WEDGE.ppt
- 3. ѽThe wedge filter is located below the ionisation chamber. ѽ It 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 60Oand 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 00 to 600.
- 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=600 Required synthesized wedge angle=200 Dt=2.00Gy F=tan(20o)/tan(600)=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