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Derivation of formalism to convert ionisation in plastic to dose to water
Converting ionisation measurement in plastic to dose in water
Phantoms irradiated with
the same MV photon
beam
In water at a reference depth
ref
z
In solid plastic at an equivalent depth
eq
z
(
)
(
)
w
s
en
ref
sp,
eq sp,
2
ref
ref
wQ,
w
air
sQ,
s
air
QQ,
Qw, D,
ref
eq Qs,
ref
ref
Qw,
) (
) (
,
,
ref
eq
o
o
β
ρ
µ
ρ
ρ
⋅
⋅
⋅
+
+
⋅
⋅
⋅
⋅
⋅
⋅
=
w
s
r S
r S
z
SSD
z
SSD
p
S
p
S
k
N r z M r z D
Phantom dose conversion factor for
the ionisation chamber
( )
( )
Qw, s,
QQ,
Qw, D,
eq Qs,
ref
Qw,
o
o
k k
N z M z D
=
Seuntjens et al, 2005, Med.Phys. 32 pp2945-