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1. The monitor signal caused by backscatter as a function of collimator aperture, can be
modelled as an additional fraction
b
of the direct signal. The formalism would be:
(
)
(
)
( )
(
)
0
0
0
;
,
;
; , ;
1
D A x y z D A x y z
M
b
M
A
=
+
(
)
(
)
(
)
( )
(
)
ref
ref
ref
ref
Measured
ref
ref
ref
ref
ref
0 Calculate
0
d
0
;
,
,
1
; ,
,
D A
x y z M
b A
x
y z
M
D A
=
+
Dose per MU formalisms
2. Alternatively the effect of the backscatter into the monitor chamber on the calculation of
MU can be accounted for indirectly through the use of a correction factor that reflects
the change in output due to this backscatter with collimator setting
(
)
( )
ref
1
1
b A
b A
+
+
Explicit as a backscatter correction factor: