BEAM MODIFYING DEVICES

2. SHIELDING
COMPENSATION
WEDGE FILTRATION
FLATTENING
TYPES OF BEAM MODIFICATION

3. • Commonly used shielding material
for photon – Lead
• Thickness depends – Energy of
radiation
• Kv range – Sheets of lead placed
directly on the surface
• Megavoltage radiation – Thicker
blocks (Placed up in shallow trays)
Choice of shielding material
• High Atomic Number
• High density
• Easily available
• Less costly
Properties of ideal shielding material
SHIELDING

4. Metal used-LIPOWITZ or cerrobend
Meltingpoint-70°c
Thickness -7.5 cm
Types
Positive blocks --------- central area blocked
Negative blocks--------- peripheral area blocked
Diverging blocks
 Follows geometric divergence of beam
 Minimizes block transmission penumbra
Custom blocks

5. Bank of large no:of collimating blocks or leaves
Can be moved automatically
Independent of each other to generate a field of any
shape
40 or more pairs of leaves having width 1cm or less
Thickness 6 - 7.5cm
Made of tungsten alloy
MULTI LEAF COLLIMATORS

6.  Are beam modifying devices which evens out the skin surface
contours, while retaining the skin sparing advantage
Allows normal depth dose data to be used for much irregular
surfaces
Also used for:
Compensate for tissue heterogeneity
Used in total body irradiation
Compensate for dose irregularities arising due to reduced
scatter near field edges
(e.g.;- Mantle fields)and horns in the beam
COMPENSATORS

7.  Special beam modification device where shadow trays made of Lucite
are kept at a distance from skin
 Based on the principle that relative surface dose increases when
surface to tray distance is reduced
 First used by DOPPKE to increase dose to superficial neck nodes in
head & neck cancers using 10MVphoton beams
BEAM SPOILERS

8.  A beam modifying device, which causes a progressive decrease in
intensity across the beam,resulting in tilting the isodose curves from
their normal positions.
 Degree of the tilt depends up on the slope of the wedge filter
 Material:tungsten,brass,lead,or steel
 Usually wedges are mounted at a distance of 15cm from the skin
surface
 The sloping surface is made either straight or sigmoid in shape
 A sigmoid shape produces a straighter isodose curve
WEDGE FILTERS

9. TYPES OF WEDGES
Individualized wedge
Universal wedge
Dynamic wedges
Virtual wedges
Pseudo wedges

10.  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 exhibit “forward peaking”
 The filter is designed so that the
thickest part is in the centre
 Material: copper or brass
FLATTENING FILTERS

11.  Tissue equalent material
 Used to reduce depth of (Dmax)
 Can be used in place of a compensator for
kilovotage radiation to even out the skin
surface contours
 In mega voltage radiation bolus is primarily
used to bring up the buildup zone near the skin
in treating superficial lesions
 the thickness of the bolus used varies according
to the energy of the radiation
 In megavoltage radiation:
 6mv: 7-8mm
 10mv: 12-14mm
 25mv: 18-20mm
 Co60: 2-3mm
BOLUS

12. A beam modifying and directing device used for 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
End plate provides compensation, enhances surface dose and
presses down the tissue
Effective shielding of lungs
BREAST CONE

13.  Consists of extensible, heavy
metal bars to attenuate the beam
in the penumbra region
 Increase the source to diaphragm
distance redusing the geometric
penumbra.
 Another method is to use
secondary blocks placed close to
the patient (15-20cms)
PENUMBRA TRIMMERS

14. Electron field shaping is done using lead cutouts.
For a low-energy electrons(<10 MeV),sheets of lead,
less than 6mm 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
ELECTRON BEAMS

15.  A device to widen the thin pencil beam (3mm) of electrons
 Metallic plates of tin, lead or aluminum are used
 Disadvantages:
Beam attenuation
Generation of bremsstrahlung radiation
 Advantages:
Less prone to mechanical errors
Less expensive
Requires less instrumentation
 Now a days dual foil systems are used, which compare well with scanning beam
systems
SCATTERING FOIL

16. 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 its favorable scatter profile.
However any beam modification necessitates a close scrutiny
on every phase of the planning and treatment process
CONCLUSION