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COMPUTER SIMULATIONS FOR INTERNAL DOSIMETRY USING
VOXEL MODELS
Sakae Kinase1,*, Akram Mohammadi1, Masa Takahashi1, Kimiaki Saito1, Maria Zankl2and
Richard Kramer3
1
Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai-mura, Ibaraki 319-1195, Japan
2
Helmholtz Zentrum Mu
¨nchen, Ingolstaedter Landstr. 1, 85764 Neuherberg, Germany
3
Universidate Federal de Pernambuco, Cidade Universitaria, CEP 50740-540, Recife, PE, Brazil
*Corresponding author: kinase.sakae@jaea.go.jp
In the Japan Atomic Energy Agency, several studies have been conducted on the use of voxel models for internal dosimetry.
Absorbed fractions (AFs) and Svalues have been evaluated for preclinical assessments of radiopharmaceuticals using human
voxel models and a mouse voxel model. Computational calibration of in vivo measurement system has been also made using
Japanese and Caucasian voxel models. In addition, for radiation protection of the environment, AFs have been evaluated
using a frog voxel model. Each study was performed by using Monte Carlo simulations. Consequently, it was concluded that
these data of Monte Carlo simulations and voxel models could adequately reproduce measurement results. Voxel models were
found to be a significant tool for internal dosimetry since the models are anatomically realistic. This fact indicates that
several studies on correction of the in vivo measurement efficiency for the variability of human subjects and interspecies
scaling of organ doses will succeed.
INTRODUCTION
There has been a considerable experience accumu-
lated in the use of voxel models for internal dosim-
etry. Inter-comparisons on Monte Carlo modelling
for in vivo measurements of radionuclides in knee
and torso voxel models were undertaken to upgrade
dosimetry programmes in the European Union. In
the Japan Atomic Energy Agency (JAEA), internal
dosimetry codes using voxel models have been devel-
oped as EGS4
(1)
user codes: UCSAF code for
internal dosimetry
(2)
and UCWBC code for calibrat-
ing in vivo measurement systems
(3)
. For preclinical
assessments of several radio-pharmaceuticals,
absorbed fractions (AFs) and Svalues for human
and mouse voxel models have been evaluated using
the UCSAF code
(4–8)
. Peak efficiencies and response
functions of an in vivo measurement system consist-
ing of Ge semiconductor detectors have been evalu-
ated for several voxel models, such as Japanese and
Caucasian models, by the UCWBC code
(3,9,10)
.In
recent years, AFs for a frog voxel model have been
evaluated for environmental protection using the
UCSAF code
(11,12)
. In this article, the use of voxel
models for internal dosimetry and two Monte Carlo
simulation codes developed in JAEA are
summarised.
MATERIALS AND METHODS
Voxel models
Several voxel models have been used for internal
dosimetry in JAEA: the Japanese adult male human
‘Otoko’
(13)
, the Japanese adult female human
‘Onago’
(14)
, the Caucasian adult male human
‘MAX06’
(15)
, the male mouse ‘Digimouse’
(16)
and
the frog. The voxel models are based on actual
image data obtained from computed tomography,
magnetic resonance and cryosections. The GSF
child female human ‘Child’
(17)
and the MIRD-type
voxel models were also used for validation of Monte
Carlo simulations. Table 1shows the characteristics
of the voxel models used in JAEA.
Monte Carlo simulation codes
Two EGS4 user codes were developed for internal
dosimetry: the UCSAF and UCWBC codes. The
EGS4 is a well-established and well-benchmarked
code for coupled electron–photon transport. The
main physical models are taken into account.
Rayleigh scattering, Doppler broadening in
Compton scattering, linearly polarised photon scat-
tering and electron impact ionisation are included by
means of options. The UCSAF and UCWBC codes
treat voxel models. In the UCWBC code, specifica-
tions of modelling are also made using the combina-
torial geometry (CG) method. The cross-sectional
data for photons are taken from PHOTX
(18)
and the
data for electrons and positrons are taken from
ICRU report 37
(19)
.
Absorbed fractions
Self-irradiation AFs for photons in the kidneys were
evaluated for the Otoko, Onago, Digimouse and
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Radiation Protection Dosimetry (2011), Vol. 146, No. 1 – 3, pp. 191– 194 doi:10.1093/rpd/ncr145
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frog voxel models using the Monte Carlo code
EGS4 in conjunction with the UCSAF. The self-
irradiation AFs were calculated by using the energy
emitted from the kidneys and energy deposited
within the kidneys. Table 2compares the masses of
the kidneys of the voxel models. The source of
photons was assumed to be monoenergetic in the
energy range of 10 keV to 4 MeV, uniformly distrib-
uted in the kidneys of the voxel models. In addition,
self-irradiation AFs for the kidneys in the Child and
MIRD-type voxel models were evaluated for com-
parison. Photon history was run at numbers suffi-
cient to reduce statistical uncertainties ,5%.
Svalues
Self-irradiation Svalues (mGy per MBq.s) to the
kidneys from uniformly distributed beta-ray emitters
within the kidneys were evaluated for the Otoko,
Onago, Digimouse and frog voxel models. The beta-
ray emitters were
18
F and
90
Y of potential interest in
the kidney dosimetry. Table 3shows the maximum
and mean energies for
18
F and
90
Y. To evaluate the
self-irradiation Svalues, the total energies deposited
in the kidneys per source particles were calculated
using the EGS4 and UCSAF codes.
In vivo measurements
Counting efficiencies of a whole-body counter
installed in JAEA were evaluated for the Otoko and
MAX06 voxel models by the EGS4 and UCWBC
codes. Figure 1shows the geometry of the simu-
lation model of the JAEA whole-body counter and
MAX06 voxel model. The model was accurately
constructed to represent the actual whole-body
counter, which has three p-type closed-ended coaxial
high-purity germanium (HPGe) semiconductor
detectors. The counting efficiencies were evaluated
by dividing the number of photons that deposited all
initial energy in the detectors, by the number of
simulated histories. The photon sources were
assumed to be isotropic, and to be homogeneously
distributed within the voxel models.
RESULTS AND DISCUSSION
Absorbed fractions and Svalues
Figure 2shows the self-irradiation AFs for photons
in the kidneys of the Otoko, Onago, Digimouse,
frog, Child and MIRD-type voxel models in the
energy range from 10 keV to 4 MeV. The self-
irradiation AFs depend on the photon energy and
decrease with an increase in photon energy on
the whole. As mentioned in the previous studies, the
self-irradiation AFs for kidneys depends on the
organ mass.
Table 1. Voxel models with their characteristics.
Name Images Organisation
Otoko CT JAEA
Onago CT JAEA
MAX06 CT Federal University
of Pernambuco
Digimouse Micro-CT and
colour cryosection
University of S
outhern California
frog Cryosection JAEA
Child CT Helmholtz
Zentrum Mu
¨nchen
MIRD type CG JAEA
Table 2. Comparison of the kidney masses for the voxel
models.
Name Kidneys (kg)
Otoko 2.710
21
Onago 2.610
21
Digimouse 5.110
24
Frog 2.210
24
Child 1.910
21
MIRD type 3.010
21
Figure 1. Geometric models of the whole-body counter
and the voxel model.
Table 3. Maximum E
max
and mean E
mean
energies for
18
F
and
90
Y.
Nuclides E
max
(MeV) E
mean
(MeV)
18
F 0.634 0.250
90
Y 2.281 0.934
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Figure 3shows the self-irradiation Svalues for the
kidneys of the Otoko, Onago, Digimouse and frog
voxel models for
18
F and
90
Y. It can be seen that the
self-irradiation Svalues for the small voxel models
are much larger than those for the large voxel
models.
Counting efficiencies of a whole-body counter
Figure 4shows the counting efficiencies of the
JAEA whole-body counter for the Otoko and
MAX06 voxel models. The counting efficiencies for
the Otoko are slightly larger than those for the
MAX06 over the whole energy range. This is due to
geometric differences between the two models.
CONCLUSIONS
Monte Carlo simulations and voxel models were
found to be a significant tool for internal dosim-
etry. In particular, voxel models were confirmed to
be useful for internal organ dose evaluations since
they are anatomically realistic. This fact indicates
that several studies on correction of the in vivo
measurement efficiency for the variability of
human subjects and interspecies scaling of organ
doses will succeed.
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Figure 4. Comparison of the counting efficiencies for the
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VOXEL MODELS FOR INTERNAL DOSIMETRY
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