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

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

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

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 “

37. THANK YOU