beam modification devices in radiation oncology

1. BEAM MODIFICATION DEVICES
IN RADIOTHERAPY
PRESENTER : DR ABDUL WAHEED
Post graduate student radiation oncology
Skims Srinagar Kashmir

2. DEFINITION
• BEAM MODIFICATION IS THE MODIFICATION
OF THE SPATIAL DISTRIBUTION OF RADIATION
WITHIN THE PATIENT BY INSERTION OF
MATERIAL INTO THE BEAM PATH.

3. RATIONALE FOR BEAM MODIFICATION
To alter the shape of the beam
Protect normal tissues.
Protect critical structures.
To alter natural beam spatial distribution
Suitable for radiotherapy.
To compensate for missing tissues
Uniform spatial distribution
Normal distribution achieved.

4. BEAM MODIFICATION DEVICES
Field blocking and shaping devices:-
• Shielding blocks:-To shield part of beam to save
normal structure from effect of radiation.
• Custom blocks.
• Asymmetrical jaws.
• Multileaf collimators:-Principle shielding.
• Internal shields:-used for Electron beam shielding.
Compensators:- To compensate for missing
tissues,when beam enters a non uniform surface.
Wedge filters:-A special tilt in isodose curves is
obtained.

5. Beam flattening filters:-To change natural spatial
distribution of beam.
 Bolus:- Principle compensator
Breast cone:- Beam shielding plus beam directing
device.
Penumbra trimmers.
Electron beam modification devices.

6. SHIELDING BLOCKS
• DEFINITION:-
Beam modification device used for altering the shape
of the beam to reduce,as far as possible eliminate the
radiation dose at some specific part or zone where the
beam is directed.
• AIMS
 To protect critical organs.
 To protect normal tissue.
 Match adjacent fields.
 Avoid unnecessary radiation to surrounding normal tissue

7. CHARACTERISTICS OF AN IDEAL MATERIAL
High atomic number
High density
Easily available
Inexpensive
The most commonly used LEAD

8. BLOCK THICKNESS
Aim of shielding is reduction of beam transmission to
5% or less of its original value.
Block thickness depends on-
 Beam quality
 Allowed transmission through the block
 Half-value layer is defined as the thickness of
material, required to attenuate the intensity of
the primary beam to half of its original value.
Practically thickness of lead between 4.5 - 5 half-value layers
results in 5% or less of primary beam transmission.

9. BLOCK THICKNESS
Beam
quality
Required
lead
thickness
Co60
5.0cm
6 MV 6.5cm
10 MV 7.0cm
15 MV 7.0cm
25 MV 7.0cm
AS NO DIFFERENCE IN ATTENUATION AT HIGHER ENERGY
ESPECIALLY AFTER 6MV , WE DON’T GET HIGHER HVL

10. CAN WE SHIELD AN AREA COMPLETELY?
• NO
• SCATTER WILL ALWAYS REACH SHIELDED AREA
• EFFECTIVE SHEILDING PRIMARY CONTRIBUTION <3%
.
P
So, also the shielding material has to be kept
near the surface so as to minimize the scatter
radiation. BUT HOW CLOSE?

11. Where to place the blocks
IN KILOVOLTAGE radiation shielding is readily
achieved by placing sheet of lead on surface directly
IN MEGAVOLTAGE RADIATION
• Thicker block used.
• Placed higher up in shadow tray (15-20cm).
• Avoid increase in skin dose due to scatter.
• Also impossible to place the heavy block on the
body.

13. CUSTOM BLOCKS
• They are the blocks used to achieve the greatest
level of conformity in order to minimize the dose to
critical structures and normal tissues.
MATERIAL USED
• Any shielding material can be used including LEAD
• Worldwide most commonly used is LIPOWITZ
metal(CERROBEND)

14. CERROBEND
 COMPOSITION
 50% Bismuth
 26.7% Lead
 13.3% Tin
 10% Cadmium
 DENSITY:-9.4g/cm (83% of lead)ᶟ
 ADVANTAGES:-Low melting point 70 Cᵒ
 Can be easily melted and casted into any shape.
 At room temperature it is harder than lead
 DISADVANTAGES-Thickness 1.21 times
 Thicker blocks than lead necessary to produce the same
attenuation ,calculated by density ratio.
 Most commonly thickness of 7.5cms used.

15. BUILDING A BLOCK
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 to form a
cavity.
The film,
wire and
styroform
blocks are
so
adjusted
that the
actual
treatment
geometry
is
obtained
STEP 1
STEP 2

16. BUILDING A BLOCK
Cavities in the Styrofoam
block being used to cast
the Cerrobend blocks
Shielding blocks can be of two types:
Positive blocks, where the central area
is blocked. e.g.- lung shields
Negative blocks, where the peripheral
area is blocked.
STEP 3

17. INDEPENDENT JAWS
When independently movable collimators are present asymmetric
fields can be created to shield a part of the field without changing
the iso-center.
 Modern machines may have one to all four independent jaws,
during operation their movements are interlocked to avoid errors.
Uses:
To block off part of field with out changing the position of the iso-
center
Beam splitting- beam is blocked of at the central axis to remove
beam divergence
Half beam block
Replace the conventional blocks of past.

18. • When they are used isodose curves tilt towards the
blocked edge, due to elimination of photon and
electron scatter in that part.
• Special care is needed to check for beam flatness
and calculation of parameters like monitor units.

19. MULTILEAF COLLIMATORS
• Multileaf collimators are a bank of 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
• 40 pairs of leaves or more.
• Width of 1cm or less as projected at the isocentre.
• Thickness along the beam between 6-7.5cm
• Made of a tungsten alloy
• Density of 17-18.5g/cm .ᶟ

20.  Primary X-ray transmission:-
• Through the leaves<2%
• Interleaf transmission<3%
• For jaws1%
• Cerrobend blocks3.5%
 Basic applications of the MLC:-
• To replace conventional blocking e.g.in analogy to the fabrication of
cerrobend blocks.
• Continuously adjusting the field shape in conformal therapy to match
the beam eye view(BEV)projection of a planning target volume (PTV)
during an arc rotation of the x-ray beam
• To achieve beam intensity modulation variants of conformal therapy
required in each field to compensated or modulated,through various
complex applications using computers.

21. • MLC system can be double focus or single focus
leaves
• Truncated pie:- ( thickness at lower end of leaf> at
upper end ) to account for beam divergence
• Single focus leaves:-Rounded at end
• Double focus leaves:-Leaf and leaf size matches
with beam divergence and difficult to manufacture.
• Both are designed to ensure sharp beam cut off at
the edge
• This objective is achieved to limited extent—dose
fall off at the edge determined by scattered
photons and electrons.

22. • In order to allow fast interleaf movement, while reducing
radiation transmission a tongue and groove design is often
used.
Beam Beam
Tongue
100
10
0

23. TECHNICAL ADVANTAGES-
 Time for shaping and inserting of custom blocks is not required.
– It’s a boon for treating large number of multiple fields because
the procedure can be automated such that complex field shaped
fields can be generated in sequence, reducing the set-up time.
– The hardening of beam, scattered radiation, and increase in skin
doses and doses outside the field, as seen with physical
compensators is avoided.
– Automation of dynamic reshaping and modulation of beam
intensity in3D-CRT and IMRT.
– MLCs can also be used as dynamic wedges and electronic
compensators (2D).

24. DISADVANTAGES
 Island blocking is not possible.
 Step edge effect
 Wider penumbra
 radiation leakage between leaves.
 The jagged boundray of the field marker matching difficult.
 Because the physical penumbra is larger than that produced by
Cerrobend blocks treatment of smaller fields is difficult, as in
shielding of critical structures, near the field.

25. COMPENSATORS
• A beam modifying device which
evens out the skin surface contours,
while retaining the skin-sparing
advantage.
• It allows normal depth dose data to
be used for such irregular surfaces.
• MATERIAL USED IS BRASS
 Compensators can also be used :-
– To compensate for tissue
heterogeneity. This was first used
by Ellis.
– To compensate for dose
irregularities arising due to
reduced scatter near the field
edges and horns in the beam
profile.

26. • 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 always has
attenuation less than that is
required for primary radiation.
• As the distance between the skin
and compensator increases the

27. • The thickness ratio depends on:
– Compensator to surface distance.
– Thickness of the missing tissue.
– Field size.
– Depth.
– Beam quality.
 Of these, the distance is the most important factor.
 when d is ≤ 20 cm, 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).

28. 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,
taking into account their effective attenuation coefficient, electron
densities and the thickness coefficient. This results in production of a
laminated filter.
 the total thickness of filter at any point is calculated to compensate
for the air gap at the point below it

29. THREE-DIMENSIONAL COMPENSATORS
3-D compensators are designed to measure tissue deficits in
both transverse and longitudinal cross sections.
Various systems in use for design of these compensators are:
Moiré Camera – SPECIALLY DESIGNED CAMERA MOUNTED ON A
SIMULATOR, ALLOWS TOPOGRAPHICAL MAPPING OF THE PATIENT AND
THE DATA IS GENERATED
Magnetic Digitizers – HANDHELD STYLUS WITH MAGNETIC SESOR WHICH
SCANS THE PATIENT BODY
CT based compensator designing systems – 3D TPS HAVE SUFFICIENT
MULTILEVEL CT DATA TO AUTOMATE COMPENSATOR DESIGN

30. BOLUS
• A tissue equivalent material used in megavoltage radiation
to decrease the depth of maximum dose i.e. Dmax.
• Properties of an ideal bolus
 Same atomic no as that of tissue
 Electron density same as that of tissue
 Same absorption and scattering properties as that of tissue
 Easily pliable.
LEADS TO LOSS OF SKIN SPARING EFFECT.

31. • MATERIALS
 Cotton soaked in waterCotton soaked in water
water acts as bolus,
cotton holds the bolus
Paraffin wax
Various other materials have been used like
– Mix- D (wax, polyethylene, mag oxide)
– Temex rubber (rubber)
– Lincolnshire bolus (sugar and mag carbonate in form of
spheres)
– Spiers Bolus (rice flour and soda bicarb)
COMERCIALLY AVAIALBLE – SUPERFLAB , SUPERSTUFF,
ELASTOGEL.

32. • The thickness of the bolus used varies according to the
energy of the radiation.
• In megavoltage radiation:
– Co60
: 2-3mm.
– 6 MV : 7-8mm.
– 10 MV : 12-14mm.
– 25 MV : 18-20mm.
• USE
 BRINGS MAXIMUM BUILT UP TO SURFACE.
 COMPENSATION.

34. WEDGE FILTERS
• A wedge shaped beam modifying device, which causes a
progressive decrease in intensity across the beam,
resulting in tilting of the isodose curves from their normal
positions.

35. • Degree of the tilt depends upon the slope of the wedge filter.
• Material: tungsten, brass, Lead or steel.
• POSITION: Usually wedges are mounted at a distance of 15 cm from
the skin surface over a plastic tray.
• 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

38. WEDGE
• Types of wedge systems:
– Individualized wedge .
– Universal wedge.
– Dynamic wedges
– Pseudo wedges
• The two dimensions of wedges
are important – “X” or width
and “Y” or length.
• All wedges are aligned so that
the central axis of the beam is
at the central axis of the wedge.
X
Y
X
Hot spot area

39. Individual wedges
• Useful in Cobalt beams
• Using bigger wedges than
necessary will reduce
output of the machine →
increased treatment time.
• The width (W) of the
wedge is fixed and
important.
• The same wedge can
however be used for fields
with lesser lengths or
breadths.
The wedge systems available
are:
6W ( x 15)
8W ( x 15)
10W ( x 15)
All systems have the following
four angles 15°, 30°, 45°, 60°.

40. UNIVERSAL WEDGES
• Universal wedges are designed so
that the same wedge can be used
with all field sizes.
• 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
(except 60°).
• Wedge angles used are: 60°, 45°, 30°
& 15°.

41. ENHANCED DYANAMIC
WEDGE-
• Treatment delivery technique in which wedged dose profile is
created electronically by the sweeping action of independent
collimator jaw from open to closed position while the beam is on.
• Because of the collimator motion, different parts of the field are
exposed to primary beam for different length of time. This
creates wedged dose gradient across the field.
• Dose rate is modified to jaw position .
• Dose is deliverd throughout the treatment.
• In VARIAN dynamic wedge ,one y jaw moves while stand still
during treatment.

42. • MOTORISED DYANAMIC WEDGE
• 60 DEGREE physical WEGDE which generates desired
wedge angle from 0-60 degree by combination of open and
wedged beam.
• It is mounted above the asymmetrical collimator and it can
be moved in and out of the radiation field to create the
different wedged profile.
• PSEUDO WEDGE
• POOR MAN WEDGE
• CREATED BY OPENNING OF SMALL FIELD IN LARGE FIELD
USUALLY
• Small field gives 2/3rd
dose while large field gives 1/3rd
dose
• Used in olden days when asymmetrical jaws opening was
not possible and no planning TPS were available.

43. WEDGE ISODOSE ANGLE
• The wedge isodose angle (θ) is the
complement of the angle through
which the isodsose curve is tilted
with respect to the central ray of
the beam at a specified depth.
• This depth is important because
the angle will decrease with
increasing depth DUE TO
SCATTERED RADIATION
• The choice of the reference depth
varies:
– 10 centimeters.
– 1/2 - 1/3rd
of the beam width.
– At the 50% isodose curve (kV).
θ

44. • WEDGE FIELD TECHNIQUES
 Wedged fields are generally used for
relatively superficial tumors.
 Beams are usually directed from the
same side of the patient.
 The broad edges of the wedges
should be aligned together.
 The wedge angle chosen depends
on the angle between the central
rays of the two beams also called
the “hinge angle”(φ).
• Uses of Wedge:
– Reduce the hot spots at the
surface
– Rapid dose fall off beyond the
region .
– Thus the 2 factors on which the
wedge angle is chosen are:
– The hinge angle.
– The wedge separation.
φ
The wedge angle that will
make the isodose curves
parallel to each other and
the hinge angle bisector is
obtained using the
equation.
Θ = 90 – φ / 2

45. WEDGE TRANSMISSION FACTOR
• The presence of the wedge decreases output of the machine.
• WTF = Dose with the wedge/ Dose without the wedge (at a point in
the phantom, along the central axis of the beam).
• Usually measured at a suitable depth below the Dmaxusually 5 -10 cms!
This minimizes the error in calculation of PDD.
• The resultant reduction in output results in an increase in the
treatment time.
• 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 Co60
beam is mono-energetic .

46. 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.
• In cobalt beams:
– The beam is almost monoenergetic.
– Source emits uniform radiation all
around.

47. HALF BEAM
BLOCKER
Half beam blocker is a modification device which blocks one
half of beam upto central axis.
Used in several types of RT treatments wherein the
divergence of beam is to be stopped.
Used in breast treatment where the lung tissue is spared by
stopping the divergence of beam.

48. HALF BEAM BLOCKER

49. 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.

51. PENUMBRA
• 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 :Depends on size of the source.
– Physical penumbra: Lateral distance between two 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)

52. PENUMBRA TRIMMERS
• Consists of extensible,
heavy metal bars to
attenuate the beam in the
penumbra region.
• Increase the source to
diaphragm distance,
reducing the geometric
penumbra.
• 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

53. Direct / Internal Shielding
• Used for electron beam
shielding.
• 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.

54. 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.

55. • To avoid variation in output and electron scatter, jaws cannot
be used to collimate electron beams.
• An electron beam cone is therefore used to provide the
collimation.
• A primary collimator is provided close to source – defines the
maximum field size.
• A secondary collimator, near the patient defines the treatment
field.

56. ELECTRON APPLICATOR

57. 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.
Primary foil
Secondary foil
Electron
cone

58. CONCLUSION
 Beam modification devices are fundamental aspect of
modern radiation therapy planning & dose delivery.
 Megavoltage radiation are suited for most form of
modification devices.
 However improper use of these devices may produce
catastrophic events in dose delivery of tumor tissue &
sparing of normal tissue.
 So clinical judgement of Radiation oncologist,planning of
Physicist & skill of technician should go hand on hand to
achieve the desired results.