Biomedical Engineering Reference
In-Depth Information
decay constant. The classifications D, W, or Y for different chemical forms of a ra-
dionuclide are provided by the ICRP along with the metabolic data. Radioactive
daughters, included in the calculations, are assumed to have the same metabolic
behavior as the original parent. While Eq. (16.3) involves only a single activity,
q
a
,
others are more complicated. For compartment d, for example, we have
d
q
d
d
t
=
ID
TB
F
d
+
λ
f
q
f
+
λ
g
q
g
-
λ
d
q
d
-
λ
R
q
d
,
(16.4)
which couples the activity
q
d
in d to those in f and g. Given a set of initial conditions,
the system of ten linear, coupled, differential equations is solved to obtain the ac-
tivities in each of the compartments a-j as functions of time. The rate of transfer of
the inhaled radionuclide into the body fluids as a function of time is then given by
BF(
t
)
=
λ
a
q
a
(
t
)+
λ
c
q
c
(
t
)+
λ
e
q
e
(
t
)+
λ
i
q
i
(
t
).
(16.5)
Similarly, the rate of transfer into the GI tract is
G
(
t
)
λ
d
q
d
(
t
).
(16.6)
The ICRP-30 respiratory-system model thus specifies the deposition, retention,
and removal of inhaled materials in various components of the pulmonary-lymph
system. It is used to calculate the number of transformations
U
for the committed
equivalent dose to the lung and to calculate source terms for the body fluids and
the GI tract.
=
λ
b
q
b
(
t
)+
16.5
ICRP-66 Human Respiratory Tract Model
The ICRP-30 lung model has served well to calculate occupational annual limits on
intake (ALI) and to be the foundation for many applied monitoring and control pro-
grams and procedures. It has been an extremely valuable tool for the dosimetry of
inhaled radionuclides. Nevertheless, there are significant problems in lung dosime-
try that the model was not designed or equipped to handle. For example, many
radioactive compounds were found to clear from the respiratory system at rates
considerably different from those assigned. Also, the lung dose was calculated as
an average over the total lung mass, whereas it is relatively rare that the respiratory
tract is uniformly irradiated by internally deposited aerosols. Moreover, different
tissues of the lung have different radio-sensitivity. Such factors are particularly rel-
evant for inhaled radon daughters or hot particles. In addition, the ICRP-30 worker-
oriented lung model lacked the flexibility to be applied generally to members of the
public, a matter related to increasing environmental concerns.
These and other considerations, including continued research and a growing
body of new information, led the ICRP to review and address lung dosimetry anew.
Rather than developing an entirely new model, efforts were aimed at improving
and building on the ICRP-30 model to meet an expanded variety of needs. The
work resulted in the 1994 Publication 66,
Human Respiratory Tract Model for Radio-
logical Protection
, with the present and considerably more sophisticated ICRP lung
