WEDGE.ppt

Wedge

2. THE WEDGE FILTER

3. ѽThe wedge filter is located below the ionisation
chamber.
ѽIt is mounted on a motorised carriage fitted
permanently in the head of the gantry.
ѽThe facility has to provide a wide range of
synthetic wedge angels with the use of only
one wedge filter

4. The wedge filter has effective wedge angles of
60
O
and can be used over field size from
1cm-1cm to 30cm-40cm.
A wedge field distribution for intermediate
angles is achieved by the combination of
the dose delivered for a wedge and an open
field of the same field size.
The use of wedged and open fields in different
ratios results in a range of intermediate
wedge angels between 0
0
to 60
0
.

5. When a wedged field treatment is used the wedge is
moved in to and out of the radiation beam area under
software control.
Movement occurs while the radiation is not switched on.
The Software control makes sure the wedge filter is in one of its
two position in or out.
The required synthesized wedge angle can be derived by the
calculating the ratio of the open to fully wedged monitor
units

6. F=tan(x)/tan(y)
Dt=Do+Dw
Dw=Dt x F
Do=Dt-Dw
Do=Dw(1-F)/F)
F=Ratio of angle of synthesized wedged field to angle of
fully wedged field
Y=Maximum wedge angle defined for a specific field size
and depth.
x=synthesized wedge angle
Dt=Total dose on central axis at depth ‘d’
Dw=wedged segment dose on central axis at depth ‘d’
Do=open segment dose on central axis at depth ‘d’

7. Eg; prescribed dose=2.0gy
Field size=10x10cm
SSD=100cm
Maximum wedge angle at 10cm depth=60
0
Required synthesized wedge angle=20
0
Dt=2.00Gy
F=tan(20
o
)/tan(60
0
)=0.210
Dw=2.0Gy x 0.210=0.420Gy
Do=2.0Gy-0.420Gy=1.580Gy

8. WEDGE FILTER WITH COLLIMATION

12. Thank you