Wedges

Question 1

What is a wedged field?

Answer 1

A field whose
distribution has
been altered so that
the isodose curves
are no longer “flat”
– but “tilted”

Question 2

What does a wedge do to the distribution?

Answer 2

Wedges are designed to modulate
the intensity of the radiation across
the field
• The dose is prescribed along the
central axis of the beam
• More dose under thin end of wedge
• Less dose under the thick end of
wedge

Question 3

Normalisation of wedged fields

Answer 3

Normalised to get 100% at dmax on the central axis
Isodose under thin end are higher than 100%
Thick end are lower than 100%

Question 4

Why are wedges used?

Answer 4

1. beam arrangement where beams overlap
2. to achieve dose homogeneity across the PTV (even dose
   distribution)
3. density variations
4. a change in contour

Question 5

Types of wedges

Answer 5

1. Physical Wedges –outdated
2. Universal Wedge
3. Enhanced Dynamic Wedges (EDW) – collimator moves

Question 6

Physical wedges

Answer 6

• acts as attenuator of the beam
• lower the amount of radiation reaching the patient along central axis
• monitor units increase

Question 7

Universal wedge

Answer 7

Single wedge fixed in centre of field
• Reduced output
• Wedge in
• Wedge out
- splits the beam 
- made out of tungsten (60 degrees)

Question 8

EDW

Answer 8

Jaws dynamically moved across field with radiation on
Speed of motion determines wedge angle
An EDW generates a sloping dose distribution by moving one of the jaws with variable speed,
while the opposite jaw remains steady

Question 9

Wedge angles

Answer 9

10, 15, 20, 25, 30, 45 and 60 degrees

• Can generate any wedge angle but commissioned for a range of standard
wedge angles

• Decreased treatment time
• No beam hardening