Basic Principles in Clinical Application Of Multileaf Collimator

Basic goal of radiotherapy treatment is the irradiation of the target volume while minimizing the amount of radiation absorbed in healthy tissue .Shaping the beam is an important way of minimizing the absorbeb dose in healthy tissue and critical structures.Conventional collimator jaws are used for shaping a rectangunal treatment field; but ,as usually treatment volume is not rectangunal , additional shaping is required.On a linear accelerator ,lead blocks or indivisually made Cerrobend blocks are attached onto the treatment head under standard collimating system.

VIETNAM NATIONAL UNIVERSITY, HA NOI VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS  Nguyen Ngoc Trang BASIC PRINCIPLES IN CLINICAL APPLICATION OF MULTILEAF COLLIMATOR Submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Physics (International Standard Program) Supervisor: Nguyen Xuan Ku, MSc HA NOI June, 2017 ACKNOWLEDGMENT This thesis would not have been possible without the support of many individuals who generously shared their wisdom and time with me First of all, I would like to express my gratidude to Master Nguyen Xuan Ku, my thesis committee chairman His input and guidance proved invaluable in the development of my ideas into a successful thesis project This thesis truly could not have been completed without him I would also like to thank Msc Kieu Thi Hong for this suggestions and for helping me learn along the way I am very grateful to all of the staff of at Military Central Hospital 108 and Phu Tho Hospital for sharing their knowledge with me and for always helping me find the answers to my endless questions Finally, I would like to thank my friends and my family for their great support I could not have done this without them The biggest thank you goes to my roomates who never left my side and through his encouragement enabled me to keep going Student, Nguyen Ngoc Trang LIST OF ABBREVIATION 2D Two Dimensions 3D Three Dimensions 3-DRTP Three dimensions radiation treatment planning BEV Beam’s Eye View DRR Digitally Reconstructed Radiograph DVHs Dose volume histograms GE General Electric HVL Half - value Layer IMRT Intensity Modulated Radiation Therapy LCD Liquid crystal display miniMLCs Miniature Multileaf Collimator MLC Multileaf Collimator MLCPPS Multileaf collimator prescription preparation system MU Moniter Unit PTV Planning Target Volume QA Quanlity Assurance R&V Recorded and Verify SAD Source-axis distance SCD Source-collimator distance TPS Treatment planning system TABLE OF CONTENTS INTRODUCTION CHAPTER 1: MULTILEAF COLLIMATOR DESIGN 1.1 Leaf Design -3 1.2 Summary of Configurations 1.2.1 Upper Jaw Replacement -4 1.2.2 Lower Jaw Replacement -5 1.2.3 Third Level Configuration -7 1.2.4 Field-Shaping Limitations -7 1.3 Attenuation -9 1.3.1 Material and Properties 1.3.2 Transmission Requirements 10 1.4 Interleaf Transmission 10 1.5 Leaf End Shape 12 1.6 MLC Control Features 14 1.7 Leaf Position Detection - 14 1.7.1 Linear Switches 14 1.7.2 Linear Encoder 14 1.7.3 Video-optical system 14 1.7.4 Driving Mechanism - 15 1.7.5 Calibration of MLC Leaf position - 15 1.7.6 The Control of Back-up Jaws 15 1.8 Summary of MLC Configurations - 16 1.9 Nonconventional MLCs 16 1.10 Computer System Configurations for MLC Leaf Prescription - 18 CHAPTER 2: MONITOR UNIT CACULATIONS - 19 2.1 The Physics of In-Air Photon Scatter - 19 2.2 MLC replaces the Upper Jaws in the Secondary Collimator - 19 2.3 MLC replaces the Lower Jaws in the Secondary Collimator 20 2.4 MLC as Tertiary Collimator 21 CHAPTER 3: MLC ACCEPTANCE TESTING, COMMISSIONING, AND SAFETY ASSESSMENT - 22 3.1 Acceptance Testing and Commissioning - 22 3.1.1 Mechanical Acceptance and Commissioning Tests - 22 3.1.2 Leaf Positioning with Collimator Rotation 22 3.1.3 Leaf Positioning with Gantry Rotation - 23 3.1.4 Coincidence of Light Field and X-ray Field - 23 3.1.5 Leaf Transmission - 24 3.1.6 Penumbra 25 3.1.7 Dosimetric Parameters - 25 3.1.8 Interlocks and File Transfer - 26 3.2 Safety Assessment - 26 CHAPTER 4: QUALITY ASSURANCE (QA) 27 4.1 Port film/Light Field Checks - 27 4.2 Record and Verify (R&V) Computer Checks 27 CHAPTER 5: CLINICAL APPLICATIONS - 29 5.1 Leaf Placement Strategies 29 5.1.1 Definition of Target Area 29 5.1.2 Optimization of MLC Conformation 30 5.2 Techniques to Determining Leaf Positions - 31 5.3 Optimization of Collimator Rotation 32 5.4 Intensity Modulated Radiotherapy (IMRT) with MLC - 33 CONCLUSION - 35 REFERENCES 36 LIST OF FIGURE Figure 1: Multileaf Collimator of Varian Millenium MLC120 Figure 2: MLC leaf design illustrating the leaf terminology Figure 3: MLC and standard collimators positions in treatment head use in third level configurations The field dimensions in the plane at isocenter are indicated Figure 4: Schematic diagram of the Philips MLC The upper jaw is replaced by the MLC leaves Figure 5: Schematic diagram of the General Electric Medical Systems MLC that replaces the lower jaws Figure 6: A comparison of the leaf travel configurations of commercially available MLCs The maximum leaf extensions are compared to a 40X40 cm maximum field size Figure 7: End view of the Siemens MLC showing the truncated pie shape of the leaves as well as the leaf side shape to reduce interleaf transmission 11 Figure 8: Illustration of different leakage paths between leaves and the effect of leaf cross-section shape on penumbra along the side of an MLC leaf 12 Figure 9: Rounded leaf ends and their influence on penumbra based on the position in the field SAD is the distance from the source to the isocenter and SCD is the distance from the source to the center of the leaf R is radius of curvature of the leaf end 13 Figure 10: Illustration of video-optical method to determining leaf position 15 Figure 11: Schematic of MLC prescription preparation workstation and it relationship to other parts of the treatment planning and delivery system employing MLCs 18 Figure 12: Three leaf coverage strategies in relation to the PTV, (a)”out-of-field strategy; (b)”in-field” strategy, (c)”cross-boundary” strategy 30 Figure 13: IMRT technique with use of MLC 33 INTRODUCTION Basic goal of radiotherapy treatment is the irradiation of a target volume while minimizing the amount of radiation absorbed in healthy tissue Shaping the beam is an important way of minimizing the absorbed dose in healthy tissue and critical structures Conventional collimator jaws are used for shaping a rectangular treatment field; but, as usually treatment volume is not rectangular, additional shaping is required On a linear accelerator, lead blocks or individually made Cerrobend blocks are attached onto the treatment head under standard collimating system Another option is the use of multileaf collimator (MLC) MLC were developed and have become a standard feature of new linear accelerator for radiation therapy They consist of two banks of 40 to 160 opposing movable leaves or shield, each leaf under individual motor control (see Figure 1) The leaves are positioned to form the field or aperture shape of the treatment beam Each leaf blocks a portion of radiation beam Figure 1: Multileaf Collimator of Varian Millenium MLC120 The hazards involved with lead-alloy blocks are eliminated Shielding preparation time and storage space is reduced As the positioning of the leaves is under computer control, the field apertures can be set remotely from outside the treatment room Thus treatment times can be reduced, and treatment with larger numbers of fields becomes feasible The treatment aperture shape can also be easily modified if necessary without having to manufacture new blocks Intensity modulated radiotherapy (IMRT), the therapy of the future, is based on the dynamic use of MLC The aim of this thesis is to provide basic information and to state fundamental concepts needed to implement the use of a multileaf collimator (MLC) in the conventional clinical setting MLCs are available from all the major therapy accelerator manufacturers The use of MLCs to replace conventional field-shaping techniques is not in itself expected to improve the local control of malignancy The rationale for using MLCs in conventional radiation oncology is to improve the efficiency of treatment delivery Thus, the intent of this thesis is to assist medical physicists, dosimetrists, and radiation oncologists with the acquisition, testing, commissioning, daily use, and quality assurance (QA) of MLCs in order to realize increased efficiency of utilization of therapy facilities There are three basic applications of the MLC 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 beam’s eye view (BEV) projection of a planning target volume (PTV) during an arc rotation of the x-ray beam (Takahashi 1965) The third application is the use of the MLC to achieve beam-intensity modulation These latter two applications of the MLC are advanced forms of conformal therapy and will not be considered in detail in this thesis The content of the thesis includes five chapters: Chapter 1: Multileaf Collimator Design Chapter 2: Monitor Unit Calculations Chapter 3: MLC Aceptance Testing, Commissioning and Safety Assessment Chapter 4: Quanlity Assurance (QA) Chapter 5: Clinical Applications CHAPTER 1: MULTILEAF COLLIMATOR DESIGN 1.1 Leaf Design An illustration of the MLC leaf design is shown in Figure The width of a leaf will be the 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 The width of the leaf determines the resolution of the field shape The length of the leaf shall refer to the leaf dimension parallel to the direction of leaf motion The surface of the leaf inserted into the field along this dimension is leaf end The surfaces is contact with adjacent leaves are leaf side The height of the leaf inserted Figure 2: MLC leaf design illustrating the leaf terminology into the field along this dimension is the leaf end The surfaces in contact with adjacent leaves are the leaf sides The leaf sides are divergent to the source to minimise penumbra The height of the leaf shall refer to the dimension of the leaf along the direction of propagation of the primary x-ray beam The leaf height extends from the top of the leaf near the x-ray source to the bottom of the leaf nearest the isocenter The height of the leaf determines its attenuation properties The reduction of dose through the full height of the leaf will be referred to as the leaf transmission The reduction of dose measured along a line passing between leaf sides will be referred to as interleaf transmission, and the reduction of dose measured along a ray passing between the ends of opposed leaves in their most closed position will be referred to as the leaf end transmission Figure 3: MLC and standard collimators positions in treatment head use in third level configurations The field dimensions in the plane at isocenter are indicated 1.2 Summary of Configurations MLC configurations may be categorized as to whether they are total or partial replacements of upper jaws, lower jaws or as tertiary collimator configurations 1.2.1 Upper Jaw Replacement This configuration entails splitting the upper jaw into a set of leaves Currently the Elekta (formerly Philips) MLC is designed in this manner (see Figure 4) In the Philips design, the 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 A different approach used by Galvin (1999) to define a half-beam or centre blocked field where one leaf bank is positioned at the zero position Film is placed at the isocentre distance with appropriate buildup The film is exposed and then the collimator rotated throughh 180 degrees and the exposure repeated The film is scanned and dose homogeneity across the abutment region demonstrates correct calibration of the leaf position whereas a low density region means the leaf is positioned too far into the field This test is repeated for the opposing leaf bank 3.1.3 Leaf Positioning with Gantry Rotation To measure the variation in the x-ray beam position due to gantry position a similar test is performed The x-ray film is positioned perpendicular to the machina axis of rotation and a slit field is formed with the MLC parallel to the axis of rotation The short dimension of the field passes through the film forming an image These images are formed at gantry angles every 45 degrees and should intersect within a circle of small (~ 1.0 mm) radius Similarly Galvin (1999) uses a centre blocked field to perform this test exposing the film on end and then rotating the gantry by 180 degrees and repeating the exposure 3.1.4 Coincidence of Light Field and X-ray Field The coincidence of the light field and the x-ray field is verified This is done by setting a known MLC field size such as 10x10 cm and marking the estimated light field border (50%) on a film placed at isocentre The film is then irradiated with this field size and the radiation field border estimated The light field border and radiation field border are compared to ensure that the difference is within the allowed tolerance This is done for all x-ray energies of the accelerator A measurement of the isodose line that corresponds to the leaf and light field setting can be performed (Klein et al., 1995) This test consists of two exposures on a film placed at isocentre distance In the first exposure the leaves from one carriage are brought to the desired position (i.e the A leafs brought to the zero position) The opposing B leaves are retracted to a position to give a 10 cm field size For the second half to the exposure the positions of the leaves are reversed, i.e the B leaves are brought to the zero position and the A leaves retracted to give a 10 cm field abutting the previous The dose profile across the junction is found and 23 normalised to the cm off-axis position The maximum or minimum isodose level found at the zero position For example a 120% value implies a 60% isodose value for each leaf bank at the zero position The test is repeated for the junctioning positions over the entire range of the leaf travel The values are likely to be higher than 50% as the tip of the leaf end defines the light field and leaf position whereas the 50% isodose line will be through a certain thickness of the leaf Similarly the tests can be performed for the leaf sides, where the effect of the tongue and groove will modify the isodose line from the light fielld projection location 3.1.5 Leaf Transmission The transmission through the leaves is specified as the transmission dose reading under the leaves divided by the dose reading in the centre of an open 10x10 cm field expressed as a precentage A measurement of this transmission can be made with an ion chamber or by film dosimetry A typical ion chamber techique involves placing the chamber at the isocentre distance (100 cm) at the centre of a 10x10 cm field formed with the secondary collimator jaws with the MLC fully retracted Several ion chamber readings are acquired at this point for a fixed number of monitor units and averaged to yield the open field (100%) transmission reference The MLC leaves are then driven so that they fully extend over the region that was previously the open field area with the secondary jaws remaining in the previous position A film is exposed with a large number of monitor units to record the transmission through the MLC leaves This film identifies by the region of highest optical density the peak transmission location The ion chamber is then placed at this location and several readings acquired for the same number of monitor units used above The averaged reading divided by the reference reading multiplied by 100 gives the transmission which must be less then the specified tolerance The test must be repeated for the opposing bank of leaves To perform the same test with film the reference 100% transmission is acquired in the same conditions as above but with film placed at isocentre distance exposed to the monitor unit setting The MLC leaves are then extended across as above and a new film placed underneath This film is exposed to a much larger number of monitor units Both films are scanned with a densitometer to determine 24 the relative optical densities The optical density at the centre of the reference field is taken as 100% level while the peak density conversion curve the relative dose to the films can be determined As the relative dose delivered is known the transmission can be calculated to ensure that it is within tolerance Galvin (1999) recommends repeating this test at the four cardinal gantry angles to ensure that any shift of leaves due to gravity is detected 3.1.6 Penumbra To quantify the penumbra, beam profiles are measured at appropriate depth (e.g 10 cm) in a water phantom with a small detector Film placed in solid water could be used provided the accuracy of the film profiles has been verified by comparision with other ion chamber measurement (Galvin et al., 1993a) This can be done for different field sizes and positions of the MLC field edge across its range of travel The penumbra for the field edge defined by the MLC leaf ends and the leaf sides is compared The 80% to 20% penumbra should not vary by a large amount (e.g less than 2mm) for different positions of the leaves The stepped field edge penumbra when the MLC define a 45 degrees field edge can be measured with film, and a densitometer with a small spot size In general though the literature information (for the same manufacturer) on these field edge distributions is utilised due to the lack of sufficient resolution densitometers in many radiation therapy departments 3.1.7 Dosimetric Parameters When introducing a MLC for clinical field shaping the monitor unit calculation when a MLC is used must be verified The monitor unit calculation for sample MLC field shapes should be verified by measurement with an ion chamber in water phantom The Varian MLC, placed as a tertiary collimator below the secondary jaws has been found to influence the dose similarly to alloy blocks The phantom scatter factor depends on the MLC field shape whereas the collimator scatter factor MLC depends on the secondary jaw setting The monitor unit calculations incorporating the MLC in this manner can be verified for representative field shapes For the Elekta design both the phantom scatter and collimator scatter and collimator scatter factors have been found to depend on the 25 MLC setting and again the accuracy of the moniter units calculated with this approach for MLC field settings must be verified 3.1.8 Interlocks and File Transfer All safety interlocks assoiated with the MLC must be tested for correct operation These interlocks ensure that when the MLC is defining the field shape that the backup secondary jaws are in the correct position to minimise leakage Also the interlocks ensure that a leaf is not inserted into the treatment field when the leaves should be retracted The faithful transfer, storage and formation by the MLC leaves of an MLC shape entered at the shaper station or derived from a treatment planning system must be verified The tests performed for the commisioning of the MLC must be done at regular intervals to ensure that the MLC continues to operate safety and within the accepted tolerances 3.2 Safety Assessment The assessment of safety with accelerators and associated devices is tested only minimally in a manufacturer’s acceptance procedures Additional safety tests are warranted because of the increased complexity of an MLC The use of multiple, conformed MLC fields in either static or dynamic modes will render the conventional use of visual inspection as a daily verification of field shapes impractical or impossible Active interlock checks should be carried out for leaf and jaw positional tolerances These measurements should include assessment of software interlocks, hardware interlocks, and other possible independent systems Nonactive interlocks designed to prevent unauthorized motions should be tested These would include procedures such as dynamic imaging of field shape, motion enable power line interrupt, etc Communication link interlocks are provided to ensure that the heavy data traffic that flows between the control computers and the accelerator hardware is not corrupted Means of intentionally corrupting the data should be carefully discussed with the manufacturer Tests should be devised to demonstrate that the interlocks are functioning to detect true positive data errors Interlock checks to ensure the software will not allow a trailing edge of a leaf to be unshielded by the jaws must be performed 26 CHAPTER 4: QUALITY ASSURANCE (QA) 4.1 Port film/Light Field Checks MLC leaf position files can be created by either digitization methods or by a direct generation of leaf positions by 3-D radiotherapy treatment planning (3DRTP) system The files are eventually transferred over a network system (or by disk transfer) to the MLC controller and workstation at the treatment machine Prior to use, each field should be compared with the original simulation film or DRR A match of light field and original shaped field should be within mm for all boundaries If the field is drawn on the patient’s skin or immobilization device, the light field of the field shaped by the MLC should also project at all boundaries to within mm Some facilities may opt to forego simulating MLC fields (in the case of a field reduction) and treat directly with the MLC-shaped field In this case, a port film should be acquired and approved before continuation of the treatment The jaw settings and field name must also be checked Inappropriate jaw settings could block a portion of the desired field, or generous settings could leave a trailing leaf region unblocked Once the fields and relevant information (patient, field name) are checked, the MLC files may then be released for treatment Electronic portal imaging devices are particularly useful for quickly and conveniently checking MLC-shaped fields 4.2 Record and Verify (R&V) Computer Checks Commercial information management systems are becoming available that offer a module designed to check the MLC fields These systems will facilitate the selection and transfer of the correct patient and field to the machine control computers The systems may also check the individual leaf settings versus actual positions The physicist must then assign tolerance levels for leaf position accuracy A tolerance of 0.5 mm is a minimum for the MLCs with widths on the order of cm For the nonconventional MLCs with leaves on the order of a few millimeters, the maximum allowable tolerance should be about 0.5 mm One institution that has an “eavesdrop” MLC Record and Verify (R&V) system found average deviations of 0.6 mm over approximately 10,000 histories (Mageras et al 1994) This system works as follows For MLC fields planned on 27 the 3-D planning computer, treatment planning software generates an MLC file containing the leaf positions This file is copied to the MLC computer and to an independent disk directory for use by the R&V system The therapists enter the treatment prescription that includes the MLC file name and the beam name for each field into the R&V system before the first treatment Before daily treatments, the therapist selects the appropriate patient and field from the MLC computer and from the R&V system The MLC is physically positioned based upon the settings in the MLC computer, although manual adjustments are permitted When an attempt is made to turn the beam on, the R&V system acquires the physical leaf positions from the treatment machine It then compares the setup values to those found in the MLC file for this field (as defined in the prescription) If there is a discrepancy, a failure occurs Also, if either the MLC file name or the beam name is not found, a failure occurs Patients not planned with the 3-D planning computer are handled differently An MLC file is copied to the MLC computer, but is not copied to R&V At the time of treatment setup, R&V acquires the settings for each field and creates an MLC file to be used for all subsequent treatments Modern commercial R&V information systems integrate the MLC fields as part of the treatment fields, avoiding the need for interaction by the accelerator operator with an additional MLC workstation Using computer files instead of physical blocks is a process that must be implemented with care Physical blocks are identified with printed labels, whereas MLC field-shape files are identified with file names Since R&V information management systems can be used to link specific MLC field-shape files to identify treatment fields, individual MLC files not need to be selected each time a patient is treated Using these systems, the MLC file is automatically loaded into the MLC controller when the identified field is selected within the R&V system Once the link between the treatment field and the MLC field-shape file has been verified, the field can be treated for the prescribed number of fractions with reasonable certainty that the correct shape will be used each time For this reason it is recommended that R&V systems be implemented whenever MLCs are used 28 CHAPTER 5: CLINICAL APPLICATIONS 5.1 Leaf Placement Strategies To realize potential benefits of MLC, it is important that its use is in corporated into treatment planning process as efficiently as possible During the treatmeant planning process, manual placement of each of the 40120 leaves is not acceptable due to time constraints Therefore some automated method must be used in a treatment planning system (TPS) That way in TPS, the position of each leaf is defined so that the field encompasses the planning target volume (PTV) More specifically, the determination of the MLC positions is carried out by means of the following steps: 5.1.1 Definition of Target Area Treatment planning system facilitates shaping leaves around PTV, as defined by a radiation oncologist An accurate definition of PTV is crucial for the success of the therapy MLC leaf positions have been based on a variety of criteria These optimization criteria can be categorized as geometric and dosimetric Rotation and translation of the collimator are often required for the best conformation The best collimator angle can be set automatically by an algorithmic search through all the possible angles, or it can be set manually The multileaf collimator prescription preparation system (MLCPPS) must consider all the physical constraints of the MLC system so that the prescribed leaf positions can be delivered Interactive manual adjustments of individual leaves and other parameters are often necessary Geometric methods align each leaf with the continuous contour of the portal aperture or with the projection of the PTV (ICRU 1993) as indicated on a simulation film or DRR by a radiation oncologist The determination of the target volume is, of course, critical to the success of the therapy The MLC should then be set to define the treated volume (ICRU 1993) It is essential that a clear understanding exist of the interpretation and significance of the contour to which the MLC leaves are set The target area is defined based on the prescription image 29 For conventional radiation therapy, the prescription image is the simulation film and the physicians draw field prescriptions directly on films Figure 12: Three leaf coverage strategies in relation to the PTV, (a)”out-of-field strategy; (b)”in-field” strategy, (c)”cross-boundary” strategy 5.1.2 Optimization of MLC Conformation To determine the optimal position of the leaves automatically with a computer algorithm, several treatment machine-dependent characteristics must be made known to the algorithm, such as the number of leaves, their widths, travel limits, source-to-MLC distance, and relative leaf travel direction Then the MLC (and jaws) may be placed relative to the target contour shape Three leaf coverage strategies that have been used are illustrated in Figure 12 In this figure the leaves are shown shaded and placed relative to the desired effective treatment field contour The three classes of strategies are the “out-of-field” strategy illustrated in panel (a), the “in-field” strategy illustrated in panel (b), and “cross-boundary” strategies typified in panel (c) Each strategy uses the intersections of the effective field contour with the projections of the trajectories of the sides of the ith leaf The out-of-field strategy avoids shielding any part of the projected treatment volume This strategy has been recommended as being the most conservative because it avoids shielding any part of the treatment volume Tighter coverage than the continuous aperture occurs when the in-field strategy illustrated in Figure 12 by panel (b) is used This approach is conservative 30 with respect to normal structures that abut the treatment volume It may be useful for 3-D multiple field techniques when other fields are added, where the isodose lines in the BEV plane for a single field shift outward The most widely used methods are cross-boundary techniques indicated in panel (c) of Figure One condition for optimizing the leaf positions was criterion that the in-field was equal to the out-of-field area 5.2 Techniques to Determining Leaf Positions The prescribed field shape and the MLC leaf positions are normally displayed for verification of correct MLC conformation It is also desirable to have dosimetric information overlaid on the prescription image If the MLCPPS contains a raster digitizer, it can be made to serve as a film dosimetry system (Du et al 1994) Isodose contours can be calculated from in-phantom film measurements of the MLC-conformed field at the prescription depth and overlaid onto the digitized simulator film image Fast BEV isodose calculation can also be incorporated into the MLCPPS to provide instant evaluation of MLC prescriptions It may also be desirable to verify the stepped field shape of MLC during treatment simulation A few models of MLC simulation have been proposed (Karlsson 1994) One approach is to project the MLC field shape using an LCD (liquid crystal display) device placed in the light field during treatment simulation Physical leaves have also been constructed for the simulator to set the desired leaf positions (Klein et al 1995) There may be occasions when cerrobend blocks must be used when the field shapes are too complex for the MLC to conform without introducing gross errors A Y-shaped field is such an example Field-splitting (i.e., division of a field into simpler sub-fields) is necessary if the MLC is required to completely replace cerrobend blocks for beam shaping The MLC flexibility in field shaping seems to make this easily applicable Good mechanical calibration of the MLC is crucial to the field-splitting approach because the border regions of the sub-fields, unlike the penumbra region, are highly sensitive to the precision of the leaf position calibration Further study on the effects of patient motion between sub-fields is required to ensure the safety and accuracy of this approach 31 Regardless of the automatic technique used, the MLC aperture shape may not be logical when evaluated by the treatment planner It is generally necessary to adjust individual leaves to ensure target coverage in a critical region or to avoid small critical structures that may be close to a target volume If this is not possible, alloy shaping may be the best solution A manual leaf adjustment facility should be provided using the BEV technique The projections of the leaves should be overlaid on the original simulation film or on a DRR such that individual leaves can be repositioned according to the judgment of the treatment planner Where BEV dose distributions and dose volume histograms (DVHs) are available, leaves can be adjusted based upon actual coverage of target and normal tissues This involves the manual adjustment of leaves in the BEV plane while the isodose distribution is updated, aided by DVHs and surface dose distributions Ideally, this adjustment would be done automatically by the treatment planning computer This approach requires a rather sophisticated treatment planning system with extremely fast computational capabilities 5.3 Optimization of Collimator Rotation One can optimize matching the leaf shape to target volume by rotating the collimator, and therefore, the direction of leaf travel An example is the alignment of the leaf faces with the cord axis when the cord is near the target Geometric relationships based upon target shape or minimization of normal tissue integral dose can drive the optimization if critical structures are not the deciding factor Brahme’s work (1988) considers the optimal choices of the collimator angle in order to optimize the leaf direction, depending on whether the field shape is convex, concave-convex, or contains multiple concavities The one conclusion drawn by Brahme is that the optimal direction for the leaf motion is in the direction along the narrower axis For a simple ellipse, the optimal leaf direction is parallel to the short axis One group (Du et al 1994) has developed a method for determining optimal leaf positioning in concert with optimal collimator angulation Their optimization schema demands the following criteria be met: (a) the desired internal area is maintained, (b) the single leaf discrepancy is minimal, (c) that criteria (a) and (b) are combined to be minimal The problem with collimator optimization is 32 that wedges cannot be used at any desired angle of rotation unless two wedges are used with weights that produce a desired wedge rotation Use of multiple weighted wedges introduces a level of complexity that decreases the feasibility of collimator optimization for MLCs 5.4 Intensity Modulated Radiotherapy (IMRT) with MLC The basic goal of IMRT treatment is precise dose delivery on any part of treated area thus avoiding the surrouding healthy tissue In IMRT treatment, the leaves of MLC, while moving during the irradiation, ensure the appropriate dose that is delivered on the parts of treated area (Figure 13) Figure 13: IMRT technique with use of MLC From the differences between the dose volume delivered during the whole treatment and the dose volumes in which the leaf is shielding some part of the treated area, we can determine what dose has been delivered on this particular part MLC for intensity modulation should be very precise, motion of leaves must be fast and constant, leaves should be precisely controlled and must have a long reach in the field Three dimension (3D) treatment planning systems must be used for IMRT 33 Two stratagies of IMRT with MLC are used One is dynamic technique, with continuous movemetn of leaves during the treatment; the second is step and shot technique with moving the leaves when radiation is stopped Both strategies with this travel determine dose delivered on the parts of treatment volume 34 CONCLUSION The multileaf collimator was developed to replace the traditional blocks MLCs are used on linear accelerators to provide conformal shaping of radiotherapy treatment beams It is a simple and useful system in the preparation and performance of radiotherapy treatment Multileaf collimators are reliable, as their manufacturers developed various mechanisms for their precision, control and reliability, together with reduction of leakage and transmission of radiation between and through the leaves The advantages of MLCs are simple and less time consuming preparation, use without needing to enter the treatment room, and simple change or correction of field shape The therapy expenses are lower because individual shielding blocks are not needed, thus eliminating the need to handle the Wood’s alloy, which is toxic With MLC, we shorten the therapy time, and thus also the period during which patient must remain in still position Other advantages are constant control and continuous adjusting of the field shape during irradiation in advanced conformal radiotherapy.1-5 MLC has also some disadvantages, which include a stepping edge effect, radiation leakage between leaves, wider penumbra, and problems with generating some complex field shapes Multileaf collimator is known today as a very useful clinical system for simple field shaping, but its use is getting even more important in dynamic radiotherapy, with the leaves moving during irradiation This enables a precise dose delivery on any part of a treated volume Specifically, conformal radiotherapy and Intensity Modulated Radiation Therapy (IMRT) can be delivered using MLC’s 35 REFERENCES [1] Boyer A, Biggs P, Galvin J, Klein E, LoSasso T, Low D, et al (2001) For the Radiation Therapy Committee Report of Task Group No.50 “Basic applications of multileaf collimators.” AAPM Report No 72 2001 [2] Boye A, T G Ochran, C E Nyerick, T J Waldron, and C J Huntzinger (1992) “Clinical dosimetry for implementation of a multileaf collimator.” Med Phys 19: 1255–1261 [3] Brahme A (1988) “Optimal setting of multileaf collimators 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