Multileaf Collimator

1. Multi Leaf Collimator
Dr Kiran Kumar BR

2. NEED OF MLC
• To achieve the aim of radiotherapy
• Maximum dose to tumor and minimum dose to OARs
• Shaping of the beam is an important way of minimizing the
absorbed dose in healthy tissue and critical structures.

3. How to shape beam
• Collimator jaws are used for shaping a regular treatment
field; but the treatment volume is not regular
• Lead blocks or individually made Cerrobend blocks are
attached onto the treatment head under standard
collimating system.

4. MLC
A multileaf collimator (MLC) is a device made up of individual
"leaves" of a high atomic numbered material, usually
tungsten, that can move independently in and out of the path
of a beam in order to block it.

5. Material
• High density
• Hard and availability
• Low coefficient of expansion
• Readily machinable

6. • Tungsten alloy
• Pure tungsten has a density of 19.3 g/cc and brittle
• alloys have densities that range from 17.0 to 18.5 g/cc3, with
varying mixtures of nickel, iron, and copper to improve
machinability.

7. CLASSIFICATION
MLC
LEAF END SHAPE
CONFIGURATION
DIVERGENT OR
DOUBLE
FOCUSED
ROUND END
LEAF
TERTIARY
LOWER JAW
REPALCEMENT
UPPER JAW
RELACEMENT

8. CONFIGURATION

9. UPPER JAW REPLACEMENT
• This configuration entails splitting the upper jaw into a set
of leaves (Elekta)

10. ADVANTAGE
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.

11. Disadvantage
•Leaf width must be somewhat smaller and the tolerances on
the dimensions of the leaves as well as the leaf travel must be
tighter than for other configurations.
•Average radiation leakage increase

12. LOWER JAW REPLACEMENT
• The lower jaws are split into individual leaves (Siemens)

13. ADVANTAGE
• Inter & Intra leaf transmission is minimum.
• Shorter leaf length & compact head diameter
• Small travel range.
.

14. Disadvantage
Lengthy downtime if mechanical problem arise.

15. Tertiary Collimator
• MLC is positioned just below the level of the standard upper
and lower adjustable jaws (Varian).

16. ADVANTAGE
• Useful to avoid lengthy downtime
• Tolerance on leaf positioning & leaf
• Dimension is relaxed

17. Disadvantage
• Collimator will have over bulk
• Higher transmission & intermediate
• leakage characteristics

18. LEAF END SHAPE

19. ROUND END

21. DOUBLE FOCUSED

22. ROUND LEAF
• Constant beam
transmission
penumbra
• Higher transmission
when leaves abut
each other (end leaf
transmission)
DOUBLE FOCUSED
• Sharp beam cut off
at edges
• Reduce
transmission
penumbra
• Machinability is
more difficult

23. Transmission through mlc
TRANSMISSION
END LEAF
INTRA LEAF
INTER LEAF

25. TONGUE AND GROOVE EFFECT

26. LEAF TRANSMISSION
• Transmission <5%
• 4 - 5 HVL

27. For upper jaw replaced by MLC :
1. Interleaf transmission : < 3%
2. Intraleaf transmission : <2%
For Tertiary MLC :
1. Interleaf transmission : <2%
2. Intraleaf transmission : <1%
However , End leaf transmission should / will be < 15-20 %
for round leaf MLC

29. Scallopian Effect

30. MINI & MICRO
• MLCs with leaves projecting to between 2 mm and
5 mm should be considered as mini
• MLCs and those below 2 mm as micro MLCs

31. LEAF POSITION DETECTION
• Limit Switches
• Linear Encoders
• Video Optical

32. • Limit switches:
• are used in bi-state MLCs such as that developed by NOMOS,Inc.
• The open or closed state can be detected depending on which switch is turned
on by the leaf.
• Linear Encoders:
• high precision potentiometers.
• Two potentiometers are used with correlated readings.
• The advantages of using linear encoders include simple read-out, less
susceptibility to radiation damage, and good linearity and accuracy.
• The drawbacks are more wiring in the head structure, and more occupied space
in the head. Since defective potentiometers are sometimes hard to detect,
redundant ones are often required to ensure correct measurements.

33. • Video Optical:

34. • The advantages of the video-optical system include real-
time display of the leaf positions, less wiring, and high
spatial resolution.
• When using solid-state devices such as charge-coupled
device (CCD) cameras, the system also provides high
position linearity.
• However, most CCDs are not radiation resistant. Frequent
camera replacements are therefore required.

35. APPLICATIONS OF MLC
APPLICATIONS
INENSITY MODULATION
CONFORMAL
DYNAMIC MLC
STEP & SHOOT

36. STEP AND SHOOT

37. DYNAMIC MLC
Leaf A Leaf B
Position
Intensity
Continuous modulation
Delivered dose with
dMLC is directly dependent
on the precision of the gap
between opposed leaves.

38. Quality assurance
• Leaf transmission
• Leaf position accuracy
• Dosimetric leaf gap

39. LEAF Transmission
• 4 quadrant method
• 24 points measurement

40. • Perspex phantom
• PPC-40
• Suitable buildup
• Energy: Higher energy available

42. • Open air measurement
• Energy: Higher energy available
• Suitable build up
• FC-65 G

43. POSITION ACCCURACY

44. Dosimetric leaf gap
• CC13
• Perspex
• Suitable buildup
• Open reading
• Transmission through A and B
• Readings with various gaps

45. THANK YOU