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Beam modification


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Beam modification devices

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Beam modification

  2. 2.  Desirable modification of the spatial distribution of radiation within the patient by insertion of material in the beam path.  Why? Protect normal tissues Uniform dose distribution Better suitable for Radiotherapy For special spatial distribution.
  3. 3. Problem : Radiation reaching any point is made up of primary and scattered photons. Introduction of a modification device alters the dose distribution :  primary radiation is attenuated  secondary radiation blurs the effect of beam modification.
  4. 4. Devices :  Shielding— shielding blocks custom blocks asymmetric jaws multi leaf collimators  Wedge  Compensators  Flattening filter  Bolus  Electron beam modifiers internal shields scattering foil electron cone  Breast cone  Penumbra trimmers
  5. 5. SHIELDING :  Definition : Process in which a beam modifying device is used for altering shape of the beam to reduce as far as possible, eliminate the radiation dose at some specific part or zone where beam is directed.  AIMS : protect critical structures avoid unnecessary radiation to normal tissues matching adjacent fields
  6. 6. Devices used for shielding :  Blocks  Custom blocks  Independent jaws  Multi leaf collimators
  7. 7. A. Blocks : An ideal shielding material should have high atomic number high density easily available inexpensive easily modifiable
  8. 8. Block thickness : Depends on the  beam quality  attenuation of shielding material
  9. 9. Half value layer :  Thickness of a material required to attenuate the intensity of primary beam to half of its original value.  Material which reduces beam transmission to 5% of original value is acceptable . 1/2n = 5% or 0.05 Thus, 2n = 1/0.05 = 20 OR, n log 2 = log 20. n = log20/log2 = 4.32 Beam energy Lead thickness Cobalt -60 5.5cms 4 MV 6cms 6 MV 6.5cms 10 MV 7 cms 25 MV 7cms
  10. 10. Placement of blocks : Kilo voltage :  sheets of lead are used  placed on the patients surface because : Scattered radiation is more and all shielding will be lost if distance between shield and the patient is large. Low penetrating power of the beam.
  11. 11. Megavoltage : placed high up in shadow tray at a distance of 15- 20cms from the skin surface to keep the dose to <50% of Dmax. Reasons :  to avoid increase in skin dose due to electron scatter  Heavy
  12. 12. Block divergence :  The blocks should be shaped or tapered so that their sides follow geometric divergence of the beam --------- to minimize block transmission penumbra.  Not much useful in cobalt due to large geometric penumbra .
  13. 13. B .Custom blocks  These are custom made to achieve the greatest level of dose conformity in order to minimize the dose to critical structures and normal tissues.  Material :most common material used is lipowitz metal ( Cerrobend ) other names : woods metal Bendalloy pewtalloy MCP 158
  14. 14.  Composition :  Density -9.4g/cm3at 20°C (83% of lead).  melting point – 70°C.  Advantage over lead – low melting point at room temperature, harder than lead  Disadvantage –thicker blocks necessary to produce same attenuation. (Most commonly thickness of 7.5 cms used).
  15. 15. Construction :
  16. 16. Types :  Positive blocks – where central area is blocked . Ex : lung , spinal cord  Negative blocks– where peripheral area is blocked Ex : tongue
  17. 17. Advantages :  Provide smooth boundaries by having continuous shape around field size  No field size limitation. Disadvantages :  Time consuming and expensive  When the materials are heated they emit toxic air due to lead and cadmium  Sometimes due to excessive weight of the blocks can cause injury to therapists.
  18. 18. C .Independent jaws :  Used when we want to block of the part of the field without changing the position of the isocenter .  Allows us to shield a part of the field and can be used for beam splitting in this beam is blocked off at the central axis to remove divergence  results in the shift of the isodose curves  Modern machines have 1-4 independent jaws
  19. 19.  Can be used as dynamic wedges also.
  20. 20. Clinical sites where asymmetric jaws are typically used include breast , head and neck, cranio-spinal , and prostate.
  21. 21. D . Multi leaf collimators (MLC) :  Are a bank of large number of collimating blocks or leaves that can be driven automatically independent of each other to generate field of any shape.  Typical MLC : 80 pairs of leaves or more width of 1 cm on less. Thickness : 6 -7.5cms Tungsten alloy. Density - 17 -18.5 g/cm3
  22. 22. A . B.
  23. 23. TG 50 Definitions :
  24. 24. Primary X-ray transmission :  Through leaves < 2%  Inter leaf <3% Jaws 1% Cerrobend blocks 3.5% The primary beam transmission can be further reduced by combining jaws with MLC in shielding areas outside field opening.
  25. 25. • In order to allow fast interleaf movement, while reducing radiation transmission a tongue and groove design is often used. • leads to some under dosing in the region of the tongue (10 – 25%). • It leads to leak of upto 10%
  26. 26.  MLC type A : Y jaws are replaced by bank of MLC’s. Elekta  MLC type B : Lower jaws X jaws are replaced By MLC Siemens – double focused leaves  MLC type C : MLC placed just below standard upper and lower jaws Varian A
  27. 27. Varian – tertiary MLC :
  28. 28. Applications of MLC :  To replace conventional blocking  Continuously adjusting the field shape in Conformal therapy  To achieve beam-intensity modulation  Dynamic wedges and electronic compensation.
  29. 29. Advantages :  Less time consuming  Easy to treat multiple fields  No problem of hardening of the beam , scattered radiation and increase in skin dose  Remote controlled– hence less radiation hazard  Constant control and continuous adjustment of field shape during irradiation is possible.
  30. 30. Disadvantages :  Wide penumbra  Radiation leakage between leaves  Island blocking not possible  Jagged boundaries makes matching difficult
  31. 31. COMPENSATORS :  It is a beam modifying device which evens out the skin surface contours while retaining the skin sparing advantage.  It compensates for the absent tissues so that normal depth dose distribution data and isodose curves can be used for irregular surfaces.
  32. 32. Material :  Kilo voltage : density wax or Lincolnshire bolus  Megavoltage : brass Dimension and shape : • Beam divergence. • Linear attenuation coefficients of the filter material and soft tissue. • Reduction in scatter at various depths due to the compensating filters, when it is placed at the distance away from the skin rather than in contact.
  33. 33. Thickness ratio : why ?? Thickness of a tissue equivalent component along ray Missing tissue thickness along same ray Depends on : Compensator surface distance Thickness of missing tissue Field size Depth Beam quality
  34. 34.  The formula used for calculation of compensator thickness is given by: TD x (τ/ρc ) TD is the tissue deficit ρc is the density of the compensator. τ is the thickness ratio.  when d is ≤ 20 cm, a fixed value of thickness ratio (τ) is used for most compensator work, (~ 0.7).  The term compensator ratio is the inverse of the thickness ratio. (ρc /τ ).
  35. 35. Types :  2D : varies along a single dimension only ex : lead, aluminum, Lucite taking into account their effective attenuation coefficient, electron densities and the thickness coefficient. This results in production of a laminated filter.
  36. 36.  3D : measures tissue deficits in both transverse and longitudinal cross sections. designed by Moire camera magnetic digitizers CT based
  37. 37. Uses :  tissue heterogeneity ( Ellis )  improve dose uniformity where dose irregularities arises due to reduced scatter near beam edges and high dose regions or horns in beam profile.  For Total Body Irradiation – as lung compensator
  38. 38. WEDGE  A wedge shaped beam modifying device, which causes a progressive decrease in intensity across the beam, resulting in tilting of the isodose curves from their normal positions.  2 classes –physical non physical
  39. 39. Physical wedge filter :  Wedge shaped absorber that causes a progressive decrease in intensity across the beam resulting in a tilt of the isodose curves from their normal positions  Tilt occurs towards the thin end  Tilt depends on slope of the wedge filter. straight sigmoid – straighter isodose curves  Material: tungsten, brass. Lead or steel.
  40. 40. Nonphysical wedge filter :  Electronic filter that generates a tilted dose distribution profile similar to a physical wedge by moving one of the collimating jaws from one end of the field to the other.  Ex : Varian's enhanced dynamic wedge Siemens virtual wedge  Advantage : automation of treatment delivery less peripheral dose
  41. 41. Placement :  Internal : internal motor slides the wedge into position placed above secondary collimating jaws.  External : manually inserted into beam placed on a transparent plastic tray that can be inserted into the head of the machine. minimum distance of 15cm is required between any absorber in the beam and surface in order to keep skin dose below 50% of Dmax
  42. 42. Wedge isodose angle :  It is the compliment of the angle through which isodose curve is tilted with respect to central ray of the beam at a specified depth .  Angle decreases with increasing depth due to scattered radiation.  The choice of the reference depth varies: • 10 centimeters. • 1/2 - 1/3rd of the beam width. • At the 50% isodose curve (kV)
  43. 43. Depends on :  Hinge angle : Angle between the central rays of two intersecting beams Θ = ϕ/2  Wedge separation – distance between thick ends of wedge filters
  44. 44. Wedge transmission factor :  The ratio of doses with wedge and without the wedge at a point in phantom along the central axis of the beam .  Presence of wedge decreases output of the machine.  Wedge factor increases with increasing depth and field size due to beam hardening and extra scatter from wedge .
  45. 45. Construction of a wedge :
  46. 46. Dimensions :  “X” is width and “Y” is length.  All wedges are aligned so that the central axis of the beam is at the central axis of the wedge.  If the X dimension of field is longer then we cant use the wedge without risking a hot spot!!
  47. 47. Parts of a wedge : Locks toehole heel width length handle
  48. 48. Types :  Physical – individual universalized motorised  Nonphysical --Enhanced dynamic dynamic  Pseudowedge  Virtual wedge  Omni wedge
  49. 49. Individual wedge :  Separate wedge is used for each beam width  Used in cobalt beams  Width is fixed  Angles--15°, 30°, 45°, 60°.  Using bigger wedge will reduce output of machine  6W x (15) 8W x (15) 10W x (15)
  50. 50. Universal wedge :  Single wedge serves for all beam widths fixed centrally in the beam irrespective of field size.  Saves time  Not useful for cobalt  Angles - 15°, 30°, 45°, 60°  Come in one size of 20 x 30 cms.
  51. 51. Motorized wedge :  Physical wedge integrated into head of the unit and operated remotely.  Single 60 physical wedge which generates desired angle from 0 to 60
  52. 52.  Dynamic wedge : Wedge effect produced by movement of one of the independently movable collimator leaf across the field.  2 types : Simple Dynamic Enhanced Dynamic Can provide wedge angles – 15,30,45,60 Wedge angles of 10,15,20,25,30,45,60 Only for symmetric fields of size upto 20cm width Both symmetric and asymmetric field size upto 30cm width
  53. 53. Pseudowedge :  created by opening of small field in large field usually  small field gives 2/3rd dose while large field gives 1/3rd dose Virtual wedge-  wedged dosimetry produced by movement of collimators planned at advanced TPS of the system  Advantage : wedge factor not needed.
  54. 54. Omni wedge : here the wedged fields are combined along the X axis and Y axis with proper weight provides diagonal wedge profile Compensating wedges : used where the contour can be approximated with a straight line for a oblique beam (for missing wedge of a tissue) Differences from wedge filters : standard isodose curves can be used no wedge transmission factor are required partial field compensation can be done
  55. 55. Wedge application : wedge pair field  For treatment using perpendicular beam arrangement the superficial region of tumor receives high dose or hot spot occurs  To avoid this wedges are placed thick edges adjacent to each other
  56. 56. Open and wedge field combination :  Dose contribution from anterior field decreases with depth  Bilateral wedges produce compensation and attenuation at thicker end  Boost to deeper area by thin end
  57. 57. Flattening filters : Why ?  Exposure rate at the center is greater that that towards the beam edge . inverse square law natural spatial distribution more scattered radiation in the center. Thus flattening filters are the devices which reduce the central exposure rate relative to that near the edge
  58. 58.  Used for megavoltage  Role in kilovoltage : initially used toavoid excess dose to normal tissues– but not preferred as it is complicated due togreater chance of scattered radiation.
  59. 59.  Material – copper or aluminum  Design – thickest at the center  It doesn't’t alter central axis percentage depth dose values  Accurate positioning is important prescribed at 10cms extent of flatness should be +_3% along central axis should cover >80% of the field
  60. 60. USES : in treatment of pituitary ,prostate Flattening filter free linacs :
  61. 61. Flattening filter free linacs: (FFF)
  62. 62. Bolus : It is a tissue equivalent material placed directly on the skin surface kilovoltage – to even out irregular contours of a patient (as a compensator) Megavoltage – to reduce the depth of Dmax— (build up bolus)
  63. 63. Ideal bolus :  Same atomic no as that of tissue  Same Electron density  Same absorption and scattering properties  Easily pliable Energy thickness Coblat -60 2-3mm 6MV 7-8mm 10MV 12-14mm 25MV 18-20mm
  64. 64. Materials:  Cotton soaked in water  Paraffin wax  Mix – D (wax , polyethylene ,MgO2)  Temex rubber  Lincolnshire bolus (87%sugar,13% MgCo3)  Spiers bolus (60%rice flour ,40% NaHCo3)
  65. 65. Commercially available :  Superflab : made of synthetic oil gel thick and doesn’t deform  Superstuff : water + powder pliable gelatin like material  Elastogel :gel enclosed in plastic sheets good adhesive property thereby minimizing air gap
  66. 66. Bolus in electron therapy :  Electron bolus is defined as water or near water equivalent material that is normally placed either in direct contact with skin surface or inside body cavity .  Uses– to flatten out irregular surface increase surface dose shape coverage of treatment volume
  67. 67.  Placement : such that 90% dose reaches skin surface  Materials : 1) flexible sheet Ex: superflab placed directly on skin surface thickness increments of 0.3-0.4cms use- chest wall irradiation
  68. 68. 2) Rigid bolus sheet  Also known as scatter plate– as it scatters electron beam in addition to decreasing energy .  Used in treating highly irregular or sensitive skin surfaces  Standard thickness of PMMA (0.125 to 0.25 cm) are placed perpendicular to beam.  Should be in contact or close to patient to avoid large penumbra which occurs due to air gap.
  69. 69. Beam spoilers :  The beam spoiler is composed of a sheet of material which has a low atomic number,typically lucite.  used to increase the build-up dose near the surface for treatment of superficial treatment areas  First used by Doppke to increase dose to superficial neck nodes in head and neck cancers using 10 MV photon beams.
  70. 70. Electron beam modifiers : Electron traveling in a medium loses energy due to : Ionization and excitation – soft tissue Bremsstrahlung radiation Electrons are generated in a similar way to photons in a linear accelerator, with the major difference being:  Lack of a tungsten target  A scattering foil instead of a flattening filter  An electron applicator or electron cone which provides added collimation of the beam
  71. 71. Electron Cone :  To avoid variation in output and electron scatter, jaws cannot be used to collimate electron beams.  An electron beam cone is therefore used to provide the collimation– fitted to treatment head.  A primary collimator is provided close to source – defines the maximum field size.  A secondary collimator, near the patient defines the treatment field.
  72. 72. Scattering foil :  A device to widen the thin pencil beam (3 mm) of electrons.  Metallic plates of tin, lead or aluminum are used.  Disadvantages: ◦ Beam attenuation. ◦ Generation of bremsstrahlung radiation.  Advantages: ◦ Less prone to mechanical errors. ◦ Less expensive. ◦ Requires less instrumentation.
  73. 73. Internal shield :  Stops electrons that enter the body surface before they reach any critical structures and deposit any significant dose  Tungsten is used for testis and ovaries  A tissue equivalent material is coated over the lead shield like wax/ dental acrylic/ aluminum.
  74. 74.  Thickness of lead required –0.5mm/Mev  When lead is used for shielding ,it needs to be supplemented by other shielding material because less efficient—hence bolus placed over skin surface.  Ex : salivary glands eyeblocks intraoperative therapy
  75. 75. Breast cone :  It is a beam modifying and directing device used for tangential fields therapy
  76. 76.  Directs beam to the central axis of the area of interest where a tangential beam is applied to a curved surface  Helps position the patient with an accurate SSD  Endplate provides compensation ,enhances surface dose and presses down the tissue  Effective shielding of lungs
  77. 77. Penumbra trimmers :  Penumbra refers to the region at the edge of beam where dose rate changes rapidly as function of distance from beam axis.  Types : geometric – source size transmission – edge of collimator block physical – lateral distance between 2 isodose curves at specified depth
  78. 78.  Width depends on : source diameter SSD depth below skin SDD (inversely related)  Trimmers consist of extensible , heavy metal bars to attenuate the beam in the penumbra region.  They increase the SDD distance ,thereby reducing geometric penumbra
  79. 79. Conclusion :  Beam modification is essentially a fundamentally important aspect of modern radiotherapy treatment planning and delivery.  Beam modification increases conformity allowing a higher dose delivery to the target while sparing more normal tissues simultaneously  Megavoltage radiation is more favorable for beam modification due to its favorable scatter profile  However necessitates close scrutiny of every phase of planning and treatment process—daily
  80. 80. “Price of safety is eternal vigilance’’ Thank you…