BEAM MODIFYING DEVICES
Dr Nanditha Kishore
DNB 1st year
Defined as desirable modification in the
spatial distribution of radiation - within the
patient - by insertion of any material in the
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
Types of beam modification
Shielding And Shaping
A. Shielding Blocks
B. Custom Blocks
C. Independent Jaws
D. Multi Leaf Collimators
Beam flattening filters
Electron beam modification
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
Matching adjacent fields.
Ideal shielding material
It should have following characteristics
1.high atomic number
Most common shielding material used for
photons is LEAD.
Choice of shielding is also
dictated by the type of beam
Thickness of shielding
It depends on
1.Attenuation of shielding material
The term half value-layer is a convenient
expression for the attenuation produced by
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
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
Also impossible to place the heavy block on
the body !!
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
Composition Of Cerrobend
1.21 times thicker blocks
necessary to produce the
Most commonly thickness
of 7.5 cms used.
Ideally the blocks should be shaped or
tapered so that their sides follow the
geometric divergence .
This minimizes the block transmission
Divergent blocks are most suited for beams
having small focal spots.
1.Drawing the outline of treatment field
including the areas to be shielded on a
simulator radiograph .
2.constructing divergent cavities in a
3.filling the cavities with cerrobend material
in liquid state.
Shielding blocks can be
of two types:
blocks, where the
central area is
blocks, where the
peripheral area is
Used when we want to block of the part of the
field without changing the position of the
Independently movable jaws, allows us to shield
a part of the field, and this can be used for
Use of independent jaws and other beam
blocking devices results in the shift of the
These have replaced half beam blocks as
Asymetric collimation produced by these
independent jaws has an effect on
2.Tilt of isodose curves
This is by eliminating photon and eletron
scatter from blocked portion of field thereby
reducing dose near the edge.
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
The dimension and shape of a compensator
must be adjusted to account for :
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
Required thickness of a
thickness along same
A tissue equivalent compensator designed
with thee same thickness f missing tissue will
To compensate for this decrease in scatter
one should use appropriate thickness of
The thickness ratio depends on:
Compensator to surface distance.
Thickness of the missing tissue.
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
The term compensator ratio is the inverse of the
thickness ratio. (ρc /τ ).
Used when proper mould room facilities are not
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.
3-D compensators are designed to measure
tissue deficits in both transverse and
longitudinal cross sections.
Various devices are used to drive a pantographic
Cavity produced in the Styrofoam block is used
to cast compensator filters.
Medium density materials are preferred to
Various systems in use for design of these
CT based compensator designing systems.
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
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
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
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
1.Conformity between the planned field
boundary which is continuous and jagged
step wise boundary created by MLC in a
2.optimization of MLC rotation.
3.The physical penumbra which is larger than
that produced by the collimator jaws or
Comparison of MLCs
1.in terms of position
it is positioned as a tertiary
system below standard
3.Non focussing leaves so
increased chance of
MLC replace the lower x-
Double focussing leaves i.e
both the leaf edges and
leaf ends are according to
4.Tongue and groove model
5.Inter leaf movements
No inter leaf movement
6.Rounded ends produce
better confirmity in
7.With of leaf varies from 0.5
to o.25cm so more
accurate delineation of
Sharp ends produce step
Width of leaf is 1cm
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
Treats by DYNAMIC ARC
METHOD OR SLIDING
system so maximum
horizontal field size is
Any repair of MLC entitles
entire machine to be
stopped for servicing.
TREATS BY STEP AND
Advantages of Multi leaf
1.beam shaping is simple and less time consuming.
2.these can be used without needing to enter
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
1.step edge effect
2.radiation leakage between the leaves
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
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.
It is the most commonly used beam modifying
device. It works by producing tilt in the isodose
Degree of the tilt depends upon the slope of the
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
Mounted on trays
which are mounted on
to the head of the
Types of wedge filters
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.
Is a physical wedge integrated into the head of
the unit and controlled remotely.
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
As the angle increases
Attenuation produced by the thicker end
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).
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
Use of wedge will result in a preferential
hardening - more pronounced in case of linear
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
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
Open And Wedged field combinations
For treatment of some tumors when open
field anteriorly and wedged field laterally is
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 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
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
A beam flattening filter reduces the central
exposure rate relative to that near the edge of
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
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.
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
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.
Other materials that have been used:
Mix- D (wax, polyethylene, mag oxide)
Temex rubber (rubber)
Spiers Bolus (rice flour and soda bicarb)
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
A beam modifying and directing device used
for a tangential fields therapy.
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
Endplate provides compensation, enhances
surface dose and presses down the tissue.
Effective shielding of lungs.
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.
Transmission penumbra: Transmission
through the edge of the collimator block.
Geometrical penumbra : Finite size of the
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:
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.
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
Beam Modification Of
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
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
Example of areas requiring these
techniques are the buccal mucosa and eye
A device to widen the thin pencil beam (3 mm) of
Metallic plates of tin, lead or aluminium are
Generation of bremsstrahlung radiation.
Less prone to mechanical errors.
Requires less instrumentation.
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
Shields can also be supported at the end of the
treatment cone if too heavy at the cost of
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
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