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Head scatter: equivalent square of a rectangular field
Empirical approach
to account for different
influence of collimator layers
primary collimator
wedge
monitor chamber
upper Y-jaws
lower X-jaws
flattening filter
Head of a treatment unit
MLC leaf
S
f
S
U
S
L
isocentre
Linac head
(
)
y xG
yx1G
y)
(x, c
e
+
+
=
Vadash P and Bjärngard B E (1993) Med. Phys. 20, 733-734.
x
➔
lower jaw
y
➔
upper jaw
G weighting factor
depends on:
➢
treatment head design
➢
beam energy
➢
beam modifiers