Brief introduction to MLC (Multileaf collimator) of Linear accelerator used for conformal radiotherapy.

1. Multileaf Collimator
Vinay Desai
M.Sc Radiation Physics
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY

2. Other PPT’s on slideshare.net
Atomic structure
Factors affecting quality and quantity of X-ray
beam
Radiation quantities and Units.
Linear Accelerator

3. Shielding and Shaping of OAR’s
• Shielding of vital organs within a radiation field is one of the major
concerns of radiation therapy.
• Considerable time and effort is spent in shaping fields to protect
critical organs, also to avoid unnecessary irradiation of the
surrounding normal tissue.
• Many organs are relatively sensitive to radiation damage (spinal
cord, salivary glands, lungs, and eyes) must be given special
consideration during radiotherapy treatment planning.
• In general, treatment planners attempt to optimize the dose
distributions achievable with a given treatment strategy to deliver a
tumorcidal dose of radiation to a target volume while minimizing
the amount of radiation absorbed in healthy tissue.

4. • A conventional treatment machine shapes x-ray fields by a set of
dense metal collimators (jaws) built into the machine.
• Conventional beam shaping is accomplished through the use of a
combination of these collimator jaws and secondary custom beam
blocks attached to the accelerator beyond the collimator jaws.
• Conventional blocks consist of either a set of lead blocks having a
range of shapes and sizes that are placed by hand at each
treatment session or Cerrobend blocks fabricated individually for a
given field applied to a specific patient.
• Beam blocks have several inherent disadvantages.
• The use of low-melting temperature blocks is time-consuming and
involves the handling of Wood’s metal (Cerrobend), a toxic
material.

5. MULTILEAF COLLIMATORS (MLC)
• MLC has replaced the beam blocks in field shaping.
• The MLC has movable leaves, or shields, which can block some
fraction of the radiation beam.
• Typical MLCs have 40 to 120 leaves, arranged in pairs.
• By using the computer controls to position a large number of
narrow, closely abutting leaves, an arbitrarily shaped field can be
generated.
• By setting the leaves to a fixed shape, the fields can be shaped to
conform to the tumor.
• Given adequate reliability of the hardware and software, the use of
MLC field shaping is likely to save time and to incur a lower
operating cost when compared to the use of beam blocks.

6. MLC

7. Materials and properties
• The material of leaf construction is ‘tungsten
alloy’ (High density).
• Thickness = 6 – 7.5 cm
• Density of tungsten alloy 17 - 18.5 g/cm3.
• Primary x-ray transmission:
Through the leaves < 2%.
Interleaf transmission < 3%.
• Tungsten alloys are also hard, simple to
fashion, reasonably inexpensive, and have a
low coefficient of thermal expansion.
6–7.5cm

8. Description of leaf:
• Width of a leaf-small dimension of the leaf perpendicular to the
direction of propagation of the x-ray beam and perpendicular to the
direction of motion of the leaf.
• Length of the leaf- leaf dimension parallel to the direction of leaf
motion.
• Leaf end-Surface of the leaf inserted into the field.
• Leaf sides -Surfaces in contact with adjacent leaves
• Height of the leaf-Dimension of the leaf along the direction of
propagation of the primary x-ray beam.

9. MLC Configurations
• MLC configurations may be categorized as to whether they are
total or partial replacements of,
• Upper jaws,
• lower jaws, or
• Tertiary collimation configurations.
ELEKTA SIEMENS VARIAN

10. Upper jaw replacement
• In this configuration the upper jaw is split into
a set of leaves. (used by Elekta)
• MLC leaves move in the Y-direction (parallel
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.
• The back-up diaphragm is essentially a thin
upper jaw that can be set to follow the leaves
if they are arranged together to form a
straight edge, or else, set to the position of
the outermost leaf if the leaves form an
irregular shape.

11. Upper jaw replacement

12. Advantage:
• The range of motion of the leaves required to traverse the
collimated field width is smaller,
• It allows for a shorter leaf length and therefore a more compact
treatment head diameter.
Disadvantage:
• 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.
• Leaves of this collimator have total travel distance 32.5 cm, which
means they can extend 12.5 cm across the centre line.

13. Lower jaw replacement
• The lower jaws can be split into a set of
leaves as well. (Siemens) and is double-
focused.
• Both leaf ends and leaf sides match the
beam divergence.
• The collimator leaves move along the
circumference of a circle centered at the x-
ray target of the linear accelerator, such that
the end of the collimator is always
tangential to the radius of the circle.
• There are no backup jaws.

14. •The leaves of Siemens MLC can extend 10cm across the field
centerline, which allows a maximum leaf travel of 30 cm.

15. Tertiary collimation configuration
• MLC are positioned just below the level of the
standard upper and lower adjustable jaws
(Varian).
• This avoids the lengthy downtime in the event
of a MLC system malfunction.
• It is possible to move leaves manually out of
the field should a failure occur.
• The treatment can be continued by using
‘Cerrobend’ individual blocks.

16. • Advantages:
• Allows retro-fitting of MLCs on existing units.
• Leaves can be manually moved out in case of system
malfunction/failure and treatment continued using customized
blocks.
• Allows larger leaf width; easier manufacturing.
• Easier Leaf positioning / Lesser positional accuracy needed.
retro-fitting set of MLC
Tertiary collimation configuration

17. Disadvantages
• Added bulk and clearance to the mechanical isocenter.
• Requires increasing the leaves size and a longer travel distance as
MLCs are moved further away from the x-ray target (Divergence
effect).
• The leaves in the Varian collimator travel on a carriage that serves
to extend their movement across the field.
• The distance between the most extended leaf and the most
retracted leaf on the same side can only be 14.5 cm.
Tertiary collimation configuration

19. MLC Leaves Size
Elekta 40 x 2 1cm
Siemens 29 x 2
1cm(27)
6.5cm(2)
Varian standard 26 x 2 1cm
Varian standard 40 x 2 1cm
Varian Millennium 26 x 2 1cm
Varian Millennium 40 x 2 1cm
Varian Millennium 60 x 2
1cm(20)
0.5cm(40)

20. Field shaping limitations
• SIEMENS – Max leaf travel = 30 cm
(extension of 20 cm to the center of the field and an
additional 10 cm across the centerline).
• ELEKTA – Max leaf travel = 32.5 cm
(Leaves can extend 12.5cm across the field
centerline).
• VARIAN – Max Leaf Travel = 14.5 cm
(distance between the most extended leaf and the
most retracted leaf on the same side can only be 14.5
cm).
Limitatons Of Varian MLC
 Carriage to extend leaf travel across midline.
 Extending the leaves out to the field center is not
possible when large fields are used.
 Leaves on one side of bank can interdigitate with
neighboring leaves on opposite bank.
(island blocks can be created)

21. Attenuation specifications
• leaf transmission: The reduction of dose through the full
height of the leaf.
• Interleaf transmission: The reduction of dose measured along
a line passing between leaf sides.
• Leaf end transmission: Reduction of dose measured along a
ray passing between the ends of opposed leaves in their most
closed position.
• Pure tungsten density- 19.3 g/cm3.
• Tungsten alloy densities- 17.0 to 18.5g/cm3 (mixture of
nickel, iron, and copper to improve machinability).
• Pure tungsten is very brittle and the machinability of tungsten
alloy improves with decreasing tungsten content.

22. Properties of tungsten alloy
(manufacturer values)

23. Transmission Requirements
 With upper / lower jaw replacement, same as collimator jaw (0.5%; <1%)
 With Tertiary collimation, same as Customized blocks (5% or 4-5 HVL);
 Achieved by 5 cm of Tungsten alloy.
 Interleaf transmission (between sides, between ends).

24. TYPE OF MLC Leaf transmission Interleaf transmission
ELEKTA 1.8% (6MV)
4.1% (6MV)
2% (20MV)
4.3% (20MV)
SIEMENS 1.0% (6MV) -
1.1% (20MV)
-
VARIAN 1.5-2.0% (6MV) +0.25-0.75%
2.0% (15MV +0.25-0.75%
1.5-2.5% (18MV) +0.25-0.75%

25. Leaf End Shape
• Rounded leaf ends (Leaf ends as part of
circumference of circle) (ELEKTA, VARIAN)
• Attenuation occurs in the rounded ends
along chords of the circle.
• Penumbra is of the same width, regardless
of position of leaf in beam
• Constant beam transmission through leaf
end, regardless of position of leaf in beam.

26. • Multileaf collimators that are double focused (Siemens design) have
flat leaf ends that follow the beam divergence.
• The leaf ends of Elekta and Varian MLC design are rounded.
• Attenuation occurs in the rounded ends along chords of the circle.
concerns of non-focused leaf ends
1. Penumbra width is larger than the penumbra generated by a focused
or divergent edge.
2. Penumbra width might change as a function of the distance of the
leaf end from the field midline.
3. Elekta and Varian configurations a little variation in the penumbra
width as a function of leaf position and,
4. The penumbra at any position is within 1-3mm of that obtained with
a focused system or with alloy blocks with divergent sides.
Leaf End Shape

27. Tongue and groove effect
• A tongue-and-groove arrangement of leaves is used to minimize
interleaf leakage.
• This can cause under dosage in step-and-shoot and dynamic MLC
delivery of IMRT, called ‘tongue-and-groove’ under dosage (TGU)
• TGU occurs when two MLC segments abut.
• The amount of ‘tongue-and-groove’ effect is related to the length of
the shared borders, and the field intensities received by the two
segments

28. CLINICAL APPLICATIONS
Leaf Placement Strategies
a) Definition of target area:
• Treatment planning system facilitates shaping leaves around PTV, as
defined by a radiation oncologist.
b) Optimization of MLC conformation
• Three leaf coverage strategies can be used to place the leaves of
MLC in conformity with the target contour shape automatically.
• Each strategy uses different position of the leaf in relation to the
contour of the field that needs to be irradiated.
1. Out of field strategy
2. In field strategy
3. Cross-boundary technique.

29. 1. The “out of field” strategy (a) avoids shielding any part of the PTV,
which is than irradiated completely.
2. In the “in field” strategy (b), PTV is not irradiated completely, but
any part out of PTV stays shielded.
3. Cross-boundary technique (c) One condition for optimizing the
leaf positions was criterion that the in-field area was equal to the
out-of-field area.

30. c) Optimization of collimator rotation:
• leaf shape to target volume is achieved by rotating the collimator
and the direction of leaf travel.
d) Intensity modulated radiotherapy (IMRT) with multileaf
collimator:
• Precise dose delivery on any part of treated area avoiding the
surrounding healthy tissue.
• MLC for IMRT - very precise, motion of leaves must be fast and
constant.
• Two strategies of IMRT with MLC
1. Dynamic technique: Continuous movement of leaves during the
treatment
2. Step and shot technique: leaves moves when radiation is stopped.

31. Quality Assurance procedure for Dynamic
Multi-leaf Collimator
• Static MLC checks:
• To verify the leaf position accuracy and leaf position repeatability
for static MLC positions.
• Test performed using calibrated front pointer and taping a piece of
graph paper to the couch top.
 leaf position accuracy with field opening of leaf positions of 5cm,
-10 cm (bank A of MLC),
-10 cm and 15 cm (bank B of MLC) field opening with MLC (Varian
test pattern).
 leaf position repeatability by marking the actual leaf position on
the graph paper and executing various standard test patterns
provided by Varian in auto cycle model.

32. • Picket fence test and the garden fence test :
• to check the stability of the dMLC and reproducibility of the gap
between leaves.
• These test patterns can be performed using I’matriXX device or film.
• The picket fence test:
• Consists of eight consecutive leaf movements of a 5-cm wide
rectangular field spaced at 5-cm intervals;
• The field information is contained in three separate test files which
are run in sequence at the accelerator treatment console.
• The test field can be exposed using I’matriXX at 94.6 cm SSD placed
over treatment couch with 5cm solid water build up (detector plane
at 100 cm).
DMLC checks:

33. • The garden fence test:
• Consists of a narrow band (2 mm wide) spaced at 2-cm intervals.
• Each leaf match line can be analyzed either visually or by measuring
the full-width half-maximum distance.
 MLC positional error should be within ±0.5mm
 Band space should be within limit.(±1mm)

34. • Treatment delivery verification or TPS plan verification:
• In TPS, the dose distribution generated for chair test and
pyramid test can be exported to verification phantoms for
quantitative evaluation with I’mariXX/film dosimetry systems.
• Measured and TPS calculated dose distribution patterns for
both the tests are measured.

35. ADVANTAGES of MLC:
– Time for shaping and inserting of custom blocks is not required.
– The hardening of beam, scattered radiation, and increase in skin
doses and doses outside the field, as seen with physical
compensators is avoided.
– Automation of reshaping and modulation of beam intensity in
IMRT.
– MLCs can also be used as electronic compensators
– Allow Intensity Modulated Radiation Therapy (IMRT).

36. Thank you
Vinay Desai
Radiation Physics Department
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
Bengaluru
E-mail:- vinaydesaimsc@gmail.com

37. Other PPT’s on slideshare.net
Atomic structure
Factors affecting quality and quantity of X-ray
beam
Radiation quantities and Units.
Linear Accelerator