However, neither quantity is universally endorsed
for dosimetric use by the nuclear medicine com-
munity [3], and some authorities still advocate use
of the equivalent radiation dose to individual
organs only.
A system for calculating the absorbed radiation
dose resulting from a radiopharmaceutical admi-
nistration has been devised by the Medical Inter-
nal Radiation Dose (MIRD) Committee of the
Society of Nuclear Medicine [4], and is the most
widely used and accepted methodology for esti-
mating the radiation dose to human subjects. This
schema specifies a set of anthropomorphic human
models, both male and female, adult and pae-
diatric. Computer simulations have been run for
many distributions of radioactivity within the
body organs of these phantoms, yielding data on
the relative irradiation between and within the
individual source organs. Thus individual organ
radiation doses may be determined for any speci-
fic administered radionuclide and observed pat-
tern of biodistribution and clearance. From these
individual organ doses the effective dose equi-
valent and effective dose may then be determined.
Sentinel Lymph Node Dosimetry
Radiation dosimetry for the technique of lym-
phoscintigraphy has been determined by Berg-
qvist et al. [5]. Their findings indicate an effective
dose equivalent of 5.32
¥
10
–3
mSv/MBq for a sub-
tracer, e.g.in the patient with renal failure or the
subject administered a thyroid “blocking” agent.
The level of radiation risk arising from differing
forms of exposure has been assessed by the Inter-
national Commission on Radiological Protection
(ICRP). In 1977 they introduced the concept of the
effective dose equivalent (EDE) (ICRP 26) [2]. This
is defined as the sum of the calculated equivalent
doses for a set of specified body organs,where each
contributing organ dose is multiplied by a tissue
weighting factor
w
T
to account for the relative
radiosensitivity for that organ, thereby arriving at
a total “effective” whole-body radiation absorbed
dose. Subsequently the effective dose (ICRP 60 [1])
has been defined by the ICRP to supersede use of
the effective dose equivalent in respect of medical
exposures. Although it has a similar underlying
concept it incorporates both revised tissue weight-
ing factors and a modified set of specified organs
better reflecting the whole patient population. The
effective dose has not been completely accepted as
a replacement for the effective dose equivalent,
however, and in practice both quantities are still in
use for dosimetric purposes. By using either, the
radiation dose resulting from different radiophar-
maceutical administrations may be compared,
both with each other and also with the dose result-
ing from an equivalent exposure caused by uni-
form irradiation of the whole body. The organs
specified in the definitions for both effective dose
equivalent and effective dose and their respective
tissue weighting factors are shown in Table 2.
chapter
8
92
Ta ble 2.
Tissue weighting factors used in the dosimetric concepts of effective dose equivalent and effective dose
Specified organ or tissue
Tissue weighting factor (w
T
)
Tissue weighting factor (w
T
)
for effec tive dose
for effective dose equivalent
Gonads
0.20
0.25
Lung
0.12
0.12
Red bone marrow
0.12
0.12
Stomach
0.12
Not specified
Colon
0.12
Not specified
Thyroid
0.05
0.03
Liver
0.05
Not specified
Oesophagus
0.05
Not specified
Breast
0.05
0.15
Bladder
0.05
Not specified
Skin
0.01
Not specified
Bone surfaces
0.01
0.03
Remaining organs
0.05
0.30
To tal
1.00
1.00
