Dose Course 2018_Flipping book

Dose per MU formalisms: factor-based dose calculations

Modulation with physical wedges

1,15

Example: Elekta Precise, 60 ° internal wedge

1,10

X 6 - op

Variation open vs wedged fields:

X 10 - op

With FS

X 25 - op

1,05

➔ ~ 10% open

X 6 - we

➔ ~ 20% wedge

X 10 - we

1,00

X 25 - we

Wedge angle

Energy

0,95

OUTPUT RATIO IN AIR

Linac specific

0,90

0 5 10 15 20 25 30 35 40

EQUIV. FS [cm]

• Physical wedges introduce changes in the beam spectrum, which are dependent on wedge material and influence dosimetric parameters that vary with depth (TPR, RDD) as well as phantom scatter, Sp • The position of the wedge, whether internal (motorised) or external (manually inserted) affects Sc

• Dosimetric parameters for wedged beams should not be confused with open beam data • Irregular wedged beams need some special considerations for MU calculation / verification

 Additional correction factors needed in the dose per MU formalism