2. • Desirable modification in the spatial
distribution of radiation - within the patient -
by insertion of any material in the beam path.
3. Problem in beam modification
• Radiation - made up of primary and scattered
photons.
• modification devices → alteration of dose
distribution
• scattering results in a “blurring” of the effect
of the beam modification
4. Types of beam modification devices
Shielding And Shaping
A. Shielding Blocks
B. Custom Blocks
C. Independent Jaws
D. Multi Leaf Collimators
Compensators
Wedge Filters
Bolus
5. Other devices
• Beam flattening filters
• Beam spoilers
• Breast cone
• Penumbra trimmers
6. Shielding
• protect the critical structures around the
treated volume by using various devices.
The aims of shielding are:
– To protect critical organs.
– Avoid unnecessary irradiation to surrounding
normal tissue.
– Matching adjacent fields.
7. Ideal shielding material
Characteristics
• 1.high atomic number
• 2.high density
• 3.easily available
• 4.inexpensive
• Most common shielding material used for
photons is LEAD.
9. Thickness of shielding material
• Depends on attenuation of shielding material
• Expressed by half value-layer
• defined as the thickness of an absorber
required to attenuate the intensity of beam to
half its original value.
10. • For practical purposes, the shielding material
which reduces beam transmission to 5% of its
original is considered acceptable.
• The number of HVL (n)
1/2n = 5% or 0.05
Thus, 2n = 1/0.05 = 20 . OR, n log 2 = log 20.
n = 4.32
• The relationship - only for mono energetic x-
ray beams.
• Practically thickness of lead between 4.5 - 5
half-value layers results in 5% or less of
primary beam transmission.
11. Shielding
100%
50%
250 KV
4 MV
Lesser amount of scattered
radiation with megavoltage
radiation means that the
attenuation produced by
shielding is also more.
The higher scatter contribution
to the overall dose results in
lower dosage adjacent to the
shielded area in kilovoltage
radiation.
12. Placement of shielding
Kilovoltage radiation shielding - by placing sheets of
lead on the surface directly.
• necessary because of the lower penetrating power
of the beam.
Megavoltage radiation
– Thicker blocks
– Placed higher up in shadow trays (15 -20 cm).
– Avoids increase in skin dose due to electron
scatter.
– impossible to place the heavy block on the body
13.
14. CUSTOM BLOCKS
• Introduced by POWER’S et al.
• Material - Lipowitz metal or Cerrobend.
• Melting point 70°C.
• Density 9.4 g /cm3 at 20°C (83% of lead).
• low melting point - enables it to cast in any shape-
advantage over lead .
• At room temperature it is harder than lead.
16. 1.21 times thicker blocks
necessary to produce the
same attenuation.
Most commonly thickness
of 7.5 cms used.
17. Constructing cerrobend blocks
Steps
• 1.Drawing the outline of treatment field including
the areas to be shielded on a simulator radiograph .
• 2.constructing divergent cavities in a styrofoam
block
• 3.filling the cavities with cerrobend material in liquid
state.
18. Custom blocks
Outline of the
treatment field
being traced on
radiograph using a
Styrofoam cutting
device.
Electrically heated wire
pivoting around a point
(simulating the source)
cutting the styrofoam block
Cavities in the
styrofoam block
being used to cast
the Cerrobend
blocks.
19. • Shielding blocks can be
of two types:
–Positive blocks,
where the central
area is blocked.
–Negative blocks,
where the
peripheral area is
blocked.
20. Independent jaws
• To block part of the field without changing the
position of the isocenter.
• used for “beam splitting”.
• results in the shift of the isodose curves.
• replaced half beam blocks as beam splitters.
21. • Asymetric collimation produced by these
independent jaws has an effect on
• 1.Physical Penumbra
• 2.Tilt of isodose curves
• by eliminating photon and eletron scatter from
blocked portion of field thereby reducing dose near
the edge.
22. Compensators
• A beam modifying device which evens out the
skin surface contours, while retaining the skin-
sparing advantage.
• normal depth dose data can be used for such
irregular surfaces.
• also be used for
– To compensate for tissue heterogeneity.
– primarily used in total body irradiation.
– To compensate for dose irregularities arising
due to reduced scatter near the field edges (eg
mantle fields), and horns in the beam profile.
24. • The dimension and shape of a compensator must
account for :
– Beam divergence.
– Linear attenuation coefficients of the filter
material and soft tissue.
– Reduction in scatter at various depths ,when it
is placed at a distance away from the skin.
25.
26. • A tissue equivalent compensator with the
same thickness of missing tissue will over
compensate.
• To compensate for decrease in scatter -
should use appropriate thickness of
compensator material.
27. Density ratio =
Reqd thickness of tissue
equivalent material along a ray
Missing tissue thickness along the
same ray
The thickness ratio depends on:
Compensator to surface distance.
Thickness of the missing tissue.
Field size.
Depth.
Beam quality
28. • distance is the most important factor when d is ≤
20 cm.
• a fixed value of thickness ratio (τ) is used for most
compensator (~ 0.7).
• The formula used for calculation of compensator
thickness is given by:
TD x (τ/ρc),
• TD is the tissue deficit and ρc is the density of the
compensator.
• The term compensator ratio is the inverse of the
thickness ratio. (ρc /τ ).
29. 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.
30. Useful in pts in whom contour varies only in one
dimension
egAP-PA fields in a sloping mediastinum
•Thin sheets of lead with known attn coefs and
glueing them together in a step wise manner.
Pt contour taken with atleast 3 ref pts
Tissue deficit = max thickness –thickness at ref
points.
31. Three-dimensional compensators
• to compensate tissue deficits in both transverse
and longitudinal cross sections.
• Cavity produced in the Styrofoam block is used to
cast compensator filters.
• Medium density materials are preferred to
reduce errors.
32. • Eg.ellis type filter,rod boxes,pantographic
devices.
• More recent devices:
• Moire camera,3D magnetic digitiser ,CT based
compensators.
33. Compensating wedges
• Used in curved surfaces and oblique beam
incidences in which contour can be approximated
with a straight line.
• fabricated from a metal such as copper, brass or
lead.
• To compensate for a missing wedge of tissue.
34. Compensating wedges
differences between compensating wedges and
wedge filters
Standard isodose curves, can be used
c-wedges are not designed to produce tilt in
isodose curves unlike standard wedges.
No wedge transmission factors are required.
Partial field compensation can be done for
only part of contour which is irregular in shape.
35. Multi leaf collimators
• Large number of collimating blocks or leaves
that can be driven automatically independent
of each other to generate a field of any shape.
• Typical MLC consists of 80 leaves or more
and number depends on sophistication of
machine.
36. Basic geometry of MLCs
• 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% .
37.
38. Advantages of Multi leaf collimators
• 1.beam shaping is simple and less time
consuming.
• 2. can be used without entering treatment room.
• 3.correction and changing of field shape is simple.
• 4.overall treatment time shortened.
• 5.constant control and continuous adjustment of
the field shape during irradiation in advanced
conformal radiotherapy is possible
39. • Eliminates the use of blocks for shielding and
field shaping.
• Ideal for treating multiple number of fields.
• Integral part of newer techniques like conformal
therapy and IMRT.
40. Wedge Filters
• Most commonly used beam modifying device.
• Works by producing tilt in the isodose curves.
• 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
cms from the skin surface.
41.
42. • 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.
43. Wedge filters
• The two dimensions of wedges are
important – “X” or width and “Y” or
length.
• If the X dimension of field is longer
then we can’t use the wedge without
risking a hot spot!!
X
This area will have a
hot spot.
44. • Wedges come in 4 angles 15,30,45 and 90
degrees.
As the angle increases
Attenuation produced by the thicker end
(heel)increases .
Dose transmission from thinner end(toe)
thus tilting of isodose curve increases.
45. Selection of wedge
• 1.Wedge isodose angle or wedge angle
The angle through which an isodose curve is
tilted at the central ray of beam at a specified
depth (1/2 or 1/3 of beam width or at 50%
isodose line).
2.Hinge angle
It is the angle between central axes of two
beams passing through the wedge.
46. 3.Degree of separation between wedges
Distance between the thick ends of wedge filters
as projected on the surface.
4. Wedge transmission factor
defined as ratio of dose with and without wedge
at a point in phantom along the central axis of
beam.
49. Classification of wedge systems:
Physical wedge : Individualised wedge
universal wedge
Motorised wedge
Virtual or dynamic wedge
Omni wedge
50. a.Individualised wedge:
Requires separate wedge for each beam width.
Optimally designed to minimise output loss.
Mechanism provided to align thin edge of the wedge
with border of the light field.
51. Universal wedge:
Single wedge for all beam widths.
Fitted centrally in the beam..Field can be opened to any size.
Only a small part of the wedge produces the wedging effect.
The rest produces reduction in machine output
52. •Individual wedge optimises on output---preferred for Co-60
units
•Universal wedge—useful for linacs.
Motorised wedge:
Universal or sliding wedge incorporated into the linac head.
53. Dynamic wedge:
Linacs have an option of allowing independent movement of
collimator jaws
This can be used to obtain wedge shaped dose distribution
by moving one of the jaws into the treatment field.
2types:
Dynamic jaw wedge:
Jaw moves while the beam is in on position
Static jaw wedge:
Jaw kept fixed when beam is turned on.
54. Omni wedge:
Motorized wedge + dynamic wedge + open field.
Provides an effective wedge direction and angle
to match the patient’s contour
Greater wedge flexibility, beyond that of a single plane motorized
wedge and independent of diaphragm rotation
55. Working principle
Wedge Pair Fields
For treatment using perpendicular beam
arrangement (gantry angles o degree and
90 degree) the superficial region of tumor
receives higher dose or hot spot occurs,
• To avoid this wedges are placed with thick
ends adjacent to each other to get uniform
distribution.
56.
57.
58. Open And Wedged field combinations
For treatment of some tumors when open
field anteriorly and wedged field laterally is
used
a. Dose contribution from anterior field decreases
with depth
b. Bilateral wedges produce compensation and
attenuation at thicker end
c. Boost to the deeper area by thinner end
59.
60.
61. Flattening filters
• 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
exhibit “forward peaking”.
• The filter is designed so that the thickest
part is in the centre.
• Material: copper or brass.
63. Bolus
• 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.
64.
65. • 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
66. • Other materials that have been used:
– Mix- D (wax, polyethylene, mag oxide)
– Temex rubber (rubber)
• Spiers Bolus (rice flour and soda bicarb)
• Unit density wax
• Lincolnshire bolus
67. • 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
68. Breast Cone
• A beam modifying and directing device used
for a tangential fields therapy.
• Advantages:
– 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.
69. Penumbra Trimmers
• Region at the edge of the beam where the dose-rate
changes rapidly as a function of distance from the
beam axis.
• Types:
– Transmission penumbra: Transmission through the
edge of the collimator block.
– Geometrical penumbra : Finite size of the source.
• Physical penumbra: Lateral distance between to
specified isodose curves at a specific depth (90% &
20% at Dmax).
• Takes scattered radiation into account.
70. • Penumbra width depends upon:
– Source diameter.
– SSD.
– Depth below skin.
– Source to diaphragm distance (inversely)
• Consists of extensible, heavy metal bars to
attenuate the beam in the penumbra region.
• Increase the source to diaphragm distance,
reducing the geometric penumbra.
71. Penumbra trimmers
• Another method is to
use secondary blocks
placed close to the
patient ( 15 – 20 cms).
3. P = AB ( SSD + d – SDD)/ SDD
d
B
1. CD/ AB = MN/ OM
SSD
SDD
C D
A
E
P
O
M
N
A
2. CD/ AB = SSD + d – SDD / SDD
72. Electron beams
• Electron field shaping is done using lead cutouts.
• For a low-energy electrons (<10 MeV), sheets of lead,
less than 6 mm thickness are used.
• The lead sheet can be placed directly on the skin
surface.
• Shields can also be supported at the end of the
treatment cone if too heavy at the cost of greater
inaccuracies.
• Design is easier, because the size is same as that of
the field on the patients skin.
73. Direct / Internal Shielding
• Used for electron beam shielding.
• A lead shield can be placed where
shielding of structures against
backscatter electrons is required.
• A tissue equivalent material is
coated over the lead shield like
wax/ dental acrylic/ aluminum.
• Example of areas requiring these
techniques are the buccal mucosa
and eye lids.
lead
wax
Tissue to be shielded
Tissue to be
treated
74. To summarize……
Beam modification essential for effective delivery of
radiation to patients.
Helps to deliver uniform dose distribution to the
treatment volume and reduce dose to surrounding
normal tissue.
The main beam modifying devices includes:
•Filters—uniform intensity across the beam.
•Shielding blocks:
To minimize dose to certain essential structures with the
treatment field and also for custom shaping of fields..
75. •Wedges-
• tilts the isodose curves to account for sloping contours and also
for treating superficial tumors with multiple beams from the same
side of the patient.
Amount of tilt depends on the wedge angle.
•Compensators:
To compensate for the effect of missing tissue. Usually higher
density material placed at a distance from the patient surface.
•Bolus: tissue density material place on the patient surface. used in
kilovoltage beams.
•Multi leaf collimators:
Replace shielding blocks.
Essential part of conformal therapy and IMRT
82. • Various systems in use for design of these
compensators are:
– Moiré Camera.
– Magnetic Digitizers.
– CT based compensator designing systems.
83.
84. Compensator set up
– At the filter-surface distance calculated ≥ 20
cm.
– Nominal SSD measured from a plane
perpendicular to beam axis touching the
highest point in the contour.
– In SAD technique the depth of the isocenter is
measured from the same elevated point only.
85. VARIAN
• Leaf carriage system
present so maximum
horizontal field opening is
upto 30cm from centre.
• Since MLC is placed in
tertiary position placement
and repair is sufficient
without disturbing machine
function.
• Treats by DYNAMIC ARC
METHOD OR SLIDING
WINDOW TECHNIQUE.
SIEMENS
• There is no leaf carriage
system so maximum
horizontal field size is 19cm.
• Any repair of MLC entitles
entire machine to be
stopped for servicing.
• TREATS BY STEP AND SHOOT
METHOD
86. Comparison of MLCs
VARIAN
1.it is positioned as a tertiary
system below standard
adjustable jaws.
2.Shape
rounded edge
3.Non focussing leaves so
increased chance of
penumbra through rounded
edges
SIEMENS
• MLC replace the lower x-
jaws.
Straight edges
Double focussing leaves i.e
both the leaf edges and leaf
ends are according to beam
divergence.
87. VARIAN
4.Tongue and groove model
5.Inter leaf movements
present.
6.Rounded ends produce
better confirmity in
treatment volume.
7.With of leaf varies from 0.5
to o.25cm so more accurate
delineation of volume
SIEMENS
• Blunt ends
• No inter leaf movement
• Sharp ends produce step
egde effect.
• Width of leaf is 1cm
