Wedges Flashcards
What is a wedged field?
Field whose distribution has been altered so that isodose curves are no longer flat and are tilted
What does a wedge do?
Progressively reduce the beam across the width of a field
Dose is prescribed along central axis
More dose under thin wedge
Less dose under thick edge
Why do we use wedges fields?
To achieve an even or homogenous dose
Wedges adjust for
Beam arrangement where beams overlap
To achieve dose homogeneity across the PTV
Density variations
Change in contour
What is a change in contour?
Compensation for tissue variation
What are combined fields used for?
Where beams overlap
Smaller wedges have what?
Smaller angles
What is the wedge transmission factor?
Presence of wedge in beam will reduce output
Accounts for this change in output
Ratio of dose on central axis with and without the wedge
What are wedges fields normalised to?
To get 100% at dmax
Isodose a under thin end above 100
Isodose under thick end
Under 100
To increase dose through field use what?
Increase MU
Types of wedges?
Physical- universal- one size fits all
Individualised wedges- 15-30-45-60
Dynamic wedges- collimator moves across field
Collimator opens and closes amount of time open for determines wedge angle
What wedge does elekta use?
Physical fixes angle
Single wedge field in centre field
Reduced output
Wedge is either in or out of field
What wedge does Varian use?
Dynamic wedge-jaws dynamically moved across the field
Speed of motion determines wedge angle
Wedges cause their effect by ?
Attenuation
Physical wedges reduce dose along central axis
To get the same dose to patient?
Increase MUs
Is the orientation of wedges important?
Yes they have some effect on beam quality due to preferential absorption of low energy photons beam hardening
What is a universal mototeised wedges
Built into accelerator made of tungsten 60 degree
Position of ionisation chambers
Effective wedge angles can be delivered by splitting the beam into two components or fractions
Wedge in
Wedge out
What are EDW?
Move across the field Y1
How does a change in monitor units affect dynamic wedges
Not as great as physical
What field sizes can be used for wedges?
Up to 30cm
To define the physical wedge angle?
Line perpendicular to central axis and 80 % isodose curve
How can superficial tumours be treated using what?
Two wedges beams directed from the same side of the patient
No wedge in the beams we get non-uniformity in the overlap region
Wedges placed in the beam can increase the uniformity
Dose uniformity from wedges?
Hotspots under thin edge of wedge
Increases with field size and wedge angle
Advantage rapid dose falloff beyond the region of overlap
Physical wedge?
Thin edge less attenuation or more transmission
Thick edge contribute more attenuation or less transmission
