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Phantom scatter: equivalent square of a circular field
Tabulated data
BJR25 in Appendix A (Day and Aird 1996): Derived from a fitting a linear scatter
function to central axis depth dose data as a function of radius
S
r or
S or
S
555 .0
90.0
00046 .0 891 .0
=
=
+ =
δ
δ
δ
Side of square field
diameter of circular field
r
S
8.1
≈
or:
•Depth and beam quality dependence is ignored
•Shown to give of an accuracy within
5% for scatter dose and 2% for total dose for points on CAX
Note: equivalence between square and circular fields, if one were to apply the area over
perimeter method...:
r S
r
r
S
S
Perimeter
Area
S
2
2 4
2
2
=⇒⇒ =→ =
K
π
π
S
eq
r
10% over-estimation!