![Show Menu](styles/mobile-menu.png)
![Page Background](./../common/page-substrates/page0762.jpg)
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:
Dose per MU formalisms
(
)
(
)
( )
(
)
0
0
0
; , ;
; , ;
1
D A x y z D A x y z
M
b
M
A
Ψ
Ψ
=
+
(
)
(
)
(
)
( )
(
)
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
Ψ
Ψ
=
+
( )
1
b A
+
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
b A
+
Explicit as a backscatter correction factor: