beam modifying devises

BEAM MODIFYING DEVICES

Dr Nanditha Kishore
DNB 1st year

Introduction
 Defined as desirable modification in the

spatial distribution of radiation - within the
patient - by insertion of any material in the
beam path.

Problem in beam modification
 Radiation reaching any point, is made up of

primary and scattered photons.

 Any introduction of the modification devices

results in alteration of dose distribution, due
to these two phenomena.

 The phenomena scattering results in an

“blurring” of the effect of the beam
modification

Types of beam modification
 1.Shielding

 2.Compensation
 3.Wedge filtration
 4.Flattening

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

Other devices
 Beam flattening filters
 Beam spoilers

 Breast cone
 Penumbra trimmers
 Electron beam modification

Shielding
 It is a method of modification of beam to

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.

Ideal shielding material
It should have following characteristics
 1.high atomic number
 2.high density
 3.easily available
 4.inexpensive
 Most common shielding material used for
photons is LEAD.

Choice of shielding is also
dictated by the type of beam
being used.

Thickness of shielding
material
 It depends on
 1.Attenuation of shielding material

 The term half value-layer is a convenient
expression for the attenuation produced by
any material.
 Half-value layer is defined as the thickness
of an absorber required to attenuate the
intensity of beam to half its original value.

 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 holds true, 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.

Placement of shielding
In kilovoltage radiation shielding is readily
achieved by placing sheets of lead on the surface
directly.
 This is necessary, because of the lower
penetrating power of the beam.
In Megavoltage radiation,
 Thicker blocks used.
 Placed higher up in shadow trays (15 -20 cm).
 Avoids increase in skin dose due to electron
scatter.
 Also impossible to place the heavy block on
the body !!

CUSTOM BLOCKS
 These are introduced by POWER’S et al.

 Material used for custom locking is known






as the Lipowitz metal or Cerrobend.
Melting point 70°C.
Density 9.4 g /cm3 at 20°C (83% of lead).
Major advantage of cerrobend block over
lead is its low melting point which enables
it to cast in any shape.
At room temperature it is harder than
lead.

Composition Of Cerrobend

Tin,
13.30%

Cadmium,
10.00%

Bismuth,
50.00%

Lead,
26.70%

Bismuth
Tin

Lead
Cadmium

1.21 times thicker blocks
necessary to produce the
same attenuation.
Most commonly thickness
of 7.5 cms used.

BLOCK DIVERGENCE
 Ideally the blocks should be shaped or

tapered so that their sides follow the
geometric divergence .
 This minimizes the block transmission
penumbra.
 Divergent blocks are most suited for beams
having small focal spots.

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.

 Shielding blocks can be
of two types:

 Positive

blocks, where the
central area is
blocked.
 Negative
blocks, where the
peripheral area is
blocked.

Independent jaws
 Used when we want to block of 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”.
 Use of independent jaws and other beam
blocking devices results in the shift of the
isodose curves.

 These have replaced half beam blocks as

beam splitters.

 Asymetric collimation produced by these

independent jaws has an effect on
 1.Physical Penumbra
 2.Tilt of isodose curves
 This is by eliminating photon and eletron
scatter from blocked portion of field thereby
reducing dose near the edge.

Compensators
 A beam modifying device which evens out the
skin surface contours, while retaining the skinsparing advantage.
 It allows normal depth dose data to be used for
such irregular surfaces.
 Compensators can also be used for
 To compensate for tissue heterogeneity. This
was first used by Ellis, and is primarily used in
total body irradiation.
 To compensate for dose irregularities arising

due to reduced scatter near the field edges
(example mantle fields), and horns in the
beam profile.

 The dimension and shape of a compensator
must be adjusted to account for :
 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.
 To compensate for these factors a tissue

compensator is always has an attenuation less
than that required for primary radiation.

Thickness ratio or
density ratio
 Required thickness of a
tissue equivqlent
compensator
 Missing tissue
thickness along same
ray

 A tissue equivalent compensator designed

with thee same thickness f missing tissue will
over compensate.
 To compensate for this decrease in scatter
one should use appropriate thickness of
compensator material.
 The thickness ratio depends on:
 Compensator to surface distance.
 Thickness of the missing tissue.
 Field size.
 Depth.
 Beam quality

 Of the above distance is the most important
factor when d is ≤ 20 cm.
 Therefore, a fixed value of thickness ratio (τ) is
used for most compensator work (~ 0.7).
 The formula used for calculation of compensator
thickness is given by:
TD x (τ/ρc),
 where TD is the tissue deficit and ρc is the
density of the compensator.
 The term τ/ρc can be directly measured by using
phantoms.
 The term compensator ratio is the inverse of the
thickness ratio. (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.

Three-dimensional compensators
 3-D compensators are designed to measure
tissue deficits in both transverse and
longitudinal cross sections.
 Various devices are used to drive a pantographic
cutting unit.

 Cavity produced in the Styrofoam block is used
to cast compensator filters.
 Medium density materials are preferred to
reduce errors.

 Various systems in use for design of these
compensators are:
 Moiré Camera.
 Magnetic Digitizers.

 CT based compensator designing systems.

Compensating wedges
 These are used in cases such as curved surfaces
and oblique beam incidences in which contour

can be approximated with a straight line.
 These are fabricated from a metal such as

copper,brass or lead.
 Designed to compensate for a missing wedge of

tissue.

Compensating wedges
Three important differences between
compensating wedges and wedge filters are:
 Standard isodose curves, can be used since

the 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.

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.

Multi leaf collimators
 A Multi leaf collimator(MLC) for photon beam

consists of a 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 this number depends on sophistication of
machine.

Basic geometry of MLCs
 Multi leaf collimators are a bank of large number
of collimating blocks or leaves
 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% .

Considerations in the use of
MLC
 1.Conformity between the planned field

boundary which is continuous and jagged
step wise boundary created by MLC in a
stationary field.
 2.optimization of MLC rotation.
 3.The physical penumbra which is larger than
that produced by the collimator jaws or
cerrobend blocks.

Comparison of MLCs
VARIAN
1.in terms of position
 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.

VARIAN
4.Tongue and groove model

SIEMENS
 Blunt ends

5.Inter leaf movements
present.

 No inter leaf movement

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

 Sharp ends produce step

egde effect.
 Width of leaf is 1cm

VARIAN

SIEMENS

 Leaf carriage system

 There is no leaf carriage

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.

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

Advantages of Multi leaf
collimators

 1.beam shaping is simple and less time consuming.

 2.these can be used without needing to enter

treatment room.
 3.correction and changing of field shape is simple.
 4.overall treatment time is shortened.
 5.constant control and continuous adjustment of the

field shape during irradiation in advanced conformal
radiotherapy is possible

Disadvantages
 1.step edge effect
 2.radiation leakage between the leaves

 3.wider penumbra

 The use of MLCs in blocking and field shaping
is ideally suited for treatments requiring large
numbers of multiple fields because of
automation of procedure and decrease in set

up time.
 Thus the importance of MLC is not just the

replacement of cerrobend blocking .
 Greater impact of this technology is in

automation of field shaping and modulation
of beam intensity.

Wedge Filters
 It is the most commonly used beam modifying
device. It 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

centimeters from the skin surface.

 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.

Types of wedge filters
Physical wedges
A physical wedge is an angled piece of lead or
steel that is placed in the beam to produce a
gradient in radiation intensity. Manual
intervention is required to place physical wedges
on the treatment unit’s collimator assembly.
Motorized wedge
Is a physical wedge integrated into the head of
the unit and controlled remotely.
Dynamic Wedge
IT produces the same wedged intensity
gradient by having one jaw close gradually while
the beam is on.

 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.

Selection of wedge
 This depends on
 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.

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.
 Usually measured at a suitable depth below
the Dmax usually 5 -10 cms.
 The resultant reduction in output results in an
increase in the treatment time

Wedge transmission factor

 In some isodose charts used in cobalt machines the







wedge transmission factor is already
incorporated, and no further correction is
necessary.
Use of wedge will result in a preferential
hardening - more pronounced in case of linear
accelerators.
This is because the Co 60 beam is monoenergetic .
For small depths (<10 cms) most of the calculation
parameters however remain unchanged.
At larger depths however, the PDD can be altered
specially in case of linear accelerator beams

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.

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

Wedge systems
 Wedge filters are of two main types
 1.Individualized wedge system

This requires a separate wedge for each
beam width optimally designed to minimize
the loss of beam output.
These are used in cobolt teletherapy .

2.Universal wedge system
Single wedge serves for all beam widths. It is
fixed centrally in the beam irrespective of field
size.

This is useful as it saves time.

However not suitable for cobalt beams
because of excessive reduction of beam output
with smaller fields.
 Come in one size of 20 x 30 cms

Flattening filters
 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.
Penetrating power should not increase as this
will alter the PDD as well as reduce the
flattening.

 The beam flatness is specified at 10 centimeters.
 The extent of flatness should be ± 3% along the

central axis of the beam at 10 centimeters.

 Should cover 80% or more of the field, or reach
closer than one centimeter from the edge.

 There is usually over flattening of isodoses, near
the surface. This results in production of “horns”
or hot spots.
 No point parallel to the surface should receive a
dose > 107% of the central axis dose.

Bolus
 A tissue equivalent material used to reduce the
depth of the maximum dose (Dmax).
 Better called a “build-up bolus”.
 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.

 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)
 Temex rubber (rubber)
Spiers Bolus (rice flour and soda bicarb)
 Commercial materials:
 Superstiff: Thick and doesn't undergo elastic
deformation. Made of synthetic oil gel.
 Superflab: Add water to powder to get a
pliable gelatin like material.
 Bolx Sheets: Gel enclosed in plastic sheet

 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

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.

Penumbra Trimmers
 Refers to the 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.

 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.

. P = AB ( SSD + d – SDD)/ SDD

SDD

SSD

d

Beam Spoilers
 First used by Doppke to increase dose to
superficial neck nodes in head and neck cancers
using 10 MV photon beams.

Special beam modification device where shadow
trays made from Lucite are kept at a certain
distance from the skin.
 Based on the principle that relative surface dose

increases when the surface to tray distance is
reduced.

Beam Modification Of
Electrons
the most clinically useful energy range for
electrons is 6 to 20 Mev.
Electron beams can be used for treating
superficial tumors with characteristic sharp
drop off in dose beyond the tumor.
Distinct advantages are dose uniformity in
target volume and sharp dose fall off beyond
the tumor.

Beam shaping
 1.Electron beam cone
 2.Internal shielding
 3.Scattering foil
 4.lead cutouts

Internal 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.

Scattering foil
 A device to widen the thin pencil beam (3 mm) of
electrons.
 Metallic plates of tin, lead or aluminium are
used.
 Disadvantages:
 Beam attenuation.
 Generation of bremsstrahlung radiation.
 Advantages:
 Less prone to mechanical errors.
 Less expensive.
 Requires less instrumentation.

Lead cut-outs
 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.

 Beam modification increases conformity
allowing a higher dose delivery to the
target, while sparing more of normal tissue
simultaneously.
 Megavoltage radiotherapy is better suited for
most forms of beam modification due to it’s
favourable scatter profile.
 However any beam modification necessitates a
close scrutiny of every phase of the planning and
treatment process.

beam modifying devises

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