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Brachy dosimetry is challenging …
f
zyD
) (
.
or an Ir-192 HDR source
K
S
,
Dose rate varies from ~20 Gy/s @
(0.5cm,90
0
) from an Ir-192 HDR source
to ~2 μGy/s @ (5cm,90
0
) from an I-125
LDR source
Sensitivity high for low uncertainty dose ,
range high, (preferably) with k
l
=k
dr
=1
Spatial dose gradient is high (i.e.
~25%/mm in the radial direction @
(0.5cm,90
0
) from an Ir-192 HDR source)
Volume must be small
positional
,
accuracy is very important
(1% rel. uncert. in D(1cm,90
0
) requires
0.05mm uncertainty in r, assuming 1/r
2
dose dependence since:
)
If solid phantoms are used to increase
r
D
r
D
2
positional accuracy a correction is
required from D
w,phant
to D
w,w
at each
meas position
.
Figure from: Taylor, Yegin, Rogers, Med. Phys. 34(2) 445 (2007)