The document discusses various beam modification devices used in radiotherapy. It describes different types of beam modifiers like shielding blocks, wedges, compensators, multileaf collimators and bolus. It provides details on the purpose and working of various wedge filters including physical wedges, universal wedges and dynamic wedges. It also explains flattening filters and different multileaf collimator designs by Elekta, Siemens and Varian.

1. Beam Modification
Devices in Radiotherapy
ALMA PETER
PGDMRS

2. Beam modifying devices are devices which when kept in path of beam produces a desirable modification in the spacial
distribution of the beam.
Types of beam modification are as follows:
• Shielding: To eliminate radiation dose to selected part of the treatment area
• Compensation: A compensator attenuates the beam based on the irregular contour of the patient
• Wedge filtration: Used to produce a desired tilt in isodose curves
• Flattening: where the spatial distribution of the natural beam is made uniform in a LINAC by reducing the central exposure
rate relative to the periphery
Types of beam modification devices are as follows:
•Shielding blocks
• Jaws
•Multileaf collimators.
•Compensators
•Beam spoilers
•Wedge filters
•Beam flattening filters
•Bolus

3. Flattening filter
The x-ray beam from a LINAC is
strongly forward peaked,
It functions to make beam intensity
distribution relatively uniform across
the field.
The flattening filter is thickest in the
middle and tapers towards the edges.
For higher beam energies thicker FF
are to be used
It is made of Copper or Brass

4. Flattening filter for MV Xrays
To ensure the flatness of the beam at appropriate depths,
it is sometimes necessary for the peripheral doses near
the surfaces to be larger than at the central axis, leading
to Horns in the beam profile.
The cross-sectional variation in the thickness of the
FF causes variation in photon spectrum and quality
of the beam owing to the selective hardening.
In general the average energy of the beam towards
the peripheral edges are some what lower
compared to the central region, hence this change
in quality of the beam causes the flatness to change
with depth.
In addition to selective hardening the increase in
scatter contribution at depths also contribute to the
change in flatness with depth.

5. Flattening filter for Electron
beams
Multiple scattering foils made of high Z materials are
used to spread the electron pencil beam evenly on the
patient plane.
Different scattering foils may be used for different
electron energies
The electron beam edges can be sharply defined only if
the collimation is extended toward the skin of the
patient by attachment of trimmers
Trimmers are optimally designed to give an electron
beam of uniform fluence, conforming to the light beam
indication at the level of the patient’s skin

6. External Beam Modifiers
Wedge Filters
Compensators
Beam Shaping Blocks
Asymmetric Collimators
Multileaf Collimators

7. Wedge Filters
Wedged beams are used for three main
purposes:
i) Combining beams from non-orthogonal
angles,
ii) Compensating for changes in surface
shape,
iii) Compensating for changes in depth dose
fall off for beams incident perpendicular to
the wedged beam.

8. Introduction of wedge filters results in,
• The decrease of dose output due to attenuation by the wedge and is characterised by the wedge
factor (WF)
• The wedge filter attenuates the lower energy xrays in a megavoltage beam causing beam
hardening effect that alters the central axis depth dose curves

9. There are three types of systems that produce wedged beams:
i) Manual fixed physical wedges,
ii) Universal physical wedge,
iii) Dynamic wedges.

10. Manual Physical wedges
These are wedge shaped pieces of aluminium, brass or steel.
A series of different wedges is usually in use e.g. 15, 30, 45, 60 degrees.
These will have different physical dimensions and may be constructed of different materials.
Usually distance kept between wedge surface and patient surface is 15cm
DISADVANTAGES
They need to be physically inserted in the position of the accessory tray.
Inserting and removing the wedge can be difficult especially at non-zero gantry and
collimator angles.
Carrying and storing them requires careful design within the treatment room.
They also block the light field used for setting up the patient, so the wedge is often
inserted after the patient is set up, increasing the manual handling problems.

11. Universal Physical Wedge
A single physical wedge can be used to create a range of wedge angles by combining the wedge
field with open field irradiation.
This design of wedge is used in Elekta accelerators. (motorized physical wedge)
The wedge is automatically positioned in the beam within the treatment head above the
position of the mirror and below the monitor chamber.
However, wedging can only occur in one direction which becomes a problem if the wedge is
combined with a MLC, when being able to wedge both in the direction of the leaf movement
and perpendicular to this may be useful.

12. Dynamic or Virtual Wedges
Dynamic or virtual wedges are created by moving a
secondary collimator jaw across the treatment field
while the beam is on.
The amount of wedging is determined by the
length of time the jaw is in the treatment field.
Wedging in different directions can be achieved by
movement of different jaws.
For Varian dynamic wedges, the wedge factor is
strongly dependent on field size and this effect
needs to be carefully modelled within the
treatment planning system.
Off-axis and half-blocked wedged fields are created
in the same way. Again care is needed in modelling
the wedge factor.
Advantages
•Fully automated treatment delivery
•Less periferal dose compared to physical wedges
Disadvantages
• greater dosimetric complexity in the acquisition
of commissioning data, beam modeling for a
treatment planning system, and MU calculations
for various field sizes and configurations.
•More elaborate QA needed

13. Wedge angle
• Wedge Angle (θ) : refers to “the angle through which
an isodose curve is titled at the central ray of a beam at
a specified depth (10 cm) or 50% isodose curve”.
• Wedge transmission factor : defined as the ratio of doses
with and without the wedge, at a point in phantom along
the central axis of the beam. This factor should be
measured in phantom at a suitable depth beyond the
depth of maximum dose (e.g., 10 cm).
• The two dimensions of wedges are important – “X” or
width and “Y” or length.

14. Asymmetric Jaws
Asymmetric collimation or independent collimation or jaws, allows the two jaws of a set to be
positioned independently of each other.
Asymmetric jaws allows rectangular fields that are not symmetric about the beam central axis to be
defined by the LINAC collimation system without the need for additional beam shaping blocks
The Asymmetric jaws helps to eliminate divergence between adjacent fields. Beam split fields with one
non divergent edge were created by positioning one of the independent jaws along the beam central
axis

15. Applications of Asymmetric collimators
◦ Non-divergent matching
◦ Change in field size for planned boosts without the need for resimulation
◦ Field size adjustment to match to a previous or concurrent divergent field.

16. Shielding Blocks
Used to block of the part of the field and is customly made using Lipowitz metal or
Cerrobend (Fig. 5.1)
Also called: Wood’s metal
• Contents: 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight
• The melting point is 70 °C (158 °F)
—main advantage of Cerrobend is that it can be easily cast into any shape
• Density 9.4 g/cm3 at 20 °C
• 1.21 times thicker blocks are necessary if made of Cerrobend when compared to lead
to get the same attenuation
• Usual thickness is 7.5 cm
• Shielding blocks types:
– When central area is blocked it is called positive blocks.
– When periphery of field is blocked it is called negative blocks.
– Divergent block—when the edge of the block follows divergence of beam. It
helps in reducing transmission penumbrae.

17. Bolus
• Also known as “build-up bolus.”
—bring up the buildup zone (reduce the skin-sparing effect) in
treating superficial lesions.
– It can act as a compensator for missing tissue or irregular
surface.
• Commonly used materials are:
– Cotton soaked with water(water acts as bolus)
– Paraffin wax – Mix- D
– Lincolnshire bolus: made up of 83 percent sugar and 13
percent magnesium carbonate
– Spiers bolus: made up of 60 percent rice flour and 40
percent calcium carbonate
• Commercial materials:
• Superflab: Thick and doesn't undergo elastic deformation. Made
of synthetic oil gel.
• Superstuff: Add water to powder to get a pliable gelatin like
Properties of an ideal bolus:
– Ideal bolus must have similar electron
density to the tissue
– Must be pliable to conform to surface
to make it uniform
– Ideal bolus must have similar
absorption and scattering properties as that of
tissue
Bolus is a tissue equivalent material used to reduce the depth of the maximum dose (Dmax) or to
bring up the surface dose

18. Multileaf collimeter
MLC has replaced the beam blocks in field shaping
The MLC has movable leaves, or shields, which can block some fraction of the radiation beam
Typical MLCs have 40 to 120 leaves, arranged in pairs
By using the computer controls to position a large number of narrow, closely abutting leaves, an arbitrarily shaped field can be generated
By setting the leaves to a fixed shape, the fields can be shaped to conform to the tumor
Give adequate reliability of the hardware and software, the use of MLC field shaping is likely to save time and to incur a lower operating
cost when compared to the use of beam blocks

19. MLC
The material of leaf construction is ‘tungsten alloy’ (High density)
Thickness: 6-7.5 cm
Density of tungsten alloy: 17-18.5 g/cm3
Primary x-ray transmission:
Through the leaves: <2%
Interleaf transmission: <3%
Tungsten alloys are also hard, simple to fashion, reasonably inexpensive, and have low coefficient of
thermal expansion

20. Upper jaw replacement
A “back-up” collimator located beneath the leaves and above the lower jaws augments (increase) the attenuation
provided by the individual leaves
In this configuration the upper jaw is split into a set of leaves (used by Elekta)
The back-up diaphragm is essentially a thin upper jaw that can be set to follow the leaves if they are arranged
together to form a straight edge, or else, set to the position of the outermost leaf if the leaves form an irregular
shape
Advantages
The range of motion of the leaves required to traverse the collimated field width is smaller
It allows for a shorter leaf length and therefore a more compact treatment head diameter
Disadvantages
Having MLC leaves so far from the accelerator isocentre needs leaf width must be somewhat smaller
Tolerances on the dimensions of the leaves as well as the leaf travel must be tighter than for other configurations

21. Elekta MLC
There are 40 Pair of Tungsten leaves each of 7.5 cm thickness.
An overlapping tongue and groove arrangement is used to minimize radiation
leakage between the leaves.
Each leaf projects a width of 10mm at isocentric plane but a total width of
11mm as the leaves move linearly they are designed with curved front faces to
minimize the penumbra variations with the field size.
Directly below the leaves there is a pair of tungsten back up collimators of 3cm
thickness.
They move in the same direction, X, as the leaves and their primary function
are to further reduce any leakage radiation between the leaves.
The field is defined in orthogonal Y direction by a pair of focused tungsten
collimator blocks of conventional design and thickness of 7cm, which allow
continuous adjustment of the field size in this direction.

22. Lower jaw replacement(siemens)
The lower jaws can be split into a set of leaves as well (used
by Siemens) and is double focused
Both leaf end and leaf sides match the beam divergence
The collimator leaves move along the circumference of a
circle centered at the x-ray target of the linear accelerator,
such that the end of the collimator is always tangential to the
radius of the circle
There are no back-up jaws
The leaves of Siemens MLC can extend 10 cm across the field
centerline, which allows a maximum leaf travel of 30 cm

23. Tertiary MLC(Varian)
MLC are positioned just below the level of the standard upper and
lower adjustable jaws (used by Varian)
This avoids the lengthy downtime in the event of a MLC system
malfunction
It is possible to move leaves manually out of the field should a failure
occur
The treatment can be continued by using ‘Cerrobend’ individual
blocks
It is single focused MLC with rounded leaf ends and no back up jaws
are provided

24. Siemens:-
Maximum leaf travel = 30 cm
Extension of 20 cm to the center of the field and an additional 10 cm across the centerline
Elekta:-
Maximum leaf travel = 32.5 cm
Leaves can extend 12.5 cm across the field centerline
Varian:-
maximum leaf travel = 14.5 cm
Distance between the most extended leaf and the most retracted leaf on the same side can only be 14.5
cm
Limitations of Varian MLC
Carriage to extend leaf travel across midline
Extending the leaves out to the field center is not possible when large fields are used
Leaves on one side of bank can interdigitate with neighboring leaves on opposite bank (island blocks can
be creat

25. Thank you!