Biomedical Engineering Reference
In-Depth Information
16.8
Specific Absorbed Fraction, Specific Effective Energy, and Committed Quantities
H
(T
The specific equivalent dose,
S), in a target organ T due to the radiation
emitted per transformation by a radionuclide in organ S (which can be the same
as T), appears in Eq. (16.2). For particular choices of T, S, and radiation type, one
can obtain this quantity from knowledge of the fraction of the energy emitted in S
that is absorbed in
T
. This absorbed fraction (AF) is designated
AF(T
←
S)
R
, where
R
denotes the radiation type. Dividing the absorbed fraction by the mass
M
T
of
the target organ gives the specific absorbed fraction,
AF(T
←
S)
R
/
M
T
.TheICRP
has published tables of specific absorbed fractions, expressed in g
-1
,forusein
computing the committed quantities given by Eqs. (16.1) and (16.2).
Table 16.2 shows an example of specific absorbed fractions in a number of target
organs for photons of several energies emitted from the thyroid as source organ.
The table illustrates the effect of the decrease in the linear attenuation coefficient
(Section 8.7) with increasing photon energy over the range considered. The specific
absorbed fractions for the thyroid as target decrease with the greater probability of
escape of the higher-energy photons from the source organ. The same holds true
for the total body. In contrast, the greater penetrability of the higher-energy pho-
tons leads to an increase in the specific absorbed fractions in tissues outside the
thyroid. Since 0.010-MeV photons are so strongly absorbed, the increase is dra-
matic at 0.100 MeV.
For most organs it is assumed that the energies of all alpha and beta particles
are absorbed in the source organ. (The exceptions, which we shall not go into, are
mineral bone and the contents of the gastrointestinal tract.) Thus AF(T
←
0
unless T and S are the same when
i
denotes alpha or beta radiation. Then also
AF(T
←
S)
i
=
1.
The absorbed fractions for gamma rays cannot be evaluated in a simple way.
Their values depend in a complicated fashion on the photon energy; the size, den-
←
S)
i
=
Table 16.2
Specific Absorbed Fraction (g
-1
) of Photon Energy in
Several Target Organs and Tissues for Monoenergetic Photon
Source in Thyroid (from ICRP Publication 23)
Photon Energy (MeV)
Target
0.010
0.100
1.00
Stomach wall
2.07 E-25
1.90 E-07
4.62 E-07
Small intestines plus contents
4.58 E-35
1.97 E-08
1.38 E-07
Lungs
1.52 E-13
3.67 E-06
3.83 E-06
Ovaries
2.33 E-23
1.09 E-08
9.62 E-08
Red marrow
2.68 E-09
4.87 E-06
2.57 E-06
Testes
2.48 E-28
7.87 E-10
2.46 E-08
Thyroid
4.29 E-02
1.44 E-03
1.54 E-03
Total body
1.43 E-05
4.71 E-06
4.26 E-06
