Biomedical Engineering Reference
In-Depth Information
Example
What is the contribution of the beta radiation from
131
I in the thyroid to the specific
effective energy values for various target organs? The mass of the thyroid is 20 g. What
will be the effect on the value of the SEE when the photons from
131
I are included?
Solution
In this example the source organ is the thyroid and the only type of radiation R that
we are to consider initially is beta rays. Because their range is small compared with
the size of the 20-g thyroid, we assume that the beta particles are completely absorbed
in the source organ. Therefore, AF(other organs
←
thyroid)
131
I(
β
-
)
=
0 and the corre-
sponding contributions to the SEE(other organs
0. (The absorbed
fractions for the other target organs are not zero for the gamma rays emitted by
131
I
in the thyroid. The SEE for the gamma photons are discussed below.) It remains to
compute the SEE for the beta rays in the thyroid itself as the target organ. The various
factors that enter Eq. (16.39) are determined as follows. The thyroid mass
M
T
=
←
thyroid)
131
I(
β
-
)
=
20 g
is given. The absorbed fraction AF(thyroid
1 (Table 14.1).
The yields
Y
R
and energies
E
R
per transformation can be obtained from Appendix D.
(We assume, when not given, that the mean beta-particle energy is one-third the max-
imum.) Thus we obtain from Eq. (16.39)
←
thyroid)
=
1, and
w
R
=
1
20
(0.006 × 0.269 + 0.89 × 0.192
+0.07
SEE (thyroid ← thyroid)
131
I(
β
-
)
=
×
0.097 + 0.02
×
0.069)
0.009 MeVg
-1
=
(16.44)
per transformation. Including the photons from
131
I will make all of the SEE(other
organs ← thyroid) = 0. Since most of the photon energy emitted inside the small
thyroid will escape from the organ, the specific effective energy SEE(thyroid
←
-
. It turns out that
thyroid)
131
I(
γ
)
is very much smaller than that of the
β
SEE(thyroid
←
thyroid)
131
I
=
SEE(thyroid
←
thyroid)
131
I(
β
-
)
+ SEE(thyroid
←
thyroid)
131
I(γ)
= 0.010 MeV g
-1
(16.45)
per transformation. As described in the next paragraph, this is the correct value as
obtained from the detailed calculations.
Complete, detailed decay-scheme data were used to calculate the specific ab-
sorbed fractions and specific effective energies used in the ICRP-30 methodology.
As an example, in place of the simple decay scheme from Appendix D that we used
for
131
I in the example just given, the ICRP calculations used the complex decay
data shown in Fig. 16.10. The yields and average energies are given for five modes
of
β
-
5)
contributing less than 0.1% to
Y
R
E
R
for the beta
particles. A total of nine gamma photons are included, along with five internal-
conversion electrons and the K
α
1
and K
α
2
daughter xenon X rays. For the partic-
ular example chosen here, the beta-particle contribution [Eq. (16.44)] turns out to
-
decay, a sixth mode
(
β
