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Air Kerma Strength S
K
Reference Air Kerma Rate /
Source Strength
air
The American Association of Physicists in Medicine in their Report No. 21 by the
AAPM Task Group No 32 introduced in 1987 the idea for the specification of
brachytherapy sources in terms of strength. That is, the
air-kerma strength
, S
K
,
defined as the product of the air kerma rate in free space at a measurement distance r
AAPM, North America
r
2
K
r)r(K S
⋅
=
α
&
from the source centre along the perpendicular bisector and the square of the
distance r.
The distance r must be chosen to be large enough so that the source can be treated
as a point source
and so that the
finite
dimensions of the detector
used for the
measurement have no influence on the result!