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Head scatter: equivalent square of a rectangular field
Linac head
Determination of weighting factor G:
a. Experimentally: set of Sc values measured for
square and rectangular fields (e.g. keeping one
collimator setting fixed and varying the other
one)
b. From treatment head geometry: relative weight
(
G
) depends on distance from the X-ray
source
primary collimator
wedge
monitor chamber
upper Y-jaws
lower X-jaws
flattening filter
MLC leaf
S
f
S
U
S
L
S
*
S
UI
S
LI
G = (S
L
/ S
U
)
2
Yu and Sloboda (1995)
isocentre
G = (S
L
/ S
LI
) (S
UI
/ S
U
)
Kim et al (1997)
Typical values: 1.4 – 2.0
(See page 30 in ESTRO no. 6)