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Beam modification

  1. 1. PRINCIPLES AND PRACTICE OF BEAM MODIFICATION G .Lakshmi Deepthi
  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…

Notas do Editor

  • Beam modification can be defined as


    To enable normal ditrbution data to be applied to treated zone when beam enters obliquely
  • This effect of blurring is more in kilo voltage than in megavoltage.
  • In this image we can see the effect of shielding on mv rad compared to kv radiation..
    In megavoltage : the attenuation produced by the shielding material is more due to less amount of scatterd radiation
    Whereas in kv : less attenuation

    And we can also see tat due to more scatter in kv there is low dose of radiation goin adjacent to shielded area
    THUS SHIELDING IS ACHIEVED BETTER IN MV THAN KV.
  • The choice of the shielding material is also dictated by the type of beam being used!!
    Most commonly used -- lead.

  • If n is the no of half value layers required to achieve this transmission of 5 % then the above is derived..

    Thus a thickness of lead greater than 4.3 hvls would give less than 5% primary beam transmission nd is recommended
    hence practical thickness of lead used is bn 4.5 tp 5 HVLs .
    As no diff in attenuation at higher energies we don’t get higher HVLS
  • Because the absorber material is a source of electron scatter there will be an increase in skin dose ---to avoid this
    so that the main advantage of skin sparin by megavoltage is not lost..


    Shadow tray – transparent plastic tray
  • not done as last minute alteration in field blockin

    Divergent blocks are most suited for linac beams – as small source size hence small geometric penumbra
  • Any material can be used including lead
  • 1.21 times thicker blocks necessary to produce the same attenuation.

    Lead meltin point – 327 c
    Low melting point so enables to cast in any shape
    At room temp harder than lead
  • Care to be taken preparation :
    Cerrobend should be poured slowly to prevent air bubble formation
    Styrofoam block should be pressed tightly against the rubber pad at bottom to avoid leakage of material
    Inside wall of cavity to be sprayed with silicone to allow easy release of styrofoam pieces from block.
  • Hence this has given rise to the advent of multi leaf collimators and independent jawss.

  • 2) In cobalt machines and older linear accelerators, where fixed jaws were present, half beam blocking was used to for beam splitting.


    3)due to the elimination of photon and electrons scatter from the blocked part of the field

  • Generated electronically by creating wedged beam profiles through the dynamic motion of an independent jaw within the treatment field.
    In cobalt machines and older linear accelerators, where fixed jaws were present, half beam blocking was used to for beam splitting.
  • In addition, the use of asymmetric jaws as beam splitters, for field reductions, and with MLCs is helpful for most sites.
    asymmetric jaws to match supraclavicular and tangential fields for breast irradiation.
    reducing overall patient setup time
    the increased attenuation by the jaws reduces the dose to the contralateral breast and lung.
    A technique for matching lateral head and neck fields and the supraclavicular field using asymmetric jaws was done

  • The material of choice for leaf construction is tungsten alloy because it has one of the highest densities of any metal. Tungsten alloys are also hard, simple to fashion, reasonably inexpensive, and have a low coefficient of thermal expansion.
  • Non-focused (rounded) Leaf Ends:
    Leaf motion restricted to a single plane
    In principle, penumbra is somewhat greater than for focused collimators of divergent custom blocks
    Potential for greater leaf end transmission when leaves are abutted
    VARIAN ND ELEKTA USE THESE

    Focused leaves – maintain geometric divergence

    leaf and leaf size matches with beam divergence


  • Leaf transmission :Reduction of dose through the full height of the leaf
    Interleaf Transmission : Reduction of dose between adjacent leaves
    Leaf End Transmission :Reduction of dose between the ends of opposed leaves
  • Rounded edges is to provide constant beam transmision through leaf edge

    In linac sharp dose fall ioff not achieved because dose fall off at the edge determined by scattered photons and electrons
    IN PGI WE HAVE SINGLE FOCUS LEAVES MLC
  • Mini – 2-5 mm leaf size
    Micro <2mm

    Secondary : replace one of the jaws—upper or lower – elekta & siemens
    Tertiary : add on device in the head of the LINAC below the jaw collimators.--varian
  • The lower jaws can be split into a set of leaves as well. This design is used by SiemensTM and is double-focused. Both leaf ends and leaf sides match the beam divergence. That means that the collimator leaves move along the circumference of a circle centred at the x-ray tar- get of the linear accelerator, such that the end of the collimator is always tangential to the radius of the circle.1
    The leaves of Siemens MLC can extend 10 cm across the field centreline, which allows a maximum leaf travel of 30 cm.
  • Advantage : MLC can be positioned just below the level of the standard upper and lower adjustable jaws . This design is used by VarianTM and was chosen to avoid lengthy downtime in the event of a MLC system malfunction. USING THIS APPROACH, IT IS POSSIBLE TO MOVE LEAVES MANUALLY OUT OF THE FIELD SHOULD A FAILURE OCCUR.
    THE MAJOR DISADVANTAGE OF PLACING THE MLC BELOW THE STANDARD JAW SYSTEM IS THE ADDED BULK AND CLEARANCE TO THE MECHANICAL ISOCENTRE.
    .
  • Electronic compensationn in head and neck
    Not useful in electron treatment and
  • because Time for shaping and inserting of custom blocks is not required.

    2) because the procedure can be automated such that complex field shaped fields can be generated in sequence, reducing the set-up time.

    3)As seen with physical compensators


  • 1) Because the physical penumbra is larger than that produced by Cerrobend blocks treatment of smaller fields is difficult, as in shielding of critical structures, near the field.
    3 done by cerrobend blocks
  • 1) In total body irradiation

    scatter near the field edges (example mantle fields), and horns in the beam profile.
  • Lincolnsire bolus nt used in mv bcoz difficult to adjust as per thickness

    To compensate for these factors a tissue compensator always has attenuation less than that is required for primary radiation.
  • Compensator surface distance is the most important
    as the dist bn skin and compensator increases –thickness decreases.as

    BCOZ A COMPENSATOR OF SAME THICKNESS AS THAT OF MISSING TISSUE WILL OVERCOMPENSATE DUUE TO DECREASE IN SCATTER ..
  • The term τ/ρc can be directly measured by using phantoms.
  • Used when proper mould room facilities are not available
    In this thin sheets of lead are glued together in a stepwise fashion..
    Series of sticks placed
    Replaced by material – aluminum-shorter length
    Glued to the base plate
  • Various devices are used to drive a pantographic cutting unit.
    Cavity produced in the Styrofoam block is used to cast compensator filters.

    Various systems in use for design of these compensators are:
    Moiré Camera – SPECIALLY DESIGNED CAMERA MOUNTED ON A SIMULATOR, ALLOWS TOPOGRAPHICAL MAPPING OF THE PATIENT AND THE DATA IS GENERATED
    Magnetic Digitizers – HANDHELD STYLUS WITH MAGNETIC SESOR WHICH SCANS THE PATIENT BODY
    CT based compensator designing systems – 3D TPS HAVE SUFFICIENT MULTILEVEL CT DATA TO AUTOMATE COMPENSATOR DESIGN
  • Need :when ones side of the pt is to treated bt more than one beam has to be used.
  • Less peripheral dose compared to physical wedge filter for example leess dose to the contralateral breast when using tangential breast irradiation technique
  • More in case of LINAC
    As cobaalt beam is monoenergetic .
  • The ratio of PDD at various points for wedged and non wedged fields are calculated
    And hence the thickness is determined.
    It also depends on the HVL of the beam for the filter material.
  • because of excessive reduction of beam output with smaller fields.
  • Using a combination of open and wedged field beams

    It is mounted above the asymmetrical collimator and can be moved in and out of the radiation fields to generate desired wedge profiless.
  • STARTING POSITION RECEIVE MAX DOSE .

    SPEED OF MOVEMENT OF COLLIMATOR LEAF DETERMINES THE ANGLE OF SLOPING ISODOSE CURVE.
    SIMPLE DYNAMIC , ENHANCED DYNAMIC
  • POOR MAN WEDGE
    Used in olden days when asymmetrical jaws opening was not possible and no planning TPS were available.

    Fixed jaws can be used to produce pseudo wedges where part of the treatment field requiring greater dose would be irradiated using smaller field sizes.


  • To get uniform distribution.
  • When open field anteriorly and wedge field laterally are usedd…
  • technique is currently available on two Elekta VersaHD linear accelerator
  • less leakage outside of beam collimation and less variation of off-axis beam hardening
    FFF has increased dose rate; 1400 MU/min for 6 MV and 2400 MU/ min for 10 MV; less ;me for motion during treatment.
    FFF is softer than FF beam but surface dose is higher for FFF. Yet field size dependence of surface dose is smaller for FFF.
    4)FFF has less photon head scatter and thus less field size dependence


    a FFF linac for SBRT of liver and lung malignancies is associated with substantial
    improvement in treatment delivery time
  • In mv cant be used as a compensator as results in loss of skin sparing effect.
  • Thickness of bolus required to achieve 90-95% build up dose is substantially less than depth of Dmax
  • Water acts as bolus
    Cotton holds it

    Lincolnshire bolus made up of tiny spheres of ¼ mm diameter
    Spiers mixture -powder
  • Designed to shape the dose distribution To conform closely to target volume
  • he potential benefit of a beam spoiler relative to bolus is preservation of skin-sparing characteristics for cases in which the skin surface does not require full dose..
    In electron therapy to spread the beam or as bolus.
  • Leadin to xray production.. Compared to high atomic number target of xray tube –soft tissue has low atomic number hence less xray production

    Produced when eectrons are slowed down by any material –more in high atomi no like lead.

    For a low-energy electrons (<10 MeV), sheets of lead, less than 6 mm thickness are used.
    Design is easier, because the size is same as that of the field on the patients skin.

  • electron field shaping is done using lead cutouts placed on skin surface.
    Shields can also be supported at the end of the treatment cone if too heavy at the cost of greater inaccuracies.
    An electron cone is fitted to the treatment head, and is available in several preset sizes. This applicator has several collimators which attenuate any lateral scatter. It usually extends to just several centimetres from the surface. The most distal aperture may have an electron cutout fitted. This allows the field to be shaped appropriately, and is located in this position to prevent lateral scatter causing blurring of the beam edge.
  • IN PLACE OF FLATTENING FILTER.
    Nowadays dual foil systems are used,
    which compare well with scanning
    beam systems.
  • Against back scatter of electrons
  • Parts :
    Steel base plate
    Horizontal bar
    Lead block
    Transparent perplex plate
    Screws for distance adjustment
  • 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

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