Multileaf Collimators & IMRT QA

Multileaf Collimators Rina TaurisiaMedical PhysicistMRCCC Siloam HospitalsOverviewBEFORE MLCs:Using jaws to shape x-ray fields (rectangular shapes)Creating custom beam blocks for a given field applied to a specific patientDISADVANTAGE OF BEAM BLOCKS:Time consumingExposure to toxic fumes during melting timeHeavy prone to patients & RTTs accidents & injury

OverviewModern MLC was used in radiotherapy starting 1980s, first developed in Japan. Philips (now Elekta) and Varian introduced the commercially available MLC in 1990s in Europe and in the USA. Main functions of MLC:To define any field shapes for radiation beamTo create beam modulationMay replace collimators or additional to normal collimators

Ideal MLCNegligible leakageSmall dosimetric penumbraFine leaf widthMaximum leaf speed and good acceleration / deceleration (Dynamic)Excellent mechanical accuracy, stability and precision

MLC Configurations

Example of MLC leaf with a curved end and a stepped side

MLC Configurations

MLC configuration may be categorized as:Total replacementPartial replacementof the upper jaws, the lower jaws, or are Tertiary collimation configuration

MLCDIAPHRAGMLower JawsMLCUpper JawsMLCLower JawsUpper JawsExit windowExit windowExit windowisocenterisocenterisocenterElektaSiemensVarianMLC Configurations

ElektaSiemensVarianMLC Configurations

MLCDIAPHRAGMLOWER JAWSMLCUPPER JAWSUPPER JAWSLOWER JAWSMLCExit windowExit windowExit window37.342.650.927.837.935.753.544.41) Upper Jaw Replacement

LOWER JAWUPPER JAW MLC

9ElektaMLC system replaces the upper movable jaws inside the linac headMLC distance is close to the target minimal range of motion required for precise field shapingAdvantages:Less wear and tear on the leaf positioning mechanismCompact linac head at 62 cm diameter reduce pontential for gantry and table collisions when non-coplanar beams are usedDisadvantage:MLC leaves are far from isocenter leaf width must be smaller and tolerances on the dimensions of the leaves as well as the leaf travel must be tighter compared to other configurations

ElektaMLC consists of 40 pairs of tungsten alloy leaves with 7.0 cm thickness (height) and each leaf projecting a 1.0 cm width and 32.5 cm length at isocenter single focus with a rounded endThe same computer controls the MLC, the leaves, and the backup jaws backup jaws automatically follow to the edge position of the outermost withdrawn leafMax. field size = 40 x 40 cm (for conventional treatment)Each leaf can travel 12.5 cm over the beam central axis max. field size for IMRT = 25 x 40 cmIMRT is delivered with step-and-shoot technique

2) Lower Jaw Replacement

SiemensMLC system replaces the lower movable jaws inside the linac headThe 82 leaf MLC has 39 pairs of inner leaves with a 1.0 cm width and two pairs of outer leaves with a 0.5 cm width cover full 40 cm IMRT field lengthThe 58 leaf MLC consists of 27 pairs of inner leaves with a 1.0 cm width and two pairs of outer leaves with a 6.5 cm width.Both MLCs leaf are 31 cm long at isocenter, with a straight edge facing toward the center of the beamA double-focus leaf design the end and the side of the leaves follow the beam divergence in both direction (along and perpendicular to the leaf motion)

SiemensBecause of the complex design for an arc trajectory, the leaf position is accurate only to within 2 mmEach leaf can travel a max distance of 15 cm over the beam central axis may limit IMRT field to 27 cmThe upper jaws must be positioned no more than 0.5 cm beyond the boundary of the MLC shape.Max. field size = 40 x 40 cm (conventional treatment)The new 160-MLC is a single focused design, consists of 80 pairs, each 9.5 cm thick (high) with a projected leaf width of 5 mm at isocenter half the size of previous predecessors

3) Tertiary Collimation System

VarianMLC system is a tertiary system mounted below the lower movable jaws inside the linac headAdvantage: reduce downtime during system malfunctionDisadvantage: added bulk to the linac head prone to gantry & table collision for non-coplanar treatmentMLC consists of 26, 40, or 60 pairs of tungsten alloy leaves with 6 cm thickness (height)For the Millennium 120 MLC, there are 40 inner pairs of leaves, with each leaves projecting 0.5 cm width, and 20 outer pairs of leaves, with a 1.0 cm width at the isocenterEach leaf is 16 cm long and single focus, with a rounded end leaf motion is along a straight line in a plane perpendicular to the beam central axis

VarianMechanical design of the MLC is simpler, with a leaf positional accuracy of 1 mm at the isocenterThe rounded end is used to ensure that the leaf end transmission is nearly independent of the leaf postionWith two MLC leaf carriages, max. conventional field size is 40 x 40 cm & travel range of each leaf is 15 cm from the end of the carriageThe carriage can be retracted up to 20 cm from the beam axis and can travel up to 2 cm beyond the axis leaf positions can vary between 20 cm away from the isocenter to 17 cm beyond the isocenter

VarianBecause the leaf length is 16 cm, the distance between the most leading and the most retracted leaves from the same bank is limited to 14.5 cm to avoid radiation leakage through the tail of the most leading leaf or the tip of the most retracted leafMLC supports all types of IMRT delivery: segmental, dynamic, combined dynamic & segmental in the same field, and conformal arc.During the delivery, the MLC leaf positions are controlled by the MLC control computer, whereas the total MU is controlled by console computer.

InterdigitationDefinition: the property whereby the tips of neighboring leaves on opposed MLC banks are allowed to pass one anotherVarian: full interdigitation is allowedSiemens: leaf tips from the opposing MLC banks are allowed to meet, but not pass each otherElekta: leaf tips from opposing leaf banks must remain 1 cm apart

Over-travel DistanceDefinition: the distance that each leaf passes over the isocenter, without leaving an uncovered regionSiemens: 10 cmElekta: 12.5 cmVarian: 17 cm*

Leaf End ShapeFlat edge: Less penumbra, butComplex mechanical arrangement to keep the flat edge of a conventional collimator aligned with the x-ray sourceRounded edge:Simpler mechanismThe curvature allows some penetration of the beam through the edge of each leaf, but this is the same for all field sizesPenumbra: the distance between the points receiving 80% and 20% of the dose on the beam axis

PenumbraMLC design with divergent and non-divergent leaves produce penumbra within 2% of each otherDistance of the collimator from the x-ray target plays an important role Varian has the smaller width despite its rounded leaf-end design

A dosimetric comparison of various multileaf collimators Huq et.al. (Phys. Med. Biol. 47, 2002)MLC Transmission

Leaf transmission:Reduction of dose through the full height of the leafInterleaf transmission:Reduction of dose measured along a line passing between leaf sidesLeaf end transmission:Reduction of dose measured along a ray passing between the ends of opposed leaves in their most closed position

Inter-leaf LeakageSiemens performs slightly better than others for inter-leaf leakage

A dosimetric comparison of various multileaf collimators Huq et.al. (Phys. Med. Biol. 47, 2002)

Tongue & Groove EffectSiemens produces reduced tongue & groove effect compared to the other two collimators

A dosimetric comparison of various multileaf collimators Huq et.al. (Phys. Med. Biol. 47, 2002)

Paper conclusion:There is no perfect MLC system that can be recommended. Each one has unique advantages & disadvantages that should be weighed with comfort, ease and cost effectiveness for clinical use.Summary

ElektaSiemensVarianDeliveryStaticStaticStatic / DynamicNo. of Leaves8082 or 16080 or 120Leaf width @ iso10 mm41 x 10 mm80 x 5 mm40 x 10 mm40x10 mm + 20x5 mmThickness (cm)7.57.56Transmission (%)1.8 2.50.9 1.251.6 1.9FocusSingle; rounded endsDouble; flat endsSingle; rounded endsMax. Field Size (cm)40 x 4040 x 4040 x 40Overtravel (cm)12.510 or 2020*Speed (cm/s)223Head to isocenter clearance (cm)4542.8 (includingfixed block tray)41.5

PROSCONSVARIANInterdigitization improves efficiency of dose deliveryInterdigitization reduces the MU, patient doseMLC closer to skin improves penumbraRobust designAdded bulkClearance to the mechanical isocenterLeaf-end leakageRounded MLC worsen penumbraNo back-up collimator following MLC edgeDifficult light-to-rad calibrationSIEMENSSmall intra and inter-leaf leakageNo need for backup collimatorsRecord & Verify each segmentExact light-to-rad calibrationLow leakageMechanical complexityLower leaf precisionOverall MLC robustnessBulky head

ELEKTASmaller and lighter MLCLess leaf movement needed, shorter MLCsCompact treatment headRobust design

Large magnification of MLCLeaf-end leakageRounded MLC worsen penumbraTighter tolerance of leaf dimensions and travelDifficult light-to-rad calibrationReferencesAAPM Task Group #50 Radiation Therapy Committee. (2001). Basic Applications of Multileaf Collimators. Madison: Medical Physics.Huq, M. S., Das, I. J., Steinberg, T., & Galvin, J. M. (2002). A dosimetric comparison of various multileaf collimators. Phys. Med. Biol., 47, N159N170.Loverock, L. (2007). Linear Accelerator. In Mayles, P., Nahum, A., and Rosenwald, J. C. (Eds.). Handbook of Radiotherapy Physics: Theory and Practice. (pp. 218 225). New York: Taylor & Francis.Mundt, A. J., & Roeske, J. C. (2005). Intensity Modulated Radiation Therapy: a Clinical Perspective. Hamilton: BC Decker.University of California, San Francisco. (January 28, 2009). IMRT dose planning and delivery with Siemens, Varian and Elekta. Retrieved November 4, 2011, from http://radonc.ucsf.edu/links/Video/Physics/.../OM_IMRTtalk.pdf