Patient contouring and beam modifying devices


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Patient contouring and beam modifying devices

  1. 1. Mayur Mayank 15.05.2012 1
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  3. 3.  PATIENT CONTOURING ◦ IntroductionA. Manual contouring - Plaster of Paris strips - Contour tubes - Contour wire - Flexible curve - Manual Contouring device - Mobile Contour Plotter - Clarke’s Model of Contouring Device ADVANTAGES AND DISADVANTAGESB. Image based contouring 3
  4. 4.  Contour : It is a cross sectional outline of the patient’s external surface. ◦ Required for outlining the Planning target Volume (PTV) ◦ Also helps in dose limitation to the normal organs 4
  5. 5. RULES FOR CONTOURING•The patient contour must be obtained with the patient in the treatment position.•A line representing the tabletop must be indicated in the contour so that thiscan be used as a reference for beam angles.•Important bony landmarks as well as beam entry points must be indicated onthe contour.•Checks of body contour should be done during the treatment course as thecontour is expected to change due to a reduction of tumor volume or a changein patient weight.•If body thickness varies significantly within the treatment field, contours shouldbe taken in more than one plane. 5
  6. 6.  Plaster of Paris strips I. Extra fast setting 4 inch wide POP strips can be used for contouring. II. The strip should be long enough to from the couch top from one side, over the patient, and down to the couch top on the opposite side. III. The strips are dipped in hot water and then placed along the transverse axis of the patient. 6
  7. 7.  Contour Tubes I. Hollow, low-temperature thermoplastic tubes. II. Heat and cool quickly to form rigid contours in minutes. III. Easy application and removal without altering the shape of the finished contour. IV. The shape is retained until reheated. 7
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  9. 9.  Contour Wire I. Thin Lead Contour Wire 2mm (0.080") Diameter used for head and neck areas II. Thick Lead Contour Wire 3mm (0.125”) Diameter used for body sections The wire is formed around the patient in the treatment position. 9
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  11. 11.  Flexible Curve I. Flexible Contour Device is covered with tough vinyl. 11
  12. 12.  Manual Contouring Device I. This device is an economical and convenient unit for making contours. II. Apparatus consists of a hinged frame assembly with two feet. III. The plastic frame guides twenty-seven rods. Small replaceable washers provide friction on the rods to prevent movement. 12
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  14. 14. -The patient is placed in a prone positionand the unit is placed over the area to becontoured.- The aluminium rods are positioned sothat the tips just touch the skin.- After all the rods are positioned, thehinge locking knob is removed. The endunit is opened and the unit is removed fromthe patient. 14
  15. 15.  Mobile Contour Plotter - Pantograph-type apparatus - A rod can be moved laterally as well as up and down. When the rod is moved over the patient contour, its motion is followed by a pen that records the outline on paper.. - This contour drawing can then be used on treatment planning computers and in conjunction with CT information for the treatment plan. 15
  16. 16. - The contour plotter has a mechanical mechanismwhich links a drawing pen to a stylus arm.- Upon contact with the body, it translates bodycontours to an overhead drawing board.- When the finger plunger is depressed, engagingthe pen, a continuous plot is drawn as the operatorfollows the physical contour of the patient.- Marks can be made along the contour to indicatebeam entry and laser light locations. 16
  18. 18. By far the most accurate of the mechanicaldevices used for manual patient contouring in Radiotherapy. 18
  19. 19.  Clarke’s Model of contouring deviceElectromechanical device in which motion of the rod over the patient contour is read by a sensing device and transferred to an X-Y recorder. ◦ Consists of a master unit with a probe which is made to follow the contour of the skin, and a recording unit which produces a life size drawing of the contour on paper. ◦ The recording unit is made to reproduce the motion of the master unit by the use of Selsyn motors. ◦ The device was readily adaptable to computerized planning system. *A contouring device for use in radiation treatment planning By Hector C. Clarke, M.Sc. Department of Radiology, The General Hospital, St. Johns, Newfoundland 19 1969, Br. J. Radiol, 42, 858-860
  20. 20. *A contouring device for use in radiation treatment planning By Hector C. Clarke, M.Sc.Department of Radiology, The General Hospital, St. Johns, Newfoundland 20 1969, Br. J. Radiol, 42, 858-860
  21. 21.  ADVANTAGES  DISADVANTAGES ◦ Cheap ◦ Inaccuracy ◦ Easy to use ◦ High chances of ◦ Readily available errors due to manual translation on the contouring paper 21
  22. 22.  Used for 3D treatment planning mostly CT/MRI/PET/Ultrasound images are used for contouring the patients anatomical landmarks, target volumes and the organs at risk Much more accurate than the manual system of contouring Expensive and requires trained professionals for the image acquisition and contouring. 22
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  25. 25.  Beam Modification Defined as desirable modification in the spatial distribution of radiation - within the patient - by insertion of any material in the beam path. 25
  26. 26.  There are four main types of beam modification: ◦ Shielding: To eliminate radiation dose to some special parts of the zone at which the beam is directed. ◦ Compensation: To allow normal dose distribution data to be applied to the treated zone, when the beam enters a or obliquely through the body or where different types of tissues are present. ◦ Wedge filtration: Where a special tilt in isodose curves is obtained. ◦ Flattening: Where the spatial distribution of the natural beam is altered by reducing the central exposure rate relative to the peripheral. 26
  27. 27.  Field blocking  Wedge filters. and shaping  Beam flattening devices: filters. ◦ Shielding blocks.  Bolus ◦ Custom blocks. ◦ Asymmetrical jaws. ◦ Multileaf collimators. Compensators. Beam spoilers 27
  28. 28.  SHEILDING BLOCKS An ideal shielding material should have the following characteristics: ◦ High atomic number. ◦ High-density. ◦ Easily available. ◦ Inexpensive. The most commonly used shielding material for photons is Lead (Pb). 28
  29. 29.  The thickness of shielding block used depends upon the energy of the radiation. The shielding material which reduces beam transmission to 5% of its original is considered acceptable. The term half value-layer is an expression for the attenuation produced by any material. Half-value layer is defined as the thickness of material, which will reduce the intensity of the primary beam by 50%. Practically thickness of lead between 4.5 - 5 half- value layers results in 5% or less of primary beam transmission. 29
  30. 30. Beam energy Required lead thicknessCo60(1.25 MeV) 5.0 cm 4 MV 6.0 cm 6 MV 6.5 cm 10 MV 7.0 cm 30
  31. 31.  CUSTOM BLOCKS Material used for custom locking is known as the Lipowitz metal or Cerrobend. Melting point 70°C. Density 9.4 g /cm3 at 20°C (83% of lead). 1.21 times thicker blocks necessary to produce the same attenuation. Most commonly thickness of 7.5 cms used. 31
  32. 32. Bismuth, 50.00% Cadmium, 10.00% Tin, 13.30%Lead, 26.70% Bismuth Lead Tin Cadmium 32
  33. 33. Outline of the Electrically heated wire Cavities in the treatment field pivoting around a styrofoam block being traced on point (simulating the being used toradiograph using source) cutting the cast the a Styrofoam styrofoam block Cerrobend cutting device. blocks. 33
  34. 34.  INDEPENDENT JAWS Used when we want to block of the part of the field without changing the position of the isocenter. Independently movable jaws, allows us to shield a part of the field, and this can be used for “beam splitting”. Here beam is blocked off at the central axis to remove the divergence. Use of independent jaws and other beam blocking devices results in the shift of the isodose curves. This is due to the elimination of photon and electrons scatter from the blocked part of the field. 34
  35. 35. Ref : The Physics of Radiation Therapy by Faiz M Khan 35
  36. 36.  MULTILEAF COLLIMATORS Multileaf collimators are a bank of large number of collimating blocks or leaves Can be moved automatically independent of each other to generate a field of any shape. 40 pairs of leaves or more having a width of 1 cm on less (projected at the isocenter). Thickness = 6 – 7.5 cm Made of a tungsten alloy. Density of 17 - 18.5 g/cm3. Primary x-ray transmission: ◦ Through the leaves < 2%. ◦ Interleaf transmission < 3%. ◦ For jaws 1% ◦ Cerrobend blocks 3.5% . 36
  37. 37. Ref : The Physics of Radiation Therapy by Faiz M Khan 37
  38. 38. MULTILEAF COLLIMATORS ADVANTAGES  DISADVANTAGES ◦ Time for shaping and ◦ Island blocking is not inserting of custom blocks possible. is not required. ◦ As the physical penumbra ◦ The hardening of is larger than that beam, scattered produced by Cerrobend radiation, and increase in blocks, treatment of skin doses and doses smaller fields is difficult, as outside the field, as seen is the shielding of critical with physical structures, near the field. compensators is avoided. ◦ The jagged boundary of ◦ Automation of reshaping the field makes matching and modulation of beam difficult. intensity in IMRT. ◦ MLCs can also be used to as dynamic wedges and electronic compensators (2D). 38
  39. 39.  A beam modifying device which evens out the skin surface contours, while retaining the skin-sparing advantage. It allows normal depth dose data to be used for such irregular surfaces. Compensators can also be used for ◦ To compensate for tissue heterogeneity. This was first used by Ellis, and is primarily used in total body irradiation. ◦ To compensate for dose irregularities arising due to reduced scatter near the field edges (example mantle fields), and horns in the beam profile. 39
  40. 40. Ref : The Physics of Radiation Therapy by Faiz M Khan 40
  41. 41. Two-dimensional compensators Used when proper mould room facilities are not available. Thickness varies, along a single dimension only. Can be constructed using thin sheets of lead, Lucite or aluminum. This results in production of a laminated filter. Constructed by gluing together sheets of lead or other material in a stepwise fashion to form a laminated filter. The total thickness of the filter at any point is calculated to compensate for the air gap at the point below it. 41
  42. 42. Three-dimensional compensators 3-D compensators are designed to measure tissue deficits in both transverse and longitudinal cross sections. Various devices are used to drive a pantographic cutting unit. Cavity produced in the Styrofoam block is used to cast compensator filters. Medium density materials are preferred to reduce errors. Various systems in use for design of these compensators are: ◦ Moiré Camera. ◦ Magnetic Digitizers. ◦ CT based compensator designing systems. 42
  43. 43. Ref : The Physics of Radiation Therapy by Faiz M Khan 43
  44. 44.  Special beam modification device where shadow trays made from Lucite are kept at a certain distance from the skin. Based on the principle that relative surface dose increases when the surface to tray distance is reduced. First used by Doppke to increase dose to superficial neck nodes in head and neck cancers using 10 MV photon beams. Also used in TBI to bring the surface dose to at least 90% of the prescribed TBI dose. 44
  45. 45. BEAMSPOILER 45
  46. 46.  A beam modifying device, which causes a progressive decrease in intensity across the beam, resulting in tilting the isodose curves from their normal positions. Degree of the tilt depends upon the slope of the wedge filter. Material: tungsten, brass. Lead or steel. Usually wedges are mounted at a distance of 15 centimeters from the skin surface. The sloping surface is made either straight or sigmoid in shade. A sigmoid shape produces a straighter isodose curve. Mounted on trays which are mounted on to the head of the gantry. 46
  47. 47. Photograph of a 45* wedge for a 4MV linear acceleratorWedge angles used are: 60°, 45°, 30° & 15°. Isodose shift due to the wedge 47
  48. 48. Ref : The Physics of Radiation Therapy by Faiz M Khan 48
  49. 49. Wedges can be of two types : - Individualized wedge - Universal wedge Universal Individualized wedge wedge 49
  50. 50.  Wedged fields are generally used for relatively superficial tumors. Beams are usually directed from the same side of the patient. The broad edges of the wedges should be aligned together. The wedge angle chosen depends on the angle between the central rays of the two beams also called the “hinge angle”(φ). Wedges: ◦ Reduce the hot spots at the surface ◦ Rapid dose falloff beyond the region of overlap. The overlap region is also called the “plateau region”. Thus the 2 factors on which the wedge angle is chosen are: ◦ The hinge angle. ◦ The wedge separation. The wedge angle that will make the isodose curves parallel to each other and the hinge angle bisector is obtained using the equation. Hinge angle = 90-Wedge angle/2 50
  51. 51. Ref : The Physics of Radiation Therapy by Faiz M Khan 51
  52. 52. Problem SolutionA different value of wedge angle is A small range of hinge angles canrequired for different beam angles be covered by a given wedge angle without producing significant variation( 5%).The body contour can be more Compensators may be used tocurved as a result, the isodose overcome the deficit or a differentcurves are not obtained in the wedge angle can be used, so thatmanner desired. part acts as a compensator. 52
  53. 53.  A beam flattening filter reduces the central exposure rate relative to that near the edge of the beam. Used for Linear accelerators. Due to the lower scatter the isodose curves are exhibit “forward peaking”. The filter is designed so that the thickest part is in the centre. Material: copper or brass.
  54. 54.  A tissue equivalent material used to reduce the depth of the maximum dose (Dmax). Better called a “build-up bolus”. A bolus can be used in place of a compensator for kilovoltage radiation to even out the skin surface contours. In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions. 54
  55. 55.  The thickness of the bolus used varies according to the energy of the radiation. In megavoltage radiation: ◦ Co60 : 2 - 3 mm ◦ 6 MV : 7- 8 mm ◦ 10 MV : 12 - 14 mm ◦ 25 MV: 18 - 20 mm Properties of an ideal bolus: ◦ Same electron density and atomic number. ◦ Pliable to conform to surface. ◦ Usual specific gravity is 1.02 -1.03 55
  56. 56.  Commonly used materials are: ◦ Cotton soaked with water. ◦ Paraffin wax. Other materials that have been used: ◦ Mix- D (wax, polyethylene, magnesium oxide) ◦ Temex rubber (rubber) ◦ Lincolnshire bolus (sugar and magnesium carbonate in form of spheres) ◦ Spiers Bolus (rice flour and soda bicarbonate) Commercial materials: ◦ Superflab: Thick and doesnt undergo elastic deformation. Made of synthetic oil gel. ◦ Superstuff: Add water to powder to get a pliable gelatin like material. ◦ Bolx Sheets: Gel enclosed in plastic sheet. 56
  57. 57. Superflab bolus Material 57
  58. 58.  Beam modification increases conformity allowing a higher dose delivery to the target, while sparing more of normal tissue simultaneously. Megavoltage radiotherapy is better suited for most forms of beam modification due to it’s favorable scatter profile. 58
  59. 59.  The Physics of Radiation therapy by Faiz M Khan Radiotherapy Planning by Gunilla C Bentel Practical Radiotherapy Planning by Dobbs, Barrett, Ash A contouring device for use in radiation treatment planning , Hector C. Clarke, M.Sc. Department of Radiology, The General Hospital, St. Johns, Newfoundland, 1969, Br. J. Radiol, 42, 858-860 59
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