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Beam Directed Radiotherapy - methods and principles


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Beam Directed Radiotherapy - methods and principles

  1. 1. Beam Directed Radiotherapy – Principles and practice
  2. 2. Definition Exact Calculations Beam Directing devices Advance Planning Beam directed radiotherapy
  3. 3. Need for Beam direction Homogenous Tumor Dose Low normal tissue dose Best therapeutic ratio
  4. 4. Steps Positioning Immobilization Localization Field Selection Dose distribution Calculations Verification Execution
  5. 5. Positioning <ul><li>Patient positioning is the most vital and often the most NEGLECTED part of beam direction: </li></ul><ul><li>Good patient position is ALWAYS : </li></ul><ul><ul><li>Stable. </li></ul></ul><ul><ul><li>Comfortable. </li></ul></ul><ul><ul><li>Minimizes movements. </li></ul></ul><ul><ul><li>Reproducible. </li></ul></ul>
  6. 6. Examples
  7. 7. Standard Positions <ul><li>MC used body position. </li></ul><ul><li>Also most comfortable. </li></ul><ul><li>Best and quickest for setup. </li></ul><ul><li>Minimizes errors due to miscommunication. </li></ul><ul><li>Best for treating posterior structures like spine </li></ul><ul><li>In some obese patients setup improved as the back is flat and less mobile. </li></ul>Supine Prone
  8. 8. Positioning aids <ul><li>Help to maintain patients in non standard positions. </li></ul><ul><li>These positions necessary to maximize therapeutic ratio. </li></ul><ul><li>Accessories allow manipulation of the non rigid human body to allow a comfortable, reproducible and stable position. </li></ul>
  9. 9. Positioning aids… Pituitary Board Prone Support 3 way support
  10. 10. Breast Boards <ul><li>Disadvantages : </li></ul><ul><ul><li>Possibility of skin reactions in the infra mammary folds </li></ul></ul><ul><ul><li>Access to CT scanners hampered </li></ul></ul><ul><li>Solutions : </li></ul><ul><ul><li>Thermoplastic brassieres. </li></ul></ul><ul><ul><li>Breast rings. </li></ul></ul><ul><ul><li>Prone treatment support. </li></ul></ul><ul><li>Allow comfortable arm up support ► brings arms out of the way of lateral beams. </li></ul><ul><li>Positions patient so that the breast / sternum is horizontal ► avoiding angulation of the collimator. </li></ul><ul><li>Pulls breast down into a better position by the pull of gravity. </li></ul>
  11. 11. Breast boards… Modern Breast Board Indexed Arm supports Indexed wrist support Head rest Carbon fiber tilt board Wedge to prevent sliding
  12. 12. Arm Support <ul><li>Also known as the T bar. </li></ul><ul><li>Allows the arm to be positioned laterally when treating the thorax using lateral beams. </li></ul>
  13. 13. Belly boards & leg immobilizer`
  14. 14. Mould making
  15. 15. Mould making : Contd..
  16. 16. Mould making : Contd..
  17. 17. Thermoplastics <ul><li>Thermoplastics are long polymers with few cross links. </li></ul><ul><li>They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated </li></ul>
  18. 18. Thermoplastics : Principle
  19. 19. Foam systems <ul><li>Made of polyurethane </li></ul><ul><li>Advantages: </li></ul><ul><ul><li>Ability to cut treatment portals into foam. </li></ul></ul><ul><ul><li>Mark treatment fields on the foam. </li></ul></ul><ul><ul><li>Rigid and holds shape. </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Chance of spillage </li></ul></ul><ul><ul><li>Environmental hazard during disposal </li></ul></ul>
  20. 20. Vacuum bags <ul><li>Consist of polystyrene beads that are locked in position with vacuum. </li></ul><ul><li>Can be reused. </li></ul><ul><li>However like former immobilization not perfect. </li></ul>
  21. 21. Bite Blocks <ul><li>A simple yet elegant design to immobilize the head. </li></ul><ul><li>A dental impression mouthpiece used. </li></ul><ul><li>The impression is attached to the base plate and is indexed . </li></ul><ul><li>Head position recorded with 3 numbers. </li></ul>
  22. 22. SRS devices <ul><li>Sterotactic frames. </li></ul><ul><li>Gill Thomas Cosman System. </li></ul><ul><li>TALON ® Systems – NOMOS corp. </li></ul>
  23. 23. Localization <ul><li>The target volume and critical normal tissues are delineated with respect to patient’s external surface contour. </li></ul><ul><li>What to localize? </li></ul><ul><ul><li>Tumor </li></ul></ul><ul><ul><li>Organ </li></ul></ul><ul><li>Methods? </li></ul><ul><ul><li>Clinical examination </li></ul></ul><ul><ul><li>Imaging </li></ul></ul>
  24. 24. Why Localize? <ul><li>Irradiate the tumor and spare the normal tissue. </li></ul><ul><li>Allow calculations and beam balancing. </li></ul><ul><li>Define radiation portals. </li></ul><ul><li>Use the beam directing devices. </li></ul>
  25. 25. Clinical localization <ul><li>Advantages: </li></ul><ul><ul><li>Available everywhere. </li></ul></ul><ul><ul><li>Cheapest and quickest(?). </li></ul></ul><ul><ul><li>Needs little additional equipment. </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Error prone in the wrong hands. </li></ul></ul><ul><ul><li>Accessible areas required. </li></ul></ul><ul><ul><li>Volumetric data not easily obtained. </li></ul></ul><ul><li>Clinical localization is mandatory despite advanced imaging – need to know what to image! </li></ul>
  26. 26. Imaging Localization <ul><li>Imaging: </li></ul><ul><ul><li>X-rays: </li></ul></ul><ul><ul><ul><li>Plain </li></ul></ul></ul><ul><ul><ul><li>Contrast Studies </li></ul></ul></ul><ul><ul><li>CT scans </li></ul></ul><ul><ul><li>MRI scans </li></ul></ul><ul><ul><li>USG scans </li></ul></ul><ul><ul><li>PET scan </li></ul></ul><ul><ul><li>Fusion imaging </li></ul></ul><ul><li>Type of study selected depends on: </li></ul><ul><ul><li>Precision desired. </li></ul></ul><ul><ul><li>Cost considerations </li></ul></ul><ul><ul><li>Time considerations </li></ul></ul><ul><ul><li>Labour considerations </li></ul></ul>
  27. 27. X rays <ul><li>The most common and cheapest modality available. </li></ul><ul><li>However 2-D data acquired only. </li></ul><ul><li>Orthogonal films can be used with appropriate contrast enhancement for localization in 3 dimensions. </li></ul>
  28. 28. Estimation of depth <ul><li>From data gained by localization studies: </li></ul><ul><ul><li>CT / MRI – Accurate data </li></ul></ul><ul><ul><li>Lateral height method </li></ul></ul><ul><ul><li>Tube shift method </li></ul></ul><ul><li>Depth estimation necessary for: </li></ul><ul><ul><li>Calculations </li></ul></ul><ul><ul><li>Selection of beam energy </li></ul></ul>
  29. 29. Lateral height method H 1 H 2 d d H 1 + H 2 2 d =
  30. 30. Tube shift method <ul><li>Image shift and tube shift are interrelated WHEN the tube to target distance remains constant. </li></ul><ul><li>Goal : To obtain a graph of different object heights against the tube shift. </li></ul><ul><li>Serial measurements of image shift measured (for same tube to film distance) while varying the height of the markers above the table. </li></ul>
  31. 31. Tube shift principles Marker d 2 y f S Tumor x 1 x 2 d 1
  32. 32. Calculation d 1 f y d 2 x 1 x 2 x 1 S = d 1 f – d 1 S x 2 S = d 2 f – d 2 y = d 2 – d 1 = f x 2 + S x 2 - x 1 + S x 1 Tumor Marker
  33. 33. CT scans <ul><li>Provides electron density data which can be directly used by the TPS. </li></ul><ul><li>Volumetric reconstruction possible. </li></ul><ul><li>Good image resolution - better where bony anatomy is to be evaluated. </li></ul><ul><li>The image is a gray scale representation of the CT numbers – related to the attenuation coefficients. </li></ul><ul><li>Hounsfield units = </li></ul><ul><li>( μ tissue – μ water ) x 1000/ ( μ water ) </li></ul>598 123 36 65 300 269 247 158 135 156 450 56 112 125 125 235 265 253
  34. 34. CT scan perquisites <ul><li>Flat table top </li></ul><ul><li>Large diameter scan aperture (≥ 70 cm). </li></ul><ul><li>Positioning, leveling and immobilization done in the treatment position. </li></ul><ul><li>Adequate internal contrast – external landmarks to be delineated too. </li></ul><ul><li>Preferably images to be transferred electronically to preserve electron density data. </li></ul>
  35. 35. MRI scans <ul><li>Advantages: </li></ul><ul><ul><li>Imaging in multiple planes without formatting. </li></ul></ul><ul><ul><li>Greater tissue contrast – essential for proper target delineation in brain and head and neck </li></ul></ul><ul><ul><li>No ionizing radiation involved. </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Lower spatial resolution </li></ul></ul><ul><ul><li>Longer scan times </li></ul></ul><ul><ul><li>Inability to image calcification or bones. </li></ul></ul>
  36. 36. Fusion Imaging <ul><li>Includes PET – CT imaging and Fusion MRI. </li></ul><ul><li>Allows “ biological modulation ” of radiation therapy. </li></ul><ul><li>Technology still in it’s infancy – (?) The future of radiotherapy. </li></ul>
  37. 37. Patient Contouring <ul><li>Contour is the representation of external body outline. </li></ul><ul><li>Methods: </li></ul><ul><ul><li>Plaster of Paris </li></ul></ul><ul><ul><li>Lead wire </li></ul></ul><ul><ul><li>Thermoplastic contouring material </li></ul></ul><ul><ul><li>Flurographic method </li></ul></ul><ul><ul><li>CT/MRI </li></ul></ul>
  38. 38. Contour Plotter
  39. 39. Radiation Field <ul><li>Types: </li></ul><ul><ul><li>Geometrical : Area DEFINED by the light beam at any given depth as projected from the point of origin of the beam. </li></ul></ul><ul><ul><li>Physical : Area encompassed by the 50% isodose curve at the isocenter. In LINACs often defined at the 80% isodose. </li></ul></ul>
  40. 40. Single Field <ul><li>Criteria for acceptability: </li></ul><ul><ul><li>Dose distribution to be uniform ( ±5% ) </li></ul></ul><ul><ul><li>Maximum dose to tissues in beam ≤ 110%. </li></ul></ul><ul><ul><li>Critical structures don’t receive dose exceeding their normal tolerance. </li></ul></ul><ul><li>Situations used: </li></ul><ul><ul><li>Skin tumors </li></ul></ul><ul><ul><li>CSI </li></ul></ul><ul><ul><li>Supraclavicular region </li></ul></ul><ul><ul><li>Palliative treatments </li></ul></ul>
  41. 41. 2 Field techniques <ul><li>Can be : </li></ul><ul><ul><li>Parallel opposed </li></ul></ul><ul><ul><li>Angled </li></ul></ul><ul><ul><ul><li>Perpendicular </li></ul></ul></ul><ul><ul><ul><li>Oblique </li></ul></ul></ul><ul><ul><li>Wedged pair </li></ul></ul><ul><li>Advantages: </li></ul><ul><ul><li>Simplicity </li></ul></ul><ul><ul><li>Reproducibility </li></ul></ul><ul><ul><li>Less chance of geometrical miss </li></ul></ul><ul><ul><li>Homogenous dose </li></ul></ul><ul><li>Dose homogeneity depends on: </li></ul><ul><ul><li>Patient thickness </li></ul></ul><ul><ul><li>Beam energy </li></ul></ul><ul><ul><li>Beam “flatness” </li></ul></ul>
  42. 42. Multiple fields <ul><li>Used to obtain a “conformal” dose distribution in the modern radiotherapy techniques. </li></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Integral dose increases </li></ul></ul><ul><ul><li>Certain beam angles are prohibited due to proximity of critical structures. </li></ul></ul><ul><ul><li>Setup accuracy better with parallel opposed arrangement. </li></ul></ul>
  43. 43. Dose distribution analysis <ul><li>Done manually or in the TPS. </li></ul><ul><li>Manual distribution gives a hands on idea of what to expect with dose distributions. </li></ul><ul><li>Inefficient and time consuming. </li></ul><ul><li>Pros: </li></ul><ul><ul><li>Cheap </li></ul></ul><ul><ul><li>Universally available </li></ul></ul><ul><ul><li>Adequate for most clinical situations. </li></ul></ul>
  44. 44. Calculations <ul><li>Techniques: </li></ul><ul><ul><li>SSD technique (PDD method) </li></ul></ul><ul><ul><li>SAD technique </li></ul></ul><ul><ul><li>Clarkson’s technique </li></ul></ul><ul><ul><li>Computerized </li></ul></ul>
  45. 45. Prescription <ul><li>Mandatory statements: </li></ul><ul><ul><li>Dose to be delivered. </li></ul></ul><ul><ul><li>Number of fractions </li></ul></ul><ul><ul><li>Number of fractions per week </li></ul></ul>
  46. 46. SSD technique <ul><li>PDD is the ratio of the absorbed dose at any point at depth d to that at a reference depth d 0 . </li></ul><ul><li>D 0 is the position of the peak absorbed dose. </li></ul><ul><li>D max is the peak absorbed dose at the central axis . </li></ul>Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 PDD Time = ID Output
  47. 47. SAD Technique <ul><li>Uses doses normalized at isocenter for calculation. </li></ul><ul><li>In this technique the impact of setup variations is minimized. </li></ul><ul><li>Dose homogeneity is better with the SAD technique. </li></ul><ul><li>Setup is easier but manual planning not possible / difficult. </li></ul>
  48. 48. SAD calculations Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 TMR/TAR Time = ID Output
  49. 49. TAR vs. SSD <ul><li>TAR = Tissue Air Ratio </li></ul><ul><li>TAR introduced by Jones for rotation therapy. </li></ul><ul><li>Allows calculation of dose at isocenter WITHOUT correcting for varying SSDs. </li></ul><ul><li>TAR is the ratio of dose at a point in the phantom to the dose in free space at the same point (D q /D 0 ) </li></ul>D q D 0
  50. 50. TAR <ul><li>TAR removes the influence of SSD as it is a ratio of two doses at the SAME point. </li></ul><ul><li>However like PDD the TAR also varies with: </li></ul><ul><ul><li>Energy </li></ul></ul><ul><ul><li>Depth </li></ul></ul><ul><ul><li>Field Size </li></ul></ul><ul><ul><li>Field Shape </li></ul></ul>
  51. 51. Verification <ul><li>Can be done using: </li></ul><ul><ul><li>Portal Films </li></ul></ul><ul><ul><li>Electronic Portal images </li></ul></ul><ul><ul><li>Cone Beam CT mounted on treatment machines (IGRT). </li></ul></ul><ul><li>Portal Films: </li></ul><ul><ul><li>Cheapest. </li></ul></ul><ul><ul><li>Legal necessity(?) </li></ul></ul><ul><ul><li>But have several disadvantages. </li></ul></ul>
  52. 52. Port film disadvantages <ul><li>Factors leading to poor image contrast: </li></ul><ul><ul><li>High beam energy (> 10 MV) </li></ul></ul><ul><ul><li>Large source size ( Cobalt) </li></ul></ul><ul><ul><li>Large patient thickness (> 20 cm) </li></ul></ul><ul><li>Slow acquisition times. </li></ul><ul><li>Image enhancement not possible. </li></ul><ul><li>Storage problems. </li></ul>
  53. 53. Electronic Portal Imaging <ul><li>Video based EPIDS </li></ul><ul><li>Fiber optic systems </li></ul><ul><li>Matrix liquid ion chambers </li></ul><ul><li>Solid state detectors </li></ul><ul><li>Amorphous Si technology * </li></ul>
  54. 54. Electronic Portal Imaging
  55. 55. Advantages of EPIDs <ul><li>Allow real time verification of patient setup. </li></ul><ul><li>Acquisition times short . </li></ul><ul><li>Multiple images possible. </li></ul><ul><li>Reasonable image quality . </li></ul><ul><li>Software assisted image enhancement . </li></ul><ul><li>Online corrections possible. </li></ul>
  56. 56. Disadvantages of EPIDs <ul><li>Cost of equipment. </li></ul><ul><li>Added service and software update requirements. </li></ul><ul><li>Fragility of the equipment – Si matrix deteriorates with time and exposure. </li></ul>
  57. 57. Cone Beam CT <ul><li>Incorporates a special CT scanner on the LINAC. </li></ul><ul><li>Useful to obtain 3 D real time images of the patient. </li></ul><ul><li>Can use kilovoltage or megavoltage CT </li></ul><ul><li>Allows IGRT. </li></ul>
  58. 58. Beam direction devices <ul><li>The main beam direction devices are: </li></ul><ul><ul><li>Collimators </li></ul></ul><ul><ul><li>Front pointer / SSD indicator </li></ul></ul><ul><ul><li>Back Pointer </li></ul></ul><ul><ul><li>Pin and arc </li></ul></ul><ul><ul><li>Isocentric mounting </li></ul></ul><ul><ul><li>Lasers </li></ul></ul>
  59. 59. Collimators <ul><li>Collimators provide beams of desired shape and size. </li></ul><ul><li>Types: </li></ul><ul><ul><li>Fixed / Master collimator. </li></ul></ul><ul><ul><li>Movable / Treatment collimator. </li></ul></ul>
  60. 60. Fixed Collimators <ul><li>Protects the patient from bulk of the radiation. </li></ul><ul><li>Dictates the maximum field size for the machine. </li></ul><ul><li>Maximum beam size is when exposure at periphery is 50% of that of the center. </li></ul><ul><li>In megavoltage radiotherapy beam angle used is 20° . </li></ul>
  61. 61. Master Collimator : Design <ul><li>In megavoltage x ray machines beam energy is maximum in forward direction. </li></ul>20 ° 20 ° <ul><li>Beam energy is equal in telecurie sources so primary collimators are spherical. </li></ul>
  62. 62. Movable Collimators <ul><li>Define the required field size and shape. </li></ul><ul><li>Placed below the master collimators results in trimming of the penumbra. </li></ul><ul><li>Types: </li></ul><ul><ul><li>Applicators </li></ul></ul><ul><ul><li>Jaws / Movable diaphragms </li></ul></ul>
  63. 63. Applicators: Design Lead Sheet Box Plastic Cap Metal Plate with hole
  64. 64. Applicators <ul><li>Advantages: </li></ul><ul><ul><li>Indicate size and shape of beam. </li></ul></ul><ul><ul><li>Distance maintained. </li></ul></ul><ul><ul><li>Direction shown. </li></ul></ul><ul><ul><li>Plastic ends allow compression . </li></ul></ul><ul><ul><li>Compression allows immobilization . </li></ul></ul><ul><ul><li>Penumbra minimized. </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Useful for low energy only. </li></ul></ul><ul><ul><li>Separate sizes and shapes required. </li></ul></ul><ul><ul><li>Costly. </li></ul></ul><ul><ul><li>Shapes may change due frequent handling. </li></ul></ul>
  65. 65. Jaws <ul><li>Handling of heavy weight not required. </li></ul><ul><li>Skin sparing effect retained. </li></ul><ul><li>Jaws moved mechanically – accurately. </li></ul>Jaw border lies along the line radiating from focal spot
  66. 66. Jaws: Disadvantages A Perspex box may be applied to the head Compression not possible SSD indicator used. Patient to source distance unknown Light beam shining through the jaws Size and shape of field remain unknown Remedy Disadvantages
  67. 67. Front & Back Pointers
  68. 68. Front Pointer/ SSD indicator <ul><li>Detachable device to measure the SSD and align the beam axis. </li></ul><ul><li>Designed so that it may be swung out of the beam path during treatment. </li></ul>
  69. 69. Back Pointer <ul><li>The pointer can be moved in the sleeve. </li></ul><ul><li>A nipple is used to allow compression. </li></ul><ul><li>The arrow lies along the central ray. </li></ul>
  70. 70. Limitations <ul><li>Requires skin marks – inherently unreliable. </li></ul><ul><li>Back pointer is unreliable when compression is desired. </li></ul><ul><li>Both front and back points must be accessible. </li></ul><ul><li>Accurate localization of tumor center is mandatory. </li></ul>
  71. 71. Pin & Arc Pin Arc Bubble
  72. 72. Pin & Arc: Principle
  73. 73. Pin & Arc : Method d D d D D This is the isocenter
  74. 74. Advantages of Pin & Arc <ul><li>Allows Isocentric treatment of </li></ul><ul><ul><li>Deep tumors. </li></ul></ul><ul><ul><li>Eccentric tumors. </li></ul></ul><ul><li>Can be used with compression e.g. in treating deep seated tumors. </li></ul><ul><li>Can be used for manual verification of Isocentric placement of machines </li></ul>
  75. 75. Isocentric Mounting <ul><li>First used by Flanders and Newberg of Hammersmith Hospital for early linear accelerators. </li></ul><ul><li>The axis of rotation of the three structures: </li></ul><ul><ul><li>Gantry </li></ul></ul><ul><ul><li>Collimator </li></ul></ul><ul><ul><li>Couch </li></ul></ul><ul><li>coincide at a point known as the Isocenter . </li></ul>
  76. 76. Principle of Isocentric mounting
  77. 77. Why Isocentric Mounting? <ul><li>Enhances accuracy. </li></ul><ul><li>Allows faster setup and is more accurate than older non isocentrically mounted machines. </li></ul><ul><li>Makes setup transfer easy from the simulator to the treatment machine. </li></ul>
  78. 78. Lasers <ul><li>LASER = Light Amplification Of stimulated Emission Of Radiation </li></ul><ul><li>Typically 3 pairs are provided with the machine and intersect at the isocenter. </li></ul><ul><li>Also define: </li></ul><ul><ul><li>Beam Entry </li></ul></ul><ul><ul><li>Beam Exit </li></ul></ul>
  79. 79. Lasers <ul><li>Other uses : </li></ul><ul><ul><li>Checking the isocenter </li></ul></ul><ul><ul><li>Reproducing the setup on the simulator at the treatment couch. </li></ul></ul><ul><li>Fallacies : </li></ul><ul><ul><li>Accurate setup depends on proper alignment of the lasers themselves </li></ul></ul><ul><ul><li>Lasers known to move  frequent adjustments needed. </li></ul></ul>
  80. 80. How to setup with LASER 5. Treat 2. Align the fiduciary marks with the laser system 3. Move the couch by to bring the planned isocenter to the machine isocenter <ul><li>Note the coordinates of the isocenter </li></ul>4. Verify the Setup
  81. 81. Conclusion Team Work Precision Quality Assurance
  82. 82. Thank You