Dose Course 2018_Flipping book

Dose per MU formalisms

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:

(

)

(

)

; D A x y z D A x y z M  = , ; ; , ;



0

(

)

( ) A

M

b

1

+

0

0

(

)

D A

, x y z M ,

;

 

)  



ref

ref

ref

ref

(

)

( ) ref

0

Measured

b A

=

1

+

(

(

)

M

 

 

D A

ref ; , x

ref y z ,



0

ref

ref

0 Calculate

d

( b A

)

1

+

( ) ref

1 + Explicit as a backscatter correction factor:

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