Evaluation of the absorbed dose to the thyroid of a
newborn due to I123 / I131 and Tc99m
M.V. Vásqueza,b, C.E. Castilloa, E. Diazc, G. R. Rojasa, J. Idrogoa, E. Rojasd, Y.
M. Sifuentesa , G. Mosqueiraa,
a
Universidad Nacional de Trujillo.Trujillo-Perú
Universidad Cesar Vallejo (UCV) Trujillo-Perú
c
Universidad Federal do Rio Grande do Sul (UFRGS).Brasil
d
Instituto Peruano de Energía Nuclear ,(IPEN) .Lima-Perú
marvva@hotmail.com
b
ABSTRACT
The absorbed dose to the thyroid of a newborn during uptake studies is estimated through analysis of the biokinetics
of radiopharmaceuticals containing I131 / I123 (iodides) or Tc99m (pertechnetate).
The study consist in determining if the dosimetric contributions due to the organs of the biokinetics of the I123 / I131
(iodide) and Tc99m (pertechnetate) , are significant in the calculation of the absorbed dose to the thyroid of a
newborn, for uptake studies.
To determine the dosimetric contributions of the organs of the biokinetics of a newborn, we use the formalism MIRD
and representation Cristy-Eckerman . The results show that the absorbed dose due to I123 / I131 are given by 5276.0
mGy / MBq / 39.15 mGy / MBq, respectively. Dosimetric contributions of the organs that are part of its biokinetics
(excluding the thyroid newborn), is not significant in the estimated doses.
The dose absorbed by the gland of a newborn, due to emissions Tc 99m is 0.306 mGy / MBq; 7.14% of this dose
corresponds to the dosimetric contributions the organs that are part of its biokinetics (excluding the thyroid
newborn), and is very significant in the estimated dose.
Keywords: Dosimetry MIRD, phantom Cristy-Eckerman, thyroid uptake iodide and pertechnetate.
1. INTRODUCTION
The estimate of the dose absorbed by the thyroid of a newborn during uptake studies can be
performed by analyzing the biokinetics of radiopharmaceuticals used, containing I131 / I123
(iodides) or Tc99m (pertechnetate).
2. MATERIAL AND METHODS
To estimate the absorbed dose to the thyroid of a newborn due to the dosimetric contributions the
organs that are part of the biokinetics, we will use the method MIRD, and the representation of
Cristy-Eckerman Medical Internal Radiation Dosimetry considered equations (Asociación,
2013):
D fotones (tiroides )
A0


   k k (tiroides  i )   i
i 1  k

Dparticle (tiroide  tiroide )
A0
 [ E particle
 tiroide
mtiroide
 E particle
 TB
mTB
rad / Ci
] x 2,13
rad / Ci
 TB  Total residence time of the body
mTB  Total body mass
The absorbed fractions, Φ k (thyroid ← i) g-1, of “i" analyzed organs (organs biokinetics ), for
photon energies "k" of I131 / I123 and Tc99m were obtained from ORNL/TM-8381/V6 (Cristy and
Eckerman, 2013a). Residence times of radiopharmaceuticals mentioned in each organ biokinetics,
given in Tables 1 and 2, were obtained from the website (HPS, 2013a).
Table 1: Residence time (hours) and biokinetics of the I123 / I131 (iodides) to newborns (25%
uptake) (HPS, 2013a)
Organ Biocinetic
Iodine
I123 :
I131 :
τᵢ
τᵢ
Thiroid
Stomach
Small
intestine
Kidney
Bladder
content
Rest of the
body
(hours)
2,94
1,08
1,08
0,062
0,833
5,03
(hours)
60,72
1,66
1,66
0,095
1,32
7,76
Table 2: Residence time (hours) and biokinetics of TC99m (pertechnetate) to newborns ( 25 %
uptake) (HPS, 2013a)
Organ Biocinetic
Radiopharm
Tc99m :τᵢ (hours)
 k  2,13 nk E k
(
Thiroid
Stomach
content
ULI content
Kidney
Bladder
content
Rest of the
body
0,037
0,154
0,743
0,033
0,345
0,363
rad  gm
)
Ci  hr
, represents average energy of the “k” photons emitted in the
decay of I131 / I123 and Tc99m, given in Table 3, were obtained from web page (HPS, 2013b).
Table 3: Nuclear Data for the emitted photons (MeV) the most significant I123 / I131 and Tc99m
(HPS, 2013b)
RFM
Photons
Radiact. gamma
I123
Characterístic
radiation
Radiact. gamma
I131
Characterístic
radiation
Radiact. gamma
99m
Tc
Characterístic
radiation
 k  2,13 n k E k
Ek
(Me V)
nk /des
0,159
0,833
0,2821
0,529
0,0139
0,0157
0,0272|
0,246
0,01415
0,0275
0,460
0,0269
0,0310
0,160
0,01056
0,080
0,026
0,0044
0,284
0,06
0,0363
0,364
0,817
0,6334
0,637
0,0717
0,097
0,723
0,0177
0,027
0,0295
0,0138
0,00088
0,0298
0,0256
0,0016
0,0336
0,009
0,0006
0,1405
0,8906
0,2665
0,1426
0,0002
0,0001
0,0183
0,021
0,0008
0,0184
0,040
0,0016
0,0206
0,012
0,0005
(
rad  gm
)
Ci  hr
E¯particle (MeV/des.) represents the average energy of particles emitted by the I131 / I123 and Tc99m,
this is, represents the electron appearing in the decay processes for capturing and Auger electrons
are given in Table 4 and were obtained from web page (HPS, 2013b).
Table 4: Nuclear Data for emitted particles (MeV) the most significant I123 / I131 and Tc99m
(HPS, 2013b)
RFM
Particles
Conversion
electrons
I123
Ek (MeV)
nk /des
nk Ek
( MeV / des )
0,1272
0,136
0,0173
0,1540
0,0177
0,0027
0,1580
0,0035
0,00055
0,0032
0,94
0,0030
0,0227
0,1235
0,0028
0,0694
0,021
0,00145
0,0966
0,073
0,007
0,1916
0,899
0,1722
0,283
0,0048
0,00135
0,0456
0,0354
0,0016
0,359
0,0025
0,00089
0,3299
0,0155
0,0051
0,2497
0,003
0,00075
0,0034
0,051
0,00017
0,0246
0,006
0,000147
0,1195
0,088
0,01052
0,1216
0,0055
0,00067
0,1375
0,0107
0,0015
0,1396
0,0017
0,00024
0,140
0,0019
0,00026
0,0016
0,746
0,0012
0,0022
0,102
0,00022
0,0155
0,0207
0,00032
Auger electrons
Beta radiation

E particle   nkEk
( MeV / des)
0,0206
0,0058
0,1818
131
I
Conversion
electrons
Auger electrons
Conversion
electrons
0,0076
0,000317
0,01439
Tc99m
Auger electrons
0,00054
Values of the thyroid mass and total body, of a newborn, were obtained from ORNL / TM-8381 /
V1 (Cristy and Eckerman, 2013b).
Table 5: Values of mass (g) thyroid and whole body of a newborn, for Cristy -Eckerman
representation (Cristy and Eckerman, 2013b)
Mass (grams)
Newborns
Thyroid
1,29
Total body (TB)
3400
Using the methodology MIRD, and representation of Cristy-Eckerman for organs the biokinetics
of the newborn, the study is to demonstrate if the dosimetric contributions of the organs that are
part of the biokinetics of the I123 / I131 (iodide) and Tc99m (pertechnetate) are significant in the
calculation of the dose absorbed during uptake studies.
3. RESULTS
Table 6: Absorbed dose to the thyroid of a newborn due to I123 / I131 (iodide) and
Tc99m (pertechnetate) in the representation of Cristy – Eckerman and formalism MIRD
(mGy / MBq)
Radiopharma-ceutical
I
123
(Iodide)
I131 (Iodide)
emissions
D(T ← T)/Ao
D(T← i)/Ao*
Photons gamma
Radiation - X
2,16 (5,52 %)
1,89 (4,82 %)
0,00015(0,07%)
Electron
conversion
Auger electron
Photons gamma
Radiation - X
Beta emission
Electron
conversion
Auger electron
‹ 0,01%
39,15
140, 4 (2,68%)
5135,6
(97,30%)
205,7 (3,89%)
5276,0
8,58 (0,16%)
Photons gamma
0,02349 (7,67%)
Radiation -X
0,00405 (1,32%)
Electron
conversion
Auger electron
35,1 (89,65%)
27,0 (68,96%)
8, 1 (20, 68%)
137, 7 (2, 6%)
2, 7 (0, 06 %)
4921,29 (93,28%)
TOTAL
mGy/MBq
4,05 (10,34%)
0,0007(0,02%)
Tc99m
(pertechnetate)
Sub-total
0,2476 (80,83%)
0,0093 (3,04%)
(*) i= all source organs except the thyroid
0,01674
0,00513
(7,14%)
-
0,0494
(16,13%)
0,2569
(83,87%)
0,3063
The results of the absorbed dose in thyroid of newborn show:
(1)
The absorbed dose to the thyroid of a newborn, due to emissions I123 (iodide) is 39.15
mGy / MBq: 99.98% of the dose corresponds to their self - doses (68.96 % due to
electron capture; 20.68% to the Auger electrons, 5.52% due to the gamma photons, and
4.82% to the characteristic radiation). Dosimetric contribution of the organs that are part
of its biokinetics (excluding the thyroid) is negligible.
(2)
The absorbed dose to the thyroid of a newborn, due to I131 (iodide) is 5 276.0 mGy /
MBq: 99.98% of the dose corresponds to their self - dose (93.28 due to beta emitters ;
3.89% to electron conversion; 0.16% to Auger electrons; 2.6% due to the gamma
photons, and 0.06% to the characteristic radiation). Dosimetric contribution of the organs
that are part of its biokinetics (excluding the thyroid) is negligible
(3)
The absorbed dose to the thyroid of a newborn, due to emissions of Tc99m (pertechnetate)
is 0.3063 mGy / MBq: 92.86% of the dose corresponds to their self - dose (80 83% due to
conversion electrons; 3.04% due to Auger electrons 7.67% due to gamma photons, and
1.32% to the characteristic radiation.
Dosimetric contribution of the organs that are part of its biokinetics (excluding newborn
thyroid), is 7.14%, very significant value to be ignored
The results obtained for the thyroid dose of a newborn, are consistent with those published in
“Radiation Dose Estimates to Adults and Children from Various Radiopharmaceuticals "
(Radiation, 2014)
Depending on the radiopharmaceutical and its biokinetics, shall the significance of their
contributions to thyroid dose. (Vasquez, Castillo, 2015, Vasquez, Quispe, 2015, Quimby,
Feitelberg and Gross, 1970)
4. CONCLUSSION
Using the methodology MIRD, and representation of Cristy-Eckerman for organs the biokinetics
of the newborn, the study shows that for thyroid uptake studies, the dosimetric contributions of
the organs that are part of the biokinetics of the I123 / I131 (iodide), excluding newborn thyroid, are
not significant; while the dosimetric contribution of the organs that are part of the biokinetics of
Tc99m (pertechnetate), excluding newborn thyroid, are very significant in the calculation of the
absorbed dose to be ignored
5. REFERENCES
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internal photon Sources. Methods.”, Oak Ridge, TN: ORNL/TM-8381/V1,(1987b).
HEALTH PHYSICS SOCIETY. Kinetic Models Used as the Basic for the Dose Estimates,
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a
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Retrieved
july
2015
from
bjrs.org.br/revista/index.php/REVISTA/article/view/140/142
PD: This work was exhibited in the X Congreso Regional Latinoamericano IRPA 2015