Wedges

Question 1

What is a wedged field?

Answer 1

Field whose distribution has been altered so that isodose curves are no longer flat and are tilted

Question 2

What does a wedge do?

Answer 2

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

Question 3

Why do we use wedges fields?

Answer 3

To achieve an even or homogenous dose

Question 4

Wedges adjust for

Answer 4

Beam arrangement where beams overlap

To achieve dose homogeneity across the PTV
Density variations
Change in contour

Question 5

What is a change in contour?

Answer 5

Compensation for tissue variation

Question 6

What are combined fields used for?

Answer 6

Where beams overlap

Question 7

Smaller wedges have what?

Answer 7

Smaller angles

Question 8

What is the wedge transmission factor?

Answer 8

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

Question 9

What are wedges fields normalised to?

Answer 9

To get 100% at dmax

Isodose a under thin end above 100

Isodose under thick end

Under 100

Question 10

To increase dose through field use what?

Answer 10

Increase MU

Question 11

Types of wedges?

Answer 11

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

Question 12

What wedge does elekta use?

Answer 12

Physical fixes angle

Single wedge field in centre field
Reduced output
Wedge is either in or out of field

Question 13

What wedge does Varian use?

Answer 13

Dynamic wedge-jaws dynamically moved across the field

Speed of motion determines wedge angle

Question 14

Wedges cause their effect by ?

Answer 14

Attenuation

Physical wedges reduce dose along central axis

Question 15

To get the same dose to patient?

Answer 15

Increase MUs

Question 16

Is the orientation of wedges important?

Answer 16

Yes they have some effect on beam quality due to preferential absorption of low energy photons beam hardening

Question 17

What is a universal mototeised wedges

Answer 17

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

Question 18

What are EDW?

Answer 18

Move across the field Y1

Question 19

How does a change in monitor units affect dynamic wedges

Answer 19

Not as great as physical

Question 20

What field sizes can be used for wedges?

Answer 20

Up to 30cm

Question 21

To define the physical wedge angle?

Answer 21

Line perpendicular to central axis and 80 % isodose curve

Question 22

How can superficial tumours be treated using what?

Answer 22

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

Question 23

Dose uniformity from wedges?

Answer 23

Hotspots under thin edge of wedge
Increases with field size and wedge angle
Advantage rapid dose falloff beyond the region of overlap

Question 24

Physical wedge?

Answer 24

Thin edge less attenuation or more transmission

Thick edge contribute more attenuation or less transmission