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potential inconsistency between dose calculations and delivery
The difference in radiation field edge and leaf tip position varies with off-axis location
isocenter plane
Source
plane
x
x
HVL
x
w
-10 -7.5 -5 -2.5
2.5 5 7.5 10
-0.2
-0.15
-0.1
-0.05
0.05
0.1
x
cm
cm
x
HVL
- x
w
Delivery (linac):
lookup table or parameterized correction to set leaf position
TPS
:
same method as used for delivery;
or
based on direct calculation of 50% transmission
From Anders Ahnesjö
Modelling the MLC leaf ends