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Dose per MU formalisms:
factor-based dose calculations
Modulation with physical wedges
• 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
0,90
0,95
1,00
1,05
1,10
1,15
0 5 10 15 20 25 30 35 40
EQUIV. FS [cm]
OUTPUT RATIO IN AIR
X 6 - op
X 10 - op
X 25 - op
X 6 - we
X 10 - we
X 25 - we
Example: Elekta Precise, 60
°
internal wedge
Variation open vs wedged fields:
With FS
➔
~ 10% open
➔
~ 20% wedge
Wedge angle
Energy
Linac specific