3: Wedge Filters

What is the most commonly used beam modifying device?

Wedge Filters

What is the tilt of the isodose curves with wedge filters

tilt towards the toe (fyi: cuz it attenuates less of the beam)1.

Materials For Wedges:

1. Brass

2. Stainless Steel

3. Lead

term. Wedge Angle

def. the angle of the tilt for an isodose curve at the CR at a specified depth, historically at the 50% isodose line

What are the most common wedge angles

15

30

45

60

How do you read the angle of the wedge?

What isodose line is used for determining wedge angle?

50% line

True/False: The 50% line is also used for megavoltage machines

False

The 50% isodose line for an 10 MV beam is 18 cm deep (not practical) however, it is the common definition/ determination

term. Hinge Angle

def. the angle between the treatment fields

(e.g. if you tx using 1 AP + 1 straight lateral, the hinge angle would be 90 degrees. For a POP hinge angle is 180 degrees.)

What is the formula for calculating a wedge angle?

Wedge angle = 90 - (hinge angle/2)

e.g. Calculate the wedge angle for an AP & a lateral

Wedge Angle = 90 - (90/2) = 45 degrees

Formula for calculating a hinge angle

Hinge Angle = 180 - (wedge angle x 2)

e.g. You are using a 15 degree wedge. What would the hinge angle be?

Hinge Angle = 180- (15 × 2) = 115 degrees

Hinge and Wedge Pair angle chart (don’t need to know but it makes it easier)

What are the 2 types of Wedge Filter systems?

1. Individual

2. Universal

Individual wedge System

Wedge is aligned so that the thin end touches the edge of the field.

REQUIRES A SEPARATE WEDGE FOR EACH FIELD WIDTH

Universal Wedge System

A single wedge works for all field widths.

It is fixed centrally so that the field can be opened to any size

True/ False: Only a small part of the universal wedge is really used in producing the given wedge angle.

True

(only black part of the wedge)

If only a small part of the wedge is used in producing the wedge angle(the black part), what does the rest do? The part that goes beyond the field edges?

It doesn’t contribute to isodose tilt

BUT does reduce beam intensity/ output more than individual systems

What is the disadvantage of using a Universal wedge?

Decreases output because the wedge is thicker

(Individual/ Universal) Wedge Systems are preferred for Co60

Individual cuz output is already lower than seen on a linac

T/F: Wedges impact on hardening of a beam

True, but minimally

What is a “Poor Man’s Wedge”?

f.s. is reduced in one axis in only one direction each day.

Dynamic Wedging