Multileaf collimator basic concepts.pptx

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 Introduction Advantages of MLC Disadvantages of MLC Materials and Properties Basic Applications Configuration of MLC 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/cm 3 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 – Upper Jaws Lower Jaws or 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 : 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 AAPM Report No-72 Task Group No 50 2. THE MULTILEAF COLLIMATOR- A COMPLETE GUIDE by James M. Galvin, D.Sc

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