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  • 1. Beam direction
    • Practice & principles
    • moderator - Mr. P. Goswami
    • department of radiotherapy
    • PGIMER, chandigarh
  • 2. Definition
    • Whole plan of the treatment is worked out in advance of the actual treatment and certain devices are used to direct the beam towards the tumor
  • 3. Why Beam direction ?
    • Homogenous tumor low normal tissue
    • dose dose
    • better therapeutic ratio
  • 4. Prerequisites for beam direction
    • Patient factors:
      • Early stage of disease
      • Good general condition
      • Good nutritional status
      • Radical treatment intent (curative)
    • Machine factors:
      • Isocentric
      • Non isocentric
  • 5. Steps
    • Localisation
    • Positioning
    • Immobilisation
    • Field selection
    • Dose distribution
    • Calculations
    • Execution & verification
  • 6. LOCALISATION
    • The target volume and critical normal tissues are delineated with respect to patient’s external surface contour.
    • What to localize?
      • Tumor
      • Organ
    • Methods?
      • Clinical examination
      • Imaging
  • 7. Why Localize?
    • Irradiate the tumor and spare the normal tissue.
    • Allow calculations and beam balancing.
    • Define radiation portals.
    • Use the beam directing devices.
  • 8. Clinical localization
    • Advantages:
      • Available everywhere.
      • Cheapest and quickest(?).
      • Needs little additional equipment.
    • Disadvantages:
      • Error prone in the wrong hands.
      • Accessible areas required.
      • Volumetric data not easily obtained.
    • Clinical localization is mandatory despite advanced imaging – need to know what to image!
  • 9. Imaging Localization
    • Imaging:
      • X-rays:
        • Plain
        • Contrast Studies
      • CT scans
      • MRI scans
      • USG scans
      • PET scan
      • Fusion imaging
    • Type of study selected depends on:
      • Precision desired.
      • Cost considerations
      • Time considerations
      • Labour considerations
  • 10. X rays
    • The most common and cheapest modality available.
    • However 2-D data acquired only.
    • Orthogonal films can be used with appropriate contrast enhancement for localization in 3 dimensions
  • 11. X rays
    • Plain films:
      • Head and neck region
      • Cervix (radio-opaque markers)
    • Contrast
      • Esophagus
      • Rectum
      • Bladder
      • Stomach
  • 12. Estimation of depth
    • From data gained by localization studies:
      • CT / MRI – Accurate data
      • Lateral height method
      • Tube shift method
    • Depth estimation necessary for:
      • Calculations
      • Selection of beam energy
  • 13. Lateral height method H 1 H 2 d d H 1 + H 2 2 d =
  • 14. Tube shift method
    • Image shift and tube shift are interrelated WHEN the tube to target distance remains constant.
    • Goal : To obtain a graph of different object heights against the tube shift.
    • Serial measurements of image shift measured (for same tube to film distance) while varying the height of the markers above the table.
  • 15. Tube shift principles Marker d 2 y f S Tumor x 1 x 2 d 1
  • 16. 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
  • 17. CT scans
    • Provides electron density data which can be directly used by the TPS.
    • Volumetric reconstruction possible.
    • Good image resolution - better where bony anatomy is to be evaluated.
    • The image is a gray scale representation of the CT numbers – related to the attenuation coefficients.
    • Hounsfield units =
    • ( μ tissue – μ water ) x 1000/ ( μ water )
    253 265 235 125 125 112 56 450 156 135 158 247 269 300 65 36 123 598
  • 18. CT scan perquisites
    • Flat table top
    • Large diameter scan aperture (≥ 70 cm).
    • Positioning, leveling and immobilization done in the treatment position.
    • Adequate internal contrast – external landmarks to be delineated too.
    • Preferably images to be transferred electronically to preserve electron density data.
  • 19. MRI scans
    • Advantages:
      • Imaging in multiple planes without formatting.
      • Greater tissue contrast – essential for proper target delineation in brain and head and neck
      • No ionizing radiation involved.
    • Disadvantages:
      • Lower spatial resolution
      • Longer scan times
      • Inability to image calcification or bones.
  • 20. Fusion Imaging
    • Includes PET – CT imaging and Fusion MRI.
    • Allows “ biological modulation ” of radiation therapy.
    • Expensive : requires additional software
    • Final clinical utility – still remains to be realized
  • 21. POSITIONING
    • Patient positioning is the most vital and often the most NEGLECTED part of beam direction:
    • Good patient position is ALWAYS :
      • Stable.
      • Comfortable.
      • Minimizes movements.
      • Reproducible.
  • 22. Standard Positions
    • Supine:
      • MC used body position.
      • Also most comfortable.
      • Best and quickest for setup.
      • Minimizes errors due to miscommunication.
    • Prone:
      • Best for treating posterior structures like spine
      • In some obese patients setup improved as the back is flat and less mobile.
  • 23. Positioning aids
    • Help to maintain patients in non standard positions.
    • These positions necessary to maximize therapeutic ratio.
    • Accessories allow manipulation of the non rigid human body to allow a comfortable, reproducible and stable position.
  • 24. Positioning aids… Pelvic Board Prone Support Breast Board
  • 25. Breast Boards
    • Disadvantages :
      • Possibility of skin reactions in the infra mammary folds
      • Access to CT scanners hampered
    • Solutions :
      • Thermoplastic brassieres.
      • Breast rings.
      • Prone treatment support.
    • Allow comfortable arm up support ► brings arms out of the way of lateral beams.
    • Positions patient so that the breast / sternum is horizontal ► avoiding angulation of the collimator.
    • Pulls breast down into a better position by the pull of gravity.
  • 26. Breast boards… Modern Breast Board Indexed Arm supports Indexed wrist support Head rest Carbon fiber tilt board Wedge to prevent sliding
  • 27. Belly boards
  • 28. Mould making
  • 29. Mould making : Contd..
  • 30. Mould making : Contd..
  • 31. Thermoplastics
    • Thermoplastics are long polymers with few cross links.
    • They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated
  • 32. Foam systems
    • Made of polyurethane
    • Advantages:
      • Ability to cut treatment portals into foam.
      • Mark treatment fields on the foam.
      • Rigid and holds shape.
    • Disadvantages:
      • Chance of spillage
      • Environmental hazard during disposal
  • 33. Vacuum bags
    • Consist of polystyrene beads that are locked in position with vacuum.
    • Can be reused.
    • However like former immobilization not perfect.
  • 34. Bite Blocks
    • A simple yet elegant design to immobilize the head.
    • A dental impression mouthpiece used.
    • The impression is attached to the base plate and is indexed.
  • 35. SRS devices
    • Sterotactic frames.
    • Gill Thomas Cosman System.
    • TALON ® Systems – NOMOS corp.
  • 36. Patient Contouring
    • Contour is the representation of external body outline.
    • Methods:
      • Plaster of Paris
      • Lead wire
      • Thermoplastic contouring material
      • Flurographic method
      • CT/MRI
  • 37. RADIATION FIELD
    • Types:
      • Geometrical : Area DEFINED by the light beam at any given depth as projected from the point of origin of the beam.
      • Physical : Area encompassed by the 50% isodose curve at the isocenter. In LINACs often defined at the 80% isodose.
  • 38. Single Field
    • Criteria for acceptability:
      • Dose distribution to be uniform ( ±5% )
      • Maximum dose to tissues in beam ≤ 110%.
      • Critical structures don ’ t receive dose exceeding their normal tolerance.
    • Situations used:
      • Skin tumors
      • CSI
      • Supraclavicular region
      • Palliative treatments
  • 39. 2 Field techniques
    • Can be :
      • Parallel opposed
      • Angled
        • Perpendicular
        • Oblique
      • Wedged pair
    • Advantages:
      • Simplicity
      • Reproducibility
      • Less chance of geometrical miss
      • Homogenous dose
    • Dose homogeneity depends on:
      • Patient thickness
      • Beam energy
      • Beam “flatness”
  • 40.
    • Disadvantage of 2 field techniques
    • -large amount of normal tissue gets radiation
    • -if separation is more there is an arc like distribution
    • so, in ca cx if separation is
    • >16 cm four fields are used.
  • 41. 3 field techniques
    • Used in
    • -deep seated tumors
    • -to save vital structures
    • Example
    • -ca esophaghus
    • -ca lung
    • -ca UB
    • -ca nasopharynx with ant extention
    • -ca maxilla with ethmoidal extention
  • 42. 4 field techniques
    • Used in
    • -ca cx
    • -ca rectum
  • 43. Multiple fields
    • Used in 3DCRT & IMRT
    • Used to obtain a “conformal” dose distribution in the modern radiotherapy techniques.
    • Disadvantages:
      • Integral dose increases
      • Certain beam angles are prohibited due to proximity of critical structures.
      • Setup accuracy better with parallel opposed arrangement.
  • 44. Magna field
    • Radical treatment of lymphoma(HD)
    • Whole body irradiation
    • Hemi body irradiation
  • 45. DOSE DISTRIBUTION ANALYSIS
    • Done manually or in the TPS.
    • Manual distribution gives a hands on idea of what to expect with dose distributions.
    • Inefficient and time consuming.
    • Pros:
      • Cheap
      • Universally available
      • Adequate for most clinical situations.
  • 46. Calculations
    • Techniques:
      • SSD technique (PDD method)
      • SAD technique
      • Clarkson’s technique
      • Computerized
  • 47. PRESCRIPTION
    • Mandatory statements:
      • Dose to be delivered.
      • Number of fractions
      • Number of fractions per week
  • 48. SSD technique
    • PDD is the ratio of the absorbed dose at any point at depth d to that at a reference depth d 0 .
    • D 0 is the position of the peak absorbed dose.
    • D max is the peak absorbed dose at the central axis .
    Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 PDD Time = ID Output
  • 49. SAD Technique
    • Uses doses normalized at isocenter for calculation.
    • In this technique the impact of setup variations is minimized.
    • Setup is easier but manual planning difficult.
    • Time taken for treatment reduced.
  • 50. SAD calculations Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 TMR/TAR Time = ID Output
  • 51. SSD vs SAD technique
    • SSD treatments:
      • Relatively less homogenous dose distribution
      • Setup possible without requiring expensive aids e.g. Laser
      • PDD charts can be used for simple dose calculations
      • More skin reactions
    • SAD treatments:
      • Less number of MUs required
      • Time taken is less
      • Impact of setup inaccuracies is minimized in 2 field techniques
      • Ease of setup reproducibility in multi field treatments.
  • 52. TAR vs. SSD
    • TAR = Tissue Air Ratio
    • TAR introduced by Jones for rotation therapy.
    • Allows calculation of dose at isocenter WITHOUT correcting for varying SSDs.
    • 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 )
    D q D 0
  • 53. TAR
    • TAR removes the influence of SSD as it is a ratio of two doses at the SAME point.
    • However like PDD the TAR also varies with:
      • Energy
      • Depth
      • Field Size
      • Field Shape
  • 54. EXECUTION & VERIFICATION
    • Can be done using:
      • Portal Films
      • Electronic Portal images
      • Cone Beam CT mounted on treatment machines (IGRT).
      • Seen during treatment
    • -CCTV camera
    • -lead glass
    • -mirror
    • -infrared camera (in imrt)
  • 55. Port films
      • Cheapest.
      • Legal necessity(?)
      • But have several disadvantages.
  • 56. Port film disadvantages
    • Factors leading to poor image contrast:
      • High beam energy (> 10 MV)
      • Large source size ( Cobalt)
      • Large patient thickness (> 20 cm)
    • Slow acquisition times.
    • Image enhancement not possible.
    • Storage problems.
  • 57. Electronic Portal Imaging
    • Video based EPIDS
    • Fiber optic systems
    • Matrix liquid ion chambers
    • Solid state detectors
    • Amorphous Si technology*
  • 58. BEAM DIRECTION DEVICES
    • The main beam direction devices are:
      • Collimators
      • Front pointer / SSD indicator
      • Back Pointer
      • Pin and arc
      • Isocentric mounting
      • Lasers
  • 59. Collimators
    • Collimators provide beams of desired shape and size.
    • Types:
      • Fixed / Master collimator.
      • Movable / Treatment collimator.
  • 60. Fixed Collimators
    • Protects the patient from bulk of the radiation.
    • Dictates the maximum field size for the machine.
    • Maximum beam size is when exposure at periphery is 50% of that of the center.
    • In megavoltage radiotherapy beam angle used is 20° .
  • 61. Master Collimator : Design
    • In megavoltage x ray machines beam energy is maximum in forward direction.
    • Beam energy is equal in telecurie sources so primary collimators are spherical.
  • 62. Movable Collimators
    • Define the required field size and shape.
    • Placed below the master collimators results in trimming of the penumbra.
    • Types:
      • Applicators
      • Jaws / Movable diaphragms
  • 63. Applicators: Design Lead Sheet Box Plastic Cap Metal Plate with hole
  • 64. Applicators
    • Advantages:
      • Indicate size and shape of beam.
      • Distance maintained.
      • Direction shown.
      • Plastic ends allow compression .
      • Compression allows immobilization .
      • Penumbra minimized.
    • Disadvantages:
      • Useful for low energy only.
      • Separate sizes and shapes required.
      • Costly.
      • Shapes may change due frequent handling.
  • 65. Jaws
    • Handling of heavy weight not required.
    • Skin sparing effect retained.
    • Jaws moved mechanically – accurately.
    Jaw border lies along the line radiating from focal spot
  • 66. Jaws: Disadvantages Disadvantages Remedy Size and shape of field remain unknown Light beam shining through the jaws Patient to source distance unknown SSD indicator used. Compression not possible A Perspex box may be applied to the head
  • 67. Front & back pointers
    • This method requires the identification & marking on the patient’s surface, of two points lying on a line passing through the tumor centre.
    • Entry point- A
    • Tumor centre- T
    • Exit point- B
  • 68.  
  • 69. Front Pointer/ SSD indicator
    • Detachable device to measure the SSD and align the beam axis.
    • Designed so that it may be swung out of the beam path during treatment.
  • 70. Back Pointer
    • The pointer can be moved in the sleeve.
    • A nipple is used to allow compression.
    • The arrow lies along the central ray.
  • 71. Sites used
    • Front pointer and back pointer used in the following situations:
      • Head and Neck
      • Breast
      • Brain tumors
  • 72. Limitations
    • Requires skin marks – inherently unreliable.
    • Back pointer is unreliable when compression is desired.
    • Both front and back points must be accessible.
    • Accurate localization of tumor center is mandatory.
  • 73. Pin & Arc Pin Arc Bubble
  • 74. Principle
    • Based on the principle of parallelogram
  • 75. How does it work?
    • arrangement of pin & arc is such that when pin is at it’s lowest position, it’s lower end is on the central axis of beam & on centre of curvature of arc.
    • Depth of the tumor is already known.
    • Pin is withdrawn the reqd. distance & it’s lower end is brought in contact with the surface mark.
  • 76.
    • So long as the pin is vertical the rest of equipment & applicator will rotate about the centre of tumor and central ray will always pass through it
    • Thus keeping the pin vertical & in contact with surface mark any particular angle can be selected
    • The oblique distance can be read off the scale or bar by applying principle of parallelogram.
  • 77. Advantages of Pin & Arc
    • Allows Isocentric treatment of
      • Deep tumors.
      • Eccentric tumors.
    • Can be used with compression e.g. in treating deep seated tumors.
    • Can be used for manual verification of Isocentric placement of machines
  • 78. Sites where used
    • Mostly in midline tumors situated at a depth
      • Esophagus
      • Cervix
      • Bladder
      • Rectum
      • Vagina
      • Lung sometimes
  • 79. Isocentric Mounting
    • First used by Flanders and Newberg of Hammersmith Hospital for early linear accelerators.
    • The axis of rotation of the three structures:
      • Gantry
      • Collimator
      • Couch
    • coincide at a point known as the Isocenter .
  • 80. Why Isocentric Mounting?
    • Enhances accuracy.
    • Allows faster setup and is more accurate than older non isocentrically mounted machines.
    • Makes setup transfer easy from the simulator to the treatment machine.
  • 81. Lasers
    • LASER = Light Amplification Of stimulated Emission Of Radiation
    • Typically 3 pairs are provided with the machine and intersect at the isocenter.
    • Also define:
      • Beam Entry
      • Beam Exit
  • 82. Lasers
    • Other uses :
      • Checking the isocenter
      • Reproducing the setup on the simulator at the treatment couch.
    • Fallacies :
      • Accurate setup depends on proper alignment of the lasers themselves
      • Lasers known to move  frequent adjustments needed.
  • 83. Conclusion
    • Beam direction devices & methods are important part of radiotherapy which aids in accurate treatment.
    • To neglect the extra accuracy that can be gained by beam direction is to throw away much of the value of the powerful and expensive apparatus now in use in radiotherapy.
  • 84.
    • Thank you.