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PRESENTED BY   MODERATED BY:
DR SANDIP      NAVIN SINGH
    Beam modification is defined as desirable
     modification to the spatial distribution of
     radiation within the patient by inserting of
     material into the beam
    TYPES OF BEAM MODIFICATIONS
1.    Shielding
2.    Compensators
3.    Wedge filters
4.    Beam flattening
   Field blocking and            Wedge filters.
    shaping devices:
    ◦ Shielding blocks.
                                  Beam flattening filters.
    ◦ Custom blocks.

    ◦ Asymmetrical jaws.
                                  Bolus
    ◦ Multileaf collimators.

   Compensators.
   The radiation reaching any point in a
    scattering medium is made up of a
    mixture of primary and scattered
    photons

   The result of introducing any beam
    modifying device depends on the
    relative amounts of primary and
    scattered radiation

   The aim of giving a point complete
    protection from radiation thus would
    not be achieved due to scattered
    radiation

   Another phenomenon called blurring is
    produced in primary beam by
    attenuation in beam modifier
DEFINITION
   Alteration to the shape of
    the beam to reduce or,as
    far as possible eliminate the
    radiation dose at some
    special parts of zone at
    which beam is directed.

   Shielding is achieved more
    easily with high energy
    radiation than low energy
    due to low scattering in
    high energies
The effect of a shielding block on a beam of kilovolt and megavolt
    An ideal shielding material should have
1.    High atomic number

2.    High density

3.    Easy availability

4.    Inexpensive

     Shielding blocks are most commonly made of lead.
     Shielding can be of two types
1.    Positive: Where central area is blocked, eg. lung block
2.    Negative :Where peripheral area is blocked.eg head & neck
    The thickness of lead required for adequate protection to
     shielded area depends on
1.    Beam quality
2.    Allowed transmission through the block

     A primary beam transmission of 5% through the block is
      considered acceptable

     A thickness of lead between 4.5 and 5.0 half value layer is
      recommended for clinical shielding

     Half value layer is defined as the thickness of material which
      will reduce the intensity of primary beam by 50 %
Beam Quality   Lead thickness


 COBALT 60      5cm

 4MV            6 cm

  6MV           6.5 cm

  10 MV         7 cm

   25MV          7cm
   Uses a low melting point alloy
    LIPOWITZ metal(cerroband)

   Made up of
    bismuth,lead,tin,cadmium

                                         Bi
   It melts at 70 c and can be easily
    cast to any shape                    Pb

   At room temperature it is harder
                                         Sn
    than lead                            Cd

   In Megavoltage range of photon
    beams the most commonly used
    thickness is 7.5 cm
   Used when we want to block 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.

   There is change in the physical penumbra .

   This causes elimination of photons and electron scatter from
    the blocked portion of field,reducing dose near the edges.
   It consists of a large number of collimating block or leaves that can be
    driven automaticallly
    independent of each other
    to generate a field of any
    shape
   Typically consists of 80 leaves

   Indivisual leaf has a width of
    1 cm or less projected at isocentre

   Leaves are made of tungsten
    alloy

   Have a thickness of 6 to 7.5 cm

   Primary x ray transmisssion through the leaves is <2%
   The degree of conformity between the planned field and the
    jagged field depends on

    1.   Projected leaf width
    2.   Shape of target volume
    3.   Angle of rotation of the collimator

         The use of multi leaf collimators
          in blocking and field shaping is
          ideally suited for treatment req
         large number of multiple fields
   The advantages are:
    ◦ Time for shaping and inserting        The disadvantages are:
      of custom blocks is not                ◦ Because the physical penumbra
      required.                                is larger than that produced by
    ◦ Reduction in set up time of              others treatment of smaller
      multiple fields                          fields is a drawback
    ◦ The hardening of
      beam, scattered radiation, and
      increase in skin doses and
      doses outside the field, as seen       ◦ Difficult when blocking is
      with physical compensators is            required close to critical
      avoided.                                 structures


    ◦ MLCs can also be used to as
      dynamic wedges and electronic          ◦ The jagged boundary of the
      compensators (2D).                       field makes matching difficult.
    ◦ Modern treatment like
      3DCRT,IMRT are dependent on
      it
DEFINITION
   Alterations to enable normal distribution data to be applied to
    all or part of the treated zone when the beam enters the body
    obliquely and/or it passes through different types of tissues
    or through curved irregular surface

   First used by Ellis

   Standard isodose charts are usually obtained from the
    measurement made in cubic phantoms at right angle to
    surface

   In kilovolt range unit density wax or lincolnshire bolus is used
   In megavoltage aluminium or brass compensators are used
    The dimentions and shape of the compensators is adjusted
     because of
1.   Beam divergence
2.   The relative linear attenuation coefficient of filter material
     and soft tissue
3.   The reduction in scatter at various depths when the
     compensator is placed at a distance from the skin

    To compensate for these factors its attenuation is less than
     that required for primary radiation only
   Compensators are made out of aluminium or brass blocks
    using a matrix of square columns corresponding to irregular
    surface

   The compensator thickness should be such that the dose at a
    given depth is same whether the missing tissue is replaced
    with the bolus in contact or with the compensator at given
    distance from skin surface
    The thickness ratio or density ratio defined as
        thickness of a tissue equivalent component along a
     ray/missing tissue thickness along the same ray

    It depends upon
1.    Compensator surface distance

2.    Thickness of missing tissue

3.    Field size

4.    Depth

5.    Beam quality
   Average value =0.7 if d> or equals to 20 cm for Co
    60,4Mev,10Mev

   Thickness ratioTc= TD x (τ/ρc),
     where TD is the tissue deficit and ρc is the density of the
    compensator.

   Compensator ratio(CR) defined as ratio of missing tissue
    thickness to compensator thickness necessary to give the
    dose for a particular field size and depth (ρc /τ ).
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.


                                Production of practical
                                compensator
Three-dimensional compensators
 3-D compensators are designed to measure tissue deficits in both
  transverse and longitudinal cross sections.


   Cavity produced in the Styrofoam block is used to cast compensator
    filters


   Various systems in use for design of these compensators are:
    ◦ Moiré Camera.
    ◦ Magnetic Digitizers.
    ◦ CT based compensator designing systems.
   DEFINITION:
     Alterations to produce special spatial distributions ,i.e to
    modify its isodose distributions

   First devised by Ellis & Miller

   It is a wedge shaped absorber thick at one end ,tapers at the
    other

   It causes progressive decrease in the intensity across the
    beam resulting in a tilt of isodose curve
   Degree of tilt depends on slope of wedge filters

   Wedge filters makes the isodose curves for two intersecting
    fields parallel,resulting in an uniform irradiation.

   Material used are tungsten,brass,lead,steel,Al,Cu
   Wedge angle or wedge isodose angle is defined as the angle
    through which the 50% isodose curve has been turned from
    its position in a normal beam

   Thus wedge angle=90-hinge angle/2

   Hinge angle is angle between the central rays of two
    intersecting fields
   Since the range of hinge angle is some what limited wedge
    angle of(35,45,55)will cope up with majority of cases

   This solves the problem of different value of wedge angle
    required for different beam angle
    TYPES OF WEDGE SYSTEMS
1.    Indivisualised wedge
2.    Universal wedge
3.    Dynamic wedge
4.    Virtual wedge
5.    Pseudo wedges

     Indivisualised wedge systems requires a separate wedge for
      each beam.
      Mainly used for cobalt systems
   Universal wedge system is a single wedge system that serves
    for all beam width
    Used for linear accelerator beams.

   Dynamic wedges or motorised wedges is a 60 degree wedge
    mounted in the treatment head to create the wedge profile
    beam directed

   Virtual wedge or dynamic enhanced are moving jaws that are
    moved by computer control to create wedge beam
   COMPENSATING WEDGES
   Used for oblique beam incident on curved surface whose
    contour can be approximated with a straight line.

   Metals of copper,brasss,lead used
   In a compensating wedge standard isodose curve can be used
    without modification.
   No transmitting factors are required for C-wedges.
   It can be used for partial field compensation i.e to
    compensate only a part of contour ,irregular in shape.
   A tissue equivalent material
    used to reduce the depth of
    the maximum dose (Dmax).

   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
   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
   Commonly used materials are:
    ◦ Cotton soaked with water.
    ◦ Paraffin wax.
   Other materials that have been used:
    ◦ Mix- D (wax, polyethylene, mag oxide)

    ◦ Lincolnshire bolus (sugar and mag carbonate in form of
      spheres)
    ◦ Spiers Bolus (rice flour and soda bicarb)
   Commercial materials:
    ◦ Superflab: Thick and doesn't undergo elastic deformation.
      Made of synthetic oil gel.
    ◦ Superstuff: Pliable gelatin like material.
    ◦ Bolx Sheets: Gel enclosed in plastic sheet.
   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.
   Beam modification allows us with the liberty to treat a
    specific part ,while protecting sensitive and vital organs.

   Although devices like wedges and compensators plays a
    pivotal role in treatment of patients ,they are likely to be
    superseded by newer technologies like multi leaf
    collimators,IMRT etc.

   And lastly I would like to mention the line in our physics book

        “THE PRICE OF SAFETY IS ETERNAL VIGILENCE “
THANK YOU

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Beam modifications

  • 1. PRESENTED BY MODERATED BY: DR SANDIP NAVIN SINGH
  • 2. Beam modification is defined as desirable modification to the spatial distribution of radiation within the patient by inserting of material into the beam  TYPES OF BEAM MODIFICATIONS 1. Shielding 2. Compensators 3. Wedge filters 4. Beam flattening
  • 3. Field blocking and  Wedge filters. shaping devices: ◦ Shielding blocks.  Beam flattening filters. ◦ Custom blocks. ◦ Asymmetrical jaws.  Bolus ◦ Multileaf collimators.  Compensators.
  • 4. The radiation reaching any point in a scattering medium is made up of a mixture of primary and scattered photons  The result of introducing any beam modifying device depends on the relative amounts of primary and scattered radiation  The aim of giving a point complete protection from radiation thus would not be achieved due to scattered radiation  Another phenomenon called blurring is produced in primary beam by attenuation in beam modifier
  • 5. DEFINITION  Alteration to the shape of the beam to reduce or,as far as possible eliminate the radiation dose at some special parts of zone at which beam is directed.  Shielding is achieved more easily with high energy radiation than low energy due to low scattering in high energies
  • 6. The effect of a shielding block on a beam of kilovolt and megavolt
  • 7. An ideal shielding material should have 1. High atomic number 2. High density 3. Easy availability 4. Inexpensive  Shielding blocks are most commonly made of lead.  Shielding can be of two types 1. Positive: Where central area is blocked, eg. lung block 2. Negative :Where peripheral area is blocked.eg head & neck
  • 8. The thickness of lead required for adequate protection to shielded area depends on 1. Beam quality 2. Allowed transmission through the block  A primary beam transmission of 5% through the block is considered acceptable  A thickness of lead between 4.5 and 5.0 half value layer is recommended for clinical shielding  Half value layer is defined as the thickness of material which will reduce the intensity of primary beam by 50 %
  • 9. Beam Quality Lead thickness COBALT 60 5cm 4MV 6 cm 6MV 6.5 cm 10 MV 7 cm 25MV 7cm
  • 10. Uses a low melting point alloy LIPOWITZ metal(cerroband)  Made up of bismuth,lead,tin,cadmium Bi  It melts at 70 c and can be easily cast to any shape Pb  At room temperature it is harder Sn than lead Cd  In Megavoltage range of photon beams the most commonly used thickness is 7.5 cm
  • 11.
  • 12. Used when we want to block 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.  There is change in the physical penumbra .  This causes elimination of photons and electron scatter from the blocked portion of field,reducing dose near the edges.
  • 13. It consists of a large number of collimating block or leaves that can be driven automaticallly independent of each other to generate a field of any shape  Typically consists of 80 leaves  Indivisual leaf has a width of 1 cm or less projected at isocentre  Leaves are made of tungsten alloy  Have a thickness of 6 to 7.5 cm  Primary x ray transmisssion through the leaves is <2%
  • 14. The degree of conformity between the planned field and the jagged field depends on 1. Projected leaf width 2. Shape of target volume 3. Angle of rotation of the collimator The use of multi leaf collimators in blocking and field shaping is ideally suited for treatment req large number of multiple fields
  • 15. The advantages are: ◦ Time for shaping and inserting  The disadvantages are: of custom blocks is not ◦ Because the physical penumbra required. is larger than that produced by ◦ Reduction in set up time of others treatment of smaller multiple fields fields is a drawback ◦ The hardening of beam, scattered radiation, and increase in skin doses and doses outside the field, as seen ◦ Difficult when blocking is with physical compensators is required close to critical avoided. structures ◦ MLCs can also be used to as dynamic wedges and electronic ◦ The jagged boundary of the compensators (2D). field makes matching difficult. ◦ Modern treatment like 3DCRT,IMRT are dependent on it
  • 16. DEFINITION  Alterations to enable normal distribution data to be applied to all or part of the treated zone when the beam enters the body obliquely and/or it passes through different types of tissues or through curved irregular surface  First used by Ellis  Standard isodose charts are usually obtained from the measurement made in cubic phantoms at right angle to surface  In kilovolt range unit density wax or lincolnshire bolus is used  In megavoltage aluminium or brass compensators are used
  • 17. The dimentions and shape of the compensators is adjusted because of 1. Beam divergence 2. The relative linear attenuation coefficient of filter material and soft tissue 3. The reduction in scatter at various depths when the compensator is placed at a distance from the skin  To compensate for these factors its attenuation is less than that required for primary radiation only
  • 18.
  • 19. Compensators are made out of aluminium or brass blocks using a matrix of square columns corresponding to irregular surface  The compensator thickness should be such that the dose at a given depth is same whether the missing tissue is replaced with the bolus in contact or with the compensator at given distance from skin surface
  • 20. The thickness ratio or density ratio defined as thickness of a tissue equivalent component along a ray/missing tissue thickness along the same ray  It depends upon 1. Compensator surface distance 2. Thickness of missing tissue 3. Field size 4. Depth 5. Beam quality
  • 21. Average value =0.7 if d> or equals to 20 cm for Co 60,4Mev,10Mev  Thickness ratioTc= TD x (τ/ρc), where TD is the tissue deficit and ρc is the density of the compensator.  Compensator ratio(CR) defined as ratio of missing tissue thickness to compensator thickness necessary to give the dose for a particular field size and depth (ρc /τ ).
  • 22. 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. Production of practical compensator
  • 23. Three-dimensional compensators  3-D compensators are designed to measure tissue deficits in both transverse and longitudinal cross sections.  Cavity produced in the Styrofoam block is used to cast compensator filters  Various systems in use for design of these compensators are: ◦ Moiré Camera. ◦ Magnetic Digitizers. ◦ CT based compensator designing systems.
  • 24. DEFINITION: Alterations to produce special spatial distributions ,i.e to modify its isodose distributions  First devised by Ellis & Miller  It is a wedge shaped absorber thick at one end ,tapers at the other  It causes progressive decrease in the intensity across the beam resulting in a tilt of isodose curve
  • 25. Degree of tilt depends on slope of wedge filters  Wedge filters makes the isodose curves for two intersecting fields parallel,resulting in an uniform irradiation.  Material used are tungsten,brass,lead,steel,Al,Cu
  • 26. Wedge angle or wedge isodose angle is defined as the angle through which the 50% isodose curve has been turned from its position in a normal beam  Thus wedge angle=90-hinge angle/2  Hinge angle is angle between the central rays of two intersecting fields
  • 27.
  • 28. Since the range of hinge angle is some what limited wedge angle of(35,45,55)will cope up with majority of cases  This solves the problem of different value of wedge angle required for different beam angle
  • 29. TYPES OF WEDGE SYSTEMS 1. Indivisualised wedge 2. Universal wedge 3. Dynamic wedge 4. Virtual wedge 5. Pseudo wedges  Indivisualised wedge systems requires a separate wedge for each beam. Mainly used for cobalt systems
  • 30. Universal wedge system is a single wedge system that serves for all beam width Used for linear accelerator beams.  Dynamic wedges or motorised wedges is a 60 degree wedge mounted in the treatment head to create the wedge profile beam directed  Virtual wedge or dynamic enhanced are moving jaws that are moved by computer control to create wedge beam
  • 31. COMPENSATING WEDGES  Used for oblique beam incident on curved surface whose contour can be approximated with a straight line.  Metals of copper,brasss,lead used  In a compensating wedge standard isodose curve can be used without modification.  No transmitting factors are required for C-wedges.  It can be used for partial field compensation i.e to compensate only a part of contour ,irregular in shape.
  • 32. A tissue equivalent material used to reduce the depth of the maximum dose (Dmax).  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
  • 33. 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
  • 34. Commonly used materials are: ◦ Cotton soaked with water. ◦ Paraffin wax.  Other materials that have been used: ◦ Mix- D (wax, polyethylene, mag oxide) ◦ Lincolnshire bolus (sugar and mag carbonate in form of spheres) ◦ Spiers Bolus (rice flour and soda bicarb)  Commercial materials: ◦ Superflab: Thick and doesn't undergo elastic deformation. Made of synthetic oil gel. ◦ Superstuff: Pliable gelatin like material. ◦ Bolx Sheets: Gel enclosed in plastic sheet.
  • 35. 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.
  • 36. Beam modification allows us with the liberty to treat a specific part ,while protecting sensitive and vital organs.  Although devices like wedges and compensators plays a pivotal role in treatment of patients ,they are likely to be superseded by newer technologies like multi leaf collimators,IMRT etc.  And lastly I would like to mention the line in our physics book “THE PRICE OF SAFETY IS ETERNAL VIGILENCE “