Volume 11, No 2
International Journal of Radiation Research, April 2013
Calculation of beta absorbed fractions for iodine
isotopes in ellipsoidal thyroid lobe
A.A. Mowlavi1*, M. Mirzaei2, M.R. Fornasier3, M. de Denaro3
1PhysicsDepartment,SchoolofSciences,HakimSabzevariUniversity,Sabzevar,Iran
2PhysicsDepartment,facultyofShahidChamran,Technical&VocationalUniversity,Kerman,Iran
3S.C.diFisicaSanitaria,A.O.U.“OspedaliRiuniti”diTrieste,Trieste,Italy
ABSTRACT
► Original article
*Correspondingauthor:
Dr.AliAsgharMowlavi,
Fax:+985714003170
E‐mail:[email protected]
Received: Feb. 2012
Accepted: May 2012
Int. J. Radiat. Res., April 2013;
11(2): 121-126
Background: The thyroid gland absorbs nearly all the iodine in the blood,
independently of its isotopic composi on. When a large enough ac vity of
radioac ve iodine like 131I, 132I, 133I is taken into the body, it can destroy the
healthy thyroid gland cells as well as the cancer’s cells. In fact, as it is well
known, some isotopes of iodine are used not only for acquiring thyroid
images but also for curing thyroid cancer or hyperthyroidism due to Graves’
disease. Moreover, some of them are released in nuclear accidents. The aim
of this work is to evaluate the absorbed frac on of beta rays from different
iodine radioisotopes in thyroid lobe, using Monte Carlo method. Materials and Methods: We have applied the MCNPX code to calculate the beta
absorbed frac ons for the most important iodine radioisotopes in the thyroid
lobes, supposed to have an ellipsoidal shape, with the volume varying from 1
ml to 25 ml and the material composi on suggested by ICRP. The beta rays
spectra of iodine isotopes have been taken from the LBNL website. Results: The result showed that the volume lobe varia on had a significant effect on
the absorbed frac on for beta rays in thyroid gland, up to 25% for 124I. The
absorbed frac ons of beta rays were decreasing from 123I to 131I, 130I, 126I, 135I,
133
I and 124I respec vely. Conclusions: Decreasing of the absorbed frac on
might be related to the beta rays energy spectra of the isotopes. Moreover,
for 131I results obtained for beta absorbed frac on in spheres was in
agreement with previously published results.
Keywords: Thyroid gland; Beta absorbed fraction; Iodine isotopes; MCNPX
code.
INTRODUCTION
life of 8.05 days, is readily available and has a
low cost. Therefore, it is widely used for a
Therearemanydifferentiodineisotopes(up
number of nuclear medicine procedures,
to 37) and some of them have an important
including monitoring and tracing the low of
usage formedicalapplications (1‐20).Radioactive
thyroxin from the thyroid, imaging of thyroid,
iodine isotopes have the same physical
evaluationoftheiodineuptakecurve,therapyof
properties as stable iodine‐127 and thyroid
thyroid benign diseases or cancer (1‐4). Iodine‐
gland cannot distinguish between radioactive
123hasapropergammaemission(159keV)for
and non‐radioactive iodine. Iodine‐131, with a
thyroidimagingbygammacamerawithaphysi‐
gamma emission of 365keV and a physical half‐
cal half‐life of 13.1 hours. Therefore, it is used
Mowlavi et al. / Beta absorbed fractions for iodine isotopes in thyroid
~
forstudiesofthyroidmetabolismandaffordsan
A  (1)
D


excellentimagequality.Aswellas,using 123Ifor
m
diagnostic purposes reduces the radiation risk,
Whereɸβisthebetaabsorbedfraction,Aisthe
because of the short half‐life, low intensity and
cumulated
activity, Δβ is the average energy of
low energy of Auger electron. Also, it hasn’t the
beta
spectrum
and m is the mass of target
same stunning effect on functioning of thyroid
region.
The
result
of our study can be used for
tissue as 131I, with an advantage for the
dose calculation of iodine isotopes in thyroid
subsequentradioiodinetreatment(5,6).
glandaccordingtoMIRDformula.
Iodine‐124 can be used to identify mediasti‐
nal micro metastases in thyroid carcinoma by
PET/CT imaging. Freudenberg et al. reported
MATERIALSANDMETHODS
that Iodine‐124 PET/CT imaging is a promising
techniquetoimprovetreatmentplanninginthy‐
For simulation of beta transport, we have
roid cancer.6‐8 Iodine–125 is used in permanent
applied the MCNPX code (22). It is a general
lowdoseradiationbrachytherapy,especiallyfor
purpose, continuous and discrete energy,
localized prostate cancer (9, 10). Farran had
132
generalized‐geometry, time‐dependent code to
reported the use of I in the diagnosis of
simulate particles transport, based on Monte
patients undergoing treatment for thyroid
Carlo method. It contains lexible source and
disordersin1969(15)aswellas,ithadbeenused
tally options and utilizes the last nuclear cross
by Goolden et al. many years ago in studies of
(16)
sectionlibraries.
thyroidfunction .
Iodine neutron rich isotopes which are Thecodehasbeenusedtocalculatethebeta
rays absorbed fractions of main radioactive
producedinsidethereactorsmaybereleasedin
nuclear accidents, like Chernobyl in 1986, or iodineisotopes,withmassnumberrangingfrom
123to135,inathyroidlobeofellipsoidalshape,
disasters, as Fukushima in 2011 (17‐19). There‐
withthemajoraxistwotimesoftheminoraxis.
fore,theunwantedcontaminationofradioactive
We must mention that Iodine‐123 decays by
iodine can cause serious thyroid problems, the
electron capture, therefore its Auger electron
primaryriskbeingthyroidcancer,withchildren
spectrum is used for our simulation. The tissue
moreatriskthanadults.Strandetal.published
densityofthyroidgland hasbeenconsideredof
theirstudyaboutthethyroiduptakenotonlyof
1.05g/cm3andthevolumevaryingfrom1mlto
iodine‐131 but also of iodine‐133 in the
25 ml. Considering thyroid lobes to have an
population of southern Sweden from Chernobyl
ellipsoidalshape,themeandistanceoftwolobes
accident(17).
has been taken from the study of the
Inapreviousarticle,wereportedthethyroid
131
experimental images of more than 50 patients.
volume’s in luence on I energy deposition
Figure 1 shows the scintigraphic image of a
evaluated using the Monte Carlo simulation
thyroid and the corresponding model of a
method (20). Recently, Amato et al. developed a
thyroidlobe.
Monte Carlo simulation in Geant4 to calculate
the absorbed fractions for some beta rays
emittersuchas199Au,177Lu,131I,153Sm,186Reand
90Y,uniformlydistributedinellipsoidalvolumes
of soft tissue (21). In this study, we have
calculated the beta absorbed fractions for main
of iodine radioisotopes in the thyroid lobes,
supposed to have an ellipsoidal shape and a
volumevaryingfrom1mlto25ml.
Following the MIRD approach, the average
Figure 1. A real image of thyroid and the model of one thyroid
betadosetoatargettissuecanbecalculatedas:
lobe.
Int. J. Radiat. Res., Vol. 11 No. 2, April 2013
122
Mowlavi et al. / Beta absorbed fractions for iodine isotopes in thyroid
Absorbed fraction
Absorbed Energy(MeV)
0.7
The beta rays spectra of the iodine isotopes
I-124
havebeentakenfromtheLBNLIsotopesProject
0.6
website (http://ie.lbl.gov/toi), whose last
0.5
version2.1hasbeenreleasedinJanuary2004by
0.4
I-133
FirestoneandEkströ m(23).
Tosimulatethebody,theadult70kghuman
I-126
0.3
I-135
MIRD5 phantom has been used, where the
I-130
0.2
source organ was the thyroid gland with a uni‐
I-131
(24)
form iodine radioisotope distribution . Also,
0.1
theneckhasbeensimulatedwithmoredetailed
I-123
0.0
organs including skin, adipose layer under the
0
2
4
6
8 10 12 14 16 18 20 22 24 26
skin, bone, spinal cord, thyroid lobes, and the
Thyroid lobe volume (ml)
remainingpartassofttissue.Thesofttissuehas
Figure 2. The absorbed energy of beta rays in ellipsoidal
been considered having 1.05 g/cm3 density and
thyroid lobe against the lobe volume.
theICRPcomposition.Wehaveusedadualcore
1.00
PCwith3GHzspeedforrunningMCNPXcode,60
0.98
0.96
minutespereachrunningtoreducetheerrorof
0.94
thecalculations.
0.92
0.90
0.88
RESULTS
Energy deposition of beta rays depends on
sizeandgeometryofthesourceregion,distribu‐
tion of the radioactive material, types of the ra‐
diations and energies emitted by the radionu‐
clide.Toobtaintheabsorbedfractionfβ,wehave
calculated the total absorbed energy per decay
forbetaraysofeachiodineisotopesasshownin
igure 2; then the absorbed fraction has been
evaluatedusingthefollowingequation:
I-123
I-131
I-130
I-126
I-135
I-133
I-124
0.86
0.84
0.82
0.80
0.78
0.76
0.74
0
2
4
6
8
10
Figure3showsthebetaabsorbedfractionfor
7differentiodineisotopesagainstthevolumeof
thyroidlobe.Theresultsshowthattheabsorbed
fractionofbetaraysincreasesbyvolumeforall
the isotopes, and the variation is higher for 124I
than for other isotopes. It can be seen that the
betaabsorbedfractionfor 124Irangesfrom0.75
for a small volume (1 ml) up to 0.91 for a high
volume (25 ml). For 131I the variation is from
0.95till0.98,andfor123Iit’saround1%,because
of low energy Auger electron emitted from this
radioisotope.
123
14
16
18
20
22
24
26
Figure 3. The varia on of absorbed frac on of beta rays by
thyroid lobe volume.
DISCUSSION
Beta' s energy absorbed in the lobe (2)
 
mean energy of beta ray
12
Thyroid lobe volume (ml)
We have used a suitable analytical function
whichwasintroducedbyAmatoetal. (25)forthe
relationship between the absorbed fraction fβ
and the generalized radius ρ, with 2 it
parametersρ0ands:
  
 ()  1  0s 
  
1
(3)
The volume and surface area of an ellipsoid
aregivenby:
4

V  3 abc


p
p
p 1/ p


S  4  (ab)  (bc)  (ac) 
; p  1.6075.
3



(4)
Int. J. Radiat. Res., Vol. 11 No. 2, April 2013
Mowlavi et al. / Beta absorbed fractions for iodine isotopes in thyroid
Based on the sphere radius, “Generalized
iscorrectandsuitableinordertoobtainthebeta
Radius” for ellipsoid can be de ined as three
absorbed fraction as a function of the general‐
timesofvolumetosurfaceratio:
izedradius.
Amato et al. (21) have reported the 131I beta
V
(5)
3
absorbed fractions for ellipsoidal shape with
S
different ratio of axes and various masses of
Figure 4 shows the absorbed fraction of the
thyroid.Ourresultsareingoodagreementwith
betarayvariationagainstthegeneralizedradius
the result of Amato el al. considering the beta
of ellipsoid. Also, in this igure it can be seen a
absorbedfractionsforspheres(listedintable2).
good match of the itted curves (drawn as
continuous lines) to the obtained data. The it
Table 2. Comparison of our result for beta absorbed
frac ons in spheres with the result of Amato el al. (21)
parametersoftheanalyticalfunctioninequation
3 and the reduced chi‐square (Χ2/DoF) values
ϕ2 ϕ1 foralloftheiodineisotopesarelistedintable1.
Amato σ = ϕ1 ‐ ϕ2 V (cm3) This work The obtained value of ρ0 ranges from 0.1747 to
et al. 0.0068 (a variation of about 25.7 times!), while
1
0.899
0.90
-0.001
thesvalueisnotvariedtoomuchanditsvalueis
0.5
0.940
0.94
0
around1.Theseresultsshowthattheequation3
1
0.953
0.96
-0.007
2
0.963
0.97
-0.007
1.00
10
0.978
0.98
-0.002
0.98
20
0.983
0.98
0.003
0.96
0.94
0.92
CONCLUSION
0.90
0.88

0.86
0.84
I-123
I-131
I-130
I-126
I-135
I-133
I-124
0.82
0.80
0.78
0.76
0.74
0.72
0.70
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
(cm)
Figure 4. Absorbed frac on of the beta rays in ellipsoids
varia on against ρ, the generalized radius (lines curves are the
fi ed func ons).
The results show that considering the lobe
volume variation has a signi icant effect on the
absorbedfractionforbetarays.Itcanbedueto
the short range of beta rays and to increasing
surfacetovolumeratioofellipsoidallobe,which
causes some beta particles near the surface to
depositafractionoftheirenergyoutsideofthe
lobe. Moreover, our results for beta absorbed
fraction in spheres are in good agreement with
theresultofAmatoelal.(21)for131I.
Table 1. Fit parameters and reduced chi‐square values for all of the iodine isotopes.
Radioisotope For 124I
For 133I
For 135I
For 126I
For 130I
For 131I
For 123I
ρ0 0.1747 ± 2.8E‐4
0.0935 ± 1.4E‐4
0.0656 ± 7E‐5
0.061 ± 7E‐5
0.0461 ± 8E‐5
0.0301 ± 1.2E‐4
0.0068 ± 2E‐5
Int. J. Radiat. Res., Vol. 11 No. 2, April 2013
s 1.1617 ± 4.72E‐3
1.0934 ± 4.37E‐3
1.0908 ± 3.12E‐3
1.0664 ± 3.3E‐3
1.0447 ± 5.1E‐3
1.0538 ± 1.13E‐2
1.0835 ± 9.04E‐5
Χ2/DoF 7.322E‐7
2.532E‐7
7.169E‐8
7.067E‐8
1.033E‐7
2.331E‐7
8.619E‐9
124
Mowlavi et al. / Beta absorbed fractions for iodine isotopes in thyroid
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