Wedge
Materials used in wedge filter: Lead or steel .
Two types:
1. Physical wedge:
Internal/Motorized/Universal wedge.
External/Manual/Individual wedge: Wedge angles – 15,
30, 45, 60 degrees.
2. Non-physical wedge:
Enhanced dynamic wedge (Varian).
Virtual wedge (Siemens).
Parts:
Thick part – Heel/base
Thin part – Toe/apex
Body
Sloping edge
Difference between Universal wedge and Individual
wedge:
Internal/Motorized/Universal External/Manual/Individual
wedge wedge
Placed above secondary Placed below secondary
collimator. collimator.
Only one 60 degree wedge is Different wedges with
used hat act as different different angles for
angle. Angle changes with different beam
field size change. characteristics.
Remote controlled. Manually used.
Placed centrally in the field. Apex is always in field
margin.
Max. field can be open upto Always half beam block is
40 cm (40×40). used. Max. field size is 20
cm (20×40).
Less time consuming. Time consuming.
Good for LINAC Good for Co60
(Polyenergetic). (Monoenergetic).
Energy:
6-15 MV energy is used for beam hardening.
>15 MV – beam softening.
Distance between wedge and surface:
Max. – 50 cm
Min. 15 cm
<15 cm will cause electron contamination and increased
surface dose.
Wedge > 60 degree can not be used because increased
wedge thickness causes increased beam attenuation and
scattered dose is also increased.
Suitable for 0-7 cm tumor depth.
Purpose of wedge use:
Homogenous dose distribution.
Decrease hot spot.
Advantages of non-physical wedge:
1. Reduction of treatment time, less scatter dose to
peripheral areas and extended field size capabilities.
2. Potential roles in intensity modulated radiation therapy.
Difference between Enhanced Dynamic wedge
(Varian) and Virtual wedge (Siemens):
Enhanced Dynamic Virtual wedge (Siemens)
wedge (Varian)
Output rate is determined Output rate is determined
using Segmental using Analytic equation.
Treatment Table (STT).
Variation in dose rate Variation in dose rate
and moving jaw speed. only.
Initial jaw – open. Initial jaw – 1 cm from
Final jaw – 0.5 cm from fixed jaw.
fixed jaw. Final jaw – open.
Wedge direction for Y Wedge direction for X or
jaw only. X jaw is fixed. Y jaw.
Treatment is prohibited if Treatment is allowed if
the fixed jaw is 0.5 cm the fixed jaw is 1 cm
beyond moving jaw. beyond moving jaw.
EDW allows for seven VW offers a continuous
possible wedge angles selection of wedge angles,
using segmented produced using an
treatment tables (STTs) exponential variation of
which govern the output the accelerator output
rate at each position of rate based on an analytic
the moving jaw during equation.
treatment.
EDW factor is strongly The VW factor is very
dependent on field size, close to unity for
varying by up to a factor symmetric fields. For
of two for the 60o EDW. asymmetric fields, the
VW factor demonstrates
a much greater variation
with off-axis distance
than that of EDW.
Gradient jaw exceed 10 No gradient jaw – no
cm beyond central axis (Y limit (X or Y).
jaw).
Upper jaw Y axis is 2 cm Upper jaw Y axis is 2 cm
beyond central axis. beyond central axis.
Lower jaw X axis is 10
cm beyond central axis.
60 degree is 15 cm from
surface.
Seven angles: 10, 15, 20, Any angle between 0-6-
25, 30, 45, 60 degree. degree.
Wedge factor:
Ratio of doses with and without wedge along the central
axis of the beam at a depth of 10 cm when wedge angle,
energy and field size are fixed.
Wedge factor depends on wedge angle and field size.
Wedge factor depends on off axis distance.
Example: Dose with wedge at 10 cm depth / Dose without
wedge at 10 cm depth.
Why there is a gap between wedge filter and skin?
To avoid electron contamination
To preserve skin sparing effect
Skin sparing effect:
When skin gets 50% dose of Dmax.
Wedge angle (Also called wedge isodose angle):
Angle between the 50 % of isodose line to the
perpendicular line to the central axis of the beam.
How to calculate wedge angle?
Hinge angle:
(180 degree – hinge angle) / 2
Two classes of wedge filters:
(a) Physical wedge filters and (b) Nonphysical wedge
filters.
Definition:
A physical wedge filter is a wedge-shaped absorber that
causes a progressive decrease in the intensity across the
beam, resulting in a tilt of the isodose curves from their
normal positions.
The isodose curves are tilted toward the thin end, and the
degree of tilt depends on the slope of the wedge filter. In
actual wedge filter design, the sloping surface is made
either straight or sigmoid in shape; the latter design is
used to produce straighter isodose curves.
A nonphysical wedge filter is an electronic filter that
generates a tilted dose distribution profile similar to a
physical wedge by moving one of the collimating jaws
from one end of the field to the other. Nonphysical
wedges are available with most accelerators.
Examples include Varian’s Enhanced Dynamic Wedge,
Siemens’ Virtual Wedge.
Advantages of nonphysical wedges:
Automation of treatment delivery.
Less peripheral dose compared to the physical wedge
filter, for example, less dose to the contra-lateral breast
when using tangential breast irradiation technique with
nonphysical wedges.
Disadvantages of nonphysical wedges:
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 procedures may be required for
nonphysical wedges to prevent the occurrence of a
treatment error.
It should be noted that wedges and compensators, which
are basically intensity-modulating devices, are superseded
by IMRT technology. However, wedges and
compensators are still used in treatment techniques that
involve combinations of individual beams of
cross-sectionally uniform intensity.
