1. Conformal Terminology
Julie Puzzonia

2. Multileaf Collimator (MLC)
Located in gantry head
Below jaws
60-80 pairs
Individual leaf widths vary by machine (1cm or less)
Made of tungsten alloy1
X-ray transmission of less than 2% (intraleaf)
Interleaf transmission less than 3%

3. Purpose Creates treatment field shapes
Alternative to blocks
Leaves can move independent of each other
Used in Step and Shoot and Volumetric Modulated Arc Therapy (VMAT)

4. Advantages of MLC versus Cast Blocks
Incorporated into head of machine and computer automated
No heavy lifting!
No entering treatment room between fields
No handling of toxic materials
Allows for segment modulation
More advanced treatment techniques
Intensity Modulated Radiation Therapy (IMRT)
VMAT
Can cool off hot spots
Protect critical structures
Lower transmission of beam
Cerrobend cast block: 7.5cm Cerrobend needed for <5% transmission1

5. Disadvantages of MLC versus Cast Blocks
More intense Quality Assurance procedures
MapCheck, ArcCheck
Limitations to MLC shapes
Tongue and groove effect
Scallop effect
Island blocking

6. Tongue and Groove Effect
MLC shape that is used to:
Limit interleaf leakage
Allow faster movement of leaves
Problems:
Poor synchronization of leaf motion can cause underdosage between overlapping adjacent leaves

7. “The MLC tongue-and-groove effect on IMRT dose distributions”
Consider the article:
“The MLC tongue-and-groove effect on IMRT dose distributions”
Previous study: Sykes and Williams (1998) showed potential for 15-28% underdosage from tongue and groove effect
Factors that matter:
Patient setup uncertainty
Co-planar vs non-coplanar beams
Number of fields
*These all contribute help to “smear” the tongue and groove effect limiting its impact on final dose delivery

8. “Smearing” Effect of increasing the number of treatment fields
Smooths out the dose distribution and field shape

9. Scallop effect
Due to the shape of the MLC, the outline of the field has scalloped appearance
Does not produce true conformal shape compared to cast block

10. Scallop effect underlap
Some areas of MLC overlap the block outline, while others underlap

11. Island Block MLC apertures cannot create island blocks easily
Treatments that heavily rely on island blocking:
TBI
Lung blocks, kidney blocks
Whole abdomen
Kidney blocks

12. References:
Khan FM. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2014.
Deng J, Pawlicki T, Chen Y, Li JS, Jiang SB, Ma C-M. The MLC tongue-and-groove effect on IMRT dose distributions. Physics in Medicine and Biology. 2001;46(4):