Mlc ppt

1. MULTILEAF COLLIMATOR
Presenter: Nipan Kalita
Intern Medical Physicist
Moderator: Shashi Bhushan Sharma
Medical physicist cum RSO
Dr. B. Borooah Cancer Institute

2. CONTENTS
1. Introduction
2. Advantages of MLC
3. Disadvantages of MLC
4. Materials and Properties
5. Basic Applications
6. Configuration of MLC
7. Collimator Geometry
8. Binary MLC
9. Field Shaping Limitations
10. Leaf End Shape
11. Transmission Requirements
12. Interleaf Transmission
13. Tongue and Groove Effect

3. INTRODUCTION
• Beam shaping devices
• Consists of two opposing banks of attenuating leaves, each of which
can be positioned independently.
• Typical MLC systems consists of 40 to 80 pairs, independently
driven.
• By using Computer controls to position a large number of narrow,
closely abutting leaves, an arbitrary shaped field can be generated .
• By setting the leaves to a fixed shape, the fields can be shaped to
conform the tumor
WHAT ARE THE MLCs?

4. ADVANTAGES OF MLC
• MLC field shaping is likely to save time
• Incur a lower operating cost when compared to the use of beam
blocks, fabrication facilities and expenses will be reduced.
• Patient setup time during treatment also decreases
• Adjustment in the field shape can be made quickly than having
to recast a new Cerrobend block.
• More conformal than jaws.

5. DISADVANTAGES OF MLC
• Island blocks cant be made.
• Positional inaccuracy of MLC may lead to high dose deviations in
IMRT or VMAT plan delivery.
• Larger Penumbra compared to conventional divergent blocks.

6. MATERIALS & PROPERTIES
• Tungsten Alloy is the material of choice for leaf construction because it has one
of the highest densities of any metal .
• The Tungsten Alloys are hard , readily machinable and reasonably inexpensive .
• The Alloys have densities that ranges from 17.0 to 18.5 g/cm3
TABLE – Properties of Tungsten alloys

7. BASIC APPLICATIONS
 The first application is to replace conventional blocking
 A second function of the MLC is an extension of the first…..
One variant of conformal therapy entails continuously adjusting the
field shape to match the be BEV projection of a PTV during an arc
rotation of the x-ray beam
 The third application is the use of the MLC to achieve beam-intensity
modulation. Variants of conformal therapy have been considered that require
each field be compensated or modulated
THREE BASIC APPLICATIONS:

8. Width : Dimension of leaf perpendicular to the direction
of propagation of the x-ray beam and direction of motion
of the leaf.
Length : It refers to the leaf dimension parallel to the
direction of leaf motion .
Leaf End : It refers to the surface of the leaf inserted into
field.
Leaf Side: It refers to the surface in contact with adjacent
leaves .
Height : It refers to the dimension of the leaf along the
direction of propagation of the primary x-ray beam.
CONFIGURATION OF MLC

9. MLC Configurations may be categorized as to whether
they are total or partial replacements of –
1. Upper Jaws
2. Lower Jaws or
3. Tertiary Collimation configuration .
CONFIGURATION OF MLC

10. 1. Upper Jaw Replacement
• Upper jaw is split into a set of leaves
• ELEKTA (Formerly Philips) MLC is
designed in this manner .
• MLC leaves move in the Y-direction (
perpendicular to the axis of rotation of the
gantry)
• A “back up” collimator located beneath the
leaves and above the lower jaws augments
the attenuation provided by the individual
leaves.

11. Advantages of upper jaw replacement-
• The range of motion of the leaves required to traverse the
collimated field width is smaller
• It allows shorter leaf length and therefore a more compact
treatment head diameter .
• Having MLC leaves so far from the accelerator isocenter
needs leaf width must be somewhat smaller .
• Tolerances on the dimensions of the leaves as well as the
leaf travel must be tighter than for other configurations .
Continued……
Disadvantages of upper jaw replacement-

12. 2. Lower Jaw Replacement
• The lower jaws can be split into a set of leaves as
well.
• SIEMENS use this configuration .
• Both leaf ends and leaf sides match the beam
divergence .
• The SIEMENS design uses 41 leaf pairs.
• The outer leaves of each leaf bank project a leaf
width of 0.5cm at isocenter.
• The inner 39 pairs projects a leaf width of 1cm at
isocenter.

13. • Speed of the leaves 1.5 cm/sec
• Double focused as the leaf ends as well as sides match
the beam divergence.

14. 3. Third level configuration (tertiary MLC)
• The VARIAN MLC is an example of a tertiary
MLC configuration
• This device is positioned just below the level
of the standard upper and the lower adjustable
jaws .
• Leaves can be manually moved out in case of
malfunction .
• Comparatively increase in size of the leaves
because of longer travel distances.
• Adds an extra bulk (30kg) (It is the major
disadvantage)
• Helpful in IMRT to cover large fields (to split
the field into two or more subfields)

15. Collimator Geometry

16. BINARY MLC
 Binary MLC provided 2
positions- open or closed
 64 leaves with 6.25mm
resolution.
 5cm×40cm maximum field
 Open close time of 20ms
 Field width varies from
5mm to 50mm.

17. FIELD SHAPING LIMITATIONS
 SIEMENS -- Maximum leaf travel = 30cm
(extension of 20 cm to the centre of the field and an
additional of 10cm across the centerline)
 ELEKTA -- Maximum leaf travel = 32.5cm
(leaves can extend 12.5cm across the field centre line)
 VARIAN -- Maximum leaf travel = 14.5cm
(distance between most extended and retracted leaf in
the same side can only be 14.5cm)
Limitations of VARIAN MLC
 Carriage to extend leaf travel across midline
 Extending the leaves out to the field centre is not
possible when large fields are used

18. Leaf End Shape
• MLCs that are double focused have flat leaf
ends that follows the beam divergence
(SIEMENS design).
• MLCs that are single focused have rounded
leaf ends ( ELEKTA & VARIAN design).
• Attenuation occurs in the rounded ends along
chords of the circle.

19. MLC Leaves Size Height Speed
ELEKTA 40*2 1 cm 7.6 cm 1.5 cm/sec
SEIMENS 29*2 (Tg -50)
41*2 (BBCI)
1 cm (27)
6.5 cm (2)
1 cm (39)
0.5 cm (2)
7.5 cm
VARIAN 60*2 1 cm (20)
0.5cm (40)
6 cm 2.4 cm/sec
DIMENSIONS OF MLCs

20. TRANSMISSION REQUREMENTS
 With upper /lower jaw
replacement: same as collimator
jaw (<1%)
 With tertiary MLC same as
customized blocks (5%; 4-5 HVL)
 Achieved by 5cm of tungsten alloy
Tansmission Interleaf
transmission
Leaf
transmission
Maximum 2% 2%
Average 0.75% 0.75%
Transmission according to AERB

21. Interleaf transmission Intraleaf transmission

22. Two situations to consider for interleaf transmission:
(1) between the sides of adjacent leaves and
(2) between the ends of the leaves.
Different leakage paths between leaves and the effect of leaf
cross-section shape on penumbra along the side of an MLC leaf.

23. Tongue and Groove Effect
• A Tongue and Groove arrangement of leaves is
used to minimize interleaf leakage.
• The leaves are so designed that their sides are
partially overlap; that is, one side of the leaf
protrudes outward (“tongue”) and the other
recesses inward (“groove”) so that the central
parts of the adjacent leaves fit like a jigsaw
puzzle.

24. CONCLUSIONS
• MLCs are the most important tools for conformal shaping of beams hence used
for almost all kind of treatment modalities like-3DCRT, IMRT, VMAT,
SRS/SRT.
• Use of MLCs in modern RT has revolutionized the Radiotherapy techniques.
• Unlike conventional techniques, these are UNIVERSAL, DYNAMIC IN USE,
EFFICIENT, COST EFFECTIVE, LESS TIME CONSUMING .
• But they do produce a larger physical penumbra than that produced by the
Cerrobend blocks.

25. REFERENCES
1. AAPM Report No-72 Task Group No 50
2. THE MULTILEAF COLLIMATOR- A COMPLETE
GUIDE by James M. Galvin, D.Sc

26. THANK YOU……