Multileaf Collimator

Vinay DesaiM.Sc Radiation Physics

KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY

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Atomic structureFactors affecting quality and quantity of X-ray

beamRadiation quantities and Units.Linear Accelerator

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.

• 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.

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.

MLC

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 – 7.5 cm

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.

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

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.

Upper jaw replacement

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.

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.

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

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.

• 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

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

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)

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)

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.

Properties of tungsten alloy (manufacturer values)

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).

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%

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.

• 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

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

CLINICAL APPLICATIONSLeaf 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 strategy2. In field strategy3. Cross-boundary technique.

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.

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 MLC1. Dynamic technique: Continuous movement of leaves during the

treatment2. Step and shot technique: leaves moves when radiation is stopped.

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.

• 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:

• 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)

• 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.

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).

Thank you

Vinay Desai

Radiation Physics DepartmentKIDWAI MEMORIAL INSTITUTE OF ONCOLOGY

BengaluruE-mail:- vinaydesaimsc@gmail.com

Other PPT’s on slideshare.net

Atomic structureFactors affecting quality and quantity of X-ray

beamRadiation quantities and Units.Linear Accelerator