient’s anatomy can be followed easily during the treatment planning. Patient setup time may also decrease.
2. One variant of conformal therapy entails continuously adjusting the field shape to match the planning target volume during the arc conformal therapy. The position of the leaves have to change during the beam is being delivered.
3. Variants of conformal therapy have been considered that required each field be compensated or modulated. The motion of the MLC leaves during irradiation create dynamic compensating filter.
Cost of production can be saved, don’t need any special place for storage and the manpower is not necessary for block insertion. Using the MLC is reduced the set up errors, RTTs can’t forget the block insertion.
The using of MLC also has some disadvantage:
1. Island blocking is not possible
2. The purchase price of MLC is, high amortization of leaf and carriages cause the relatively higher service costs while conventional blocks are cheaper.
3. During the treatment planning different type of x-ray transmission should be considered (through the leaves < 2%, interleaf transmission < 3%, and for jaws <1%.)

MLC configuration may be categorized as to whether they are total or partial replacements of the upper, the lower jaws, or else are tertiary collimation configuration.

 
Upper jaw replacement – this configuration entails splitting the upper jaw into a set of leaves. The primary advantage of the upper jaw replacement configuration is that the range of motion of the leaves required to traverse the collimated field width is smaller, allowing for a shorter leaf length and therefore a more compact treatment head diameter. The disadvantage: leaves are so far from the isocenter and the leaf width must be smaller and the leaf travel must be tighter than for other configuration. Lower jaw replacement –the lower jaws can be split into a set of leaves. The leaf ends and the leaf sides match the beam divergence, making the configuration double-focused. Third level configurations – this device are positioned just below the level of the standard upper and lower adjustable jaws. This design can reduce the downtime in the event of a system malfunction. Using this system, it is possible to move leaves manually out of the field should a failure occur. In this case the MLC is farther from source, therefore the size of leaves is increased and the travel distance to move from one side of the field to the other is longer.
Schematic of the MLC as shown in next figure:

 

Literature

1. Mould, R.F.: Radiotherapy Treatment Planning, Med. Phys. Handbooks 14. 1985.
2. Dutreix at all.: Physics for Clinical Radiotherapy, Booklet No.3., ESTRO, Leuven- Apeldoom 1997
3. Mageras, G.S.: Experience with Computer-Controlled Accelerator Systems, Teletherapy: Present and Future, Proceedings of the 1996 Summer School, American Association of Physicists in Medicine, 1996.
4. Kutcher, G.J., Georg, D.: Description of MLC, ESTRO-Teaching Course, Santorini 1998.
5. Purdy, J.A.: Virtual Simulation and 3-D Conformal Radiation Therapy Planning, AAPM/IOMP International Course on Radiation Therapy Physics, Cluj, 1999

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