Beam directed radiotherapy aims to deliver a homogenous tumor dose while minimizing radiation to normal tissues. It involves careful patient positioning, immobilization, tumor localization, field selection, dose calculations, and verification. Key steps include using positioning aids and molds to reproducibly position the patient, imaging such as CT to delineate the tumor volume, contouring to define external body outlines, and dose calculations and verification to ensure accurate delivery.

1. Beam Directed Radiotherapy – Principles and practice

2. Definition Exact Calculations Beam Directing devices Advance Planning Beam directed radiotherapy

3. Need for Beam direction Homogenous Tumor Dose Low normal tissue dose Best therapeutic ratio

4. Steps Positioning Immobilization Localization Field Selection Dose distribution Calculations Verification Execution

5. 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.

6. Examples

7. Standard Positions MC used body position. Also most comfortable. Best and quickest for setup. Minimizes errors due to miscommunication. Best for treating posterior structures like spine In some obese patients setup improved as the back is flat and less mobile. Supine Prone

8. 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.

9. Positioning aids… Pituitary Board Prone Support 3 way support

10. 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.

11. Breast boards… Modern Breast Board Indexed Arm supports Indexed wrist support Head rest Carbon fiber tilt board Wedge to prevent sliding

12. Arm Support Also known as the T bar. Allows the arm to be positioned laterally when treating the thorax using lateral beams.

13. Belly boards & leg immobilizer`

14. Mould making

15. Mould making : Contd..

16. Mould making : Contd..

17. Thermoplastics Thermoplastics are long polymers with few cross links. They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated

18. Thermoplastics : Principle

19. 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

20. Vacuum bags Consist of polystyrene beads that are locked in position with vacuum. Can be reused. However like former immobilization not perfect.

21. 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 . Head position recorded with 3 numbers.

22. SRS devices Sterotactic frames. Gill Thomas Cosman System. TALON ® Systems – NOMOS corp.

23. Localization 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

24. Why Localize? Irradiate the tumor and spare the normal tissue. Allow calculations and beam balancing. Define radiation portals. Use the beam directing devices.

25. 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!

26. 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

27. 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.

28. 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

29. Lateral height method H 1 H 2 d d H 1 + H 2 2 d =

30. 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.

31. Tube shift principles Marker d 2 y f S Tumor x 1 x 2 d 1

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. 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 ) 598 123 36 65 300 269 247 158 135 156 450 56 112 125 125 235 265 253

34. 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.

35. 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.

36. Fusion Imaging Includes PET – CT imaging and Fusion MRI. Allows “ biological modulation ” of radiation therapy. Technology still in it’s infancy – (?) The future of radiotherapy.

37. Patient Contouring Contour is the representation of external body outline. Methods: Plaster of Paris Lead wire Thermoplastic contouring material Flurographic method CT/MRI

38. Contour Plotter

39. 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.

40. 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

41. 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”

42. Multiple fields 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.

43. 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.

44. Calculations Techniques: SSD technique (PDD method) SAD technique Clarkson’s technique Computerized

45. Prescription Mandatory statements: Dose to be delivered. Number of fractions Number of fractions per week

46. 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

47. SAD Technique Uses doses normalized at isocenter for calculation. In this technique the impact of setup variations is minimized. Dose homogeneity is better with the SAD technique. Setup is easier but manual planning not possible / difficult.

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. 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

50. 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

51. Verification Can be done using: Portal Films Electronic Portal images Cone Beam CT mounted on treatment machines (IGRT). Portal Films: Cheapest. Legal necessity(?) But have several disadvantages.

52. 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.

53. Electronic Portal Imaging Video based EPIDS Fiber optic systems Matrix liquid ion chambers Solid state detectors Amorphous Si technology *

54. Electronic Portal Imaging

55. Advantages of EPIDs Allow real time verification of patient setup. Acquisition times short . Multiple images possible. Reasonable image quality . Software assisted image enhancement . Online corrections possible.

56. Disadvantages of EPIDs Cost of equipment. Added service and software update requirements. Fragility of the equipment – Si matrix deteriorates with time and exposure.

57. Cone Beam CT Incorporates a special CT scanner on the LINAC. Useful to obtain 3 D real time images of the patient. Can use kilovoltage or megavoltage CT Allows IGRT.

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. 20 ° 20 ° 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 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. Front & Back Pointers

68. 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.

69. Back Pointer The pointer can be moved in the sleeve. A nipple is used to allow compression. The arrow lies along the central ray.

70. 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.

71. Pin & Arc Pin Arc Bubble

72. Pin & Arc: Principle

73. Pin & Arc : Method d D d D D This is the isocenter

74. 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

75. 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 .

76. Principle of Isocentric mounting

77. 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.

78. 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

79. 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.

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 Note the coordinates of the isocenter 4. Verify the Setup

81. Conclusion Team Work Precision Quality Assurance

82. Thank You