Introduction
Thorax computed tomography (CCT) is an important
diagnostic method that is frequently used for the diagnosis
of lung diseases. CCT can be performed for
those who are unaware of their pregnancy or who have
a false positive pregnancy test.[] While the targeted
area in CCT is exposed to primary radiation, the fetus
is mostly exposed to only scattered radiation. The biological
effects of radiation are divided into two groups
as deterministic and stochastic effects.[] Tissue damage, cataracts, alopecia, and infertility are associated
with deterministic effects caused by exposure to certain
radiation doses. On the other hand, teratogenic and
carcinogenic developments are stochastic effects that
can increase in proportion to the radiation dose level.
In diagnostic imaging, fetal radiation exposure is accidental,
but exposure time and radiation intensity determine
the magnitude of the embryo/fetal absorbed dose.
The most harmful biological effects of ionizing radiation
came from long-term studies of atomic bomb survivors
in Hiroshima and Nagasaki. It has been reported that 2800 pregnant women survived radiation doses
above 10 mGy. International Commission on Radiological
Protection (ICRP), in its 84th report, states that if the
fetus is exposed to >10 mGy, the risk of leukemia and
cancer may increase by 40%, however, in the 0-15 age
group, the cancer risk is around 1/1700.[,] Whereas,
therapeutic abortion is considered significant at an exposure
above 100 mGy. On the other hand, the FETDose
V4 code is a computer code dedicated to calculating the
absorbed dose and risk to the fetus from computed tomography
(CT) using the uterine dose at an average fetal
depth normalized to the airborne free entry surface
dose. In addition, anthropomorphic Alderson Rando
phantom Alderson Radiation Therapy (ART) and TLDs
are widely used in the experimental evaluation of embryo/
fetal radiation dose.[,] The female Rando Phantom
is 155 cm tall and weighs 50 kg. It is made of 0.985
g/cm3 density, 32 plates, and 2.5 cm thick tissue-simulating
plastic resembling the female body geometry.
Since radiation damage is dependent on the developmental
period of the fetus, the critical periods in
pregnancy are classified into: pre-implantation or blastogenesis
(0-2 weeks), organogenesis (3-8 weeks), and
embryo/fetal development from the 9th week until birth.
The all-or-none rule is valid when the embryo is exposed
to radiation before implantation and radiationinduced
spontaneous abortion is accordingly expected.
[,] However, the increasing risk of malformation is a
matter of concern when the radiation dose exceeds 100
mGy.[] This study, it was aimed to investigate the embryo/
fetal radiation doses of pregnant women who had
CCT examined from the thorax and to draw attention
to possible technical errors in CCT imaging.
Methods
The study was approved by the Istanbul University-Cerrahpasa,
Cerrahpasa Faculty of Medicine Ethics Committee
(No: 219488, Date: July 09, 2015). In Türkiye, the
Genetic Research and Teratology Counseling Center
(GETAM) at Istanbul University-Cerrahpasa, Cerrahpasa
Medical Faculty is dedicated to providing the necessary
assessment to pregnant patients. Female patients
who underwent CCT and learned to be pregnant applied
to teratological counseling services. A group of 9
pregnant patients with symptomatic coronavirus during
the Covid-19 pandemic were referred to GETAM
because they were accidentally exposed to CCT. The
screening and exposure techniques of CCT including
make, model, peak kilo-voltage, mAs, number of slices, tube rotation (sec), pitch, CTDIvol (mGy), and
dose length product (mGy×cm) were retrospectively
gathered. In particular, CT images of Covid-19 patients
(6/9 patients) revealed an excessive scan length covering
the thorax and abdomen. In addition to the patient
study, a phantom study was performed to measure
embryo/fetal site dose from both thorax and thorax +
abdominal CCT scans using anthropomorphic Alderson
Rando phantom (ART) and TLD-100. Ethics committee
approval numbered 219488 was obtained from
Cerrahpaşa Medical Faculty for this study. The consent
of the participants was obtained using an assigned form.
Patient Population
Twenty-two pregnant patients who had an accidental
CCT performed between March 2015 and December
2022 were included in this study. CCT of 9/22 (41%)
patients were done during the Covid-19 outbreak. The
mean number of pregnancies per patient was 2.36±0.8,
the mean height of the patients was 162.2±4.48 cm, and
the mean weight of the patients was 71.9±15.08 kg.
CT Examination and Calculation of Embryo/Fetal
Doses
Diagnostic CT scans were performed using various CT
devices from different manufacturers (6, 16, 128, and
640 slices). Pregnant women who applied to GETAM
were routinely referred to the medical physics department
for the estimation of embryo/fetal radiation
doses. Detailed information was obtained about the
procedure, CT images, and exposure techniques. Information
about the patients and CT scan parameters
were obtained from a backup site called e-pulse or from
CDs brought with the patients. FetDose V4 computer
software was used to calculate the dose absorbed by the
fetus. Embryo/fetal dose absorbed during CT examination
was calculated with the following formula.
Df = (NUDV × CTDIsoft tissue×[mAs/100])/Pitch
Where Df refers to embryo/fetal absorbed dose,
CTDIsoft tissue (mGy/100 mAs) is the CTDIair to the ICRU
muscle (CTDIsoft tissue=CTDIair×1.07) used as approximations
for the dose to soft tissue and NUDV is the
sum of the normalized uterus doses for all 5 mm slabs
lying within the scan volume.[-]
Phantom Study
The anthropomorphic Alderson Rando Phantom is
broadly used to simulate the human body. A tissuesimulating
plastic with a density of 0.985 cm3 is the basic
composition of a female Rando Phantom with 32
slices. The Phantom is designed to match a female human
body type of 155 cm in height and 55 kg in weight.
The Rando Phantom is equipped with many holes in
the slabs that allow the placement of TLDs. TLD calibration
and post-irradiation reading were done in the
Secondary Standard Dosimetry Laboratory (SSDL).
The SSDL has a Harshaw 4500 model reader that connects
a computer with WinREMS software, which can
read the TLD card and chip. The TLD heating process is
carried out by hot nitrogen gas. The TLD chips are specially
designed with doped lithium fluoride (LiF: Mg,
Ti) crystals by Harshaw. The reader calibration factor
for the TLD and the element correction coefficients of
the TLD chips were determined using the standard Cs-
137 gamma source in the SSDL according to the manual
of WinREMS software. Dose measurements were
made on the transverse planes of the phantoms corresponding
to the embryo/fetal position, and the dose
was measured at 5 locations for each plane (Fig. 1). The
Rando Phantom represents the location of the uterus
and the approximate location of a developing conceptus.
This position was 7.5 cm from the anterior and
14.5 cm from the posterior side of the Phantom. TLDs
were placed in various slots in the embryo/fetal slice.
The imaging protocols were likely set as determined by
the hospital. After exposure, the dose readings were obtained
from the TLD detectors at the Cekmece Nuclear
Research Center. Measurements were made once and
then the average of the measurements was taken.
Fig. 1. Axial pelvic image of the anthropomorphic phantom
showing the inserted TLDs, (red arrows).
TLDs: Thermoluminescent dosimeters.
Results
Patients Study
Pregnant women who underwent CCT scans were sent
to GETAM for teratogenic counseling. Embryo/fetal
radiation doses were calculated in the department of
medical physics. The gestational ages of those who underwent
CCT scans were recorded as 6-21 weeks. Of 22 women, 21 (95%) were in the first trimester and
one patient (5%) was in the second trimester. The frequency
of pregnant women as a function of gestational
age ranges is shown in Figure 2. The mean number of
pregnancies was 2.36±0.8 and ranged from 1 to 3. The
mean height was 162.2±4.48 cm and the mean weight
was 71.9±15.08 kg. Only 6 patients showed thorax area
on CCT, whereas thorax + abdomen area was shown on
CCT of 16 patients. As a result, it was determined that
the mean embryo/fetal dose of all participants varied
between 0.62 and 8.84 mGy (mean 4.28±2.25 mGy).
However, the mean embryo/fetal dose in women with
CCT involving one breast is 1.54±0.71 mGy, ranging
from 0.62 to 2.48 mGy (Fig. 3). In addition, the mean
dose was as high as 5.31±1.67 mGy (range 3.01-8.84
mGy) in those with CCT covering the thorax+abdomen
(Table 1) as summarized and the amount of radiation
dose received by fetal participants is shown 100.
Fig. 2. Number of CCT examinations according to gestational
age of pregnant women.
CCT: Thorax computed tomography.
Fig. 3. Number of embryos/fetus by absorbed doses following
thorax CT examination of pregnant women.
CT: Computed tomography.
Table 1 Pregnant female information and embryo/fetus doses
Phantom Study
On the Rando Phantom (female), 5 TLDs were placed at
various points along the embryo/fetal slices. The imaging
setup and protocols were most likely used as determined
by the hospital, as shown in Figure 4. Phantom CT examinations were performed separately according
to the two protocols: thorax CT only and thorax+abdomen.
Screening parameters and measured embryo/
fetal doses are reported in Table 2. As expected, the
embryo/fetal absorbed doses were 6, 16, 128, and 640
sliced CT in patients with thorax CT only. The mean
embryo/fetal dose was also calculated as 0.173±0.018
mGy. However, thorax + abdominal CT scans showed
higher embryo/fetal doses up to 5.05 mGy, 4.91 mGy,
4.85 mGy, and 5.27 mGy corresponding to the same
slice count, with a mean of 5.05±0.18 mGy.
Fig. 4. Phantom positioning for CT scans.
CT: Computed tomography.
Table 2 Thorax+upper abdomen scan parameters in Alderson Rando Phantom
Discussion
Excess collimation was observed in thorax CT scans
of Covid-19 patients compared to previously scanned
pregnant women. In general, embryo/fetal radiation
doses obtained from thorax CTs have been previously
reported as 1.0 and 1.4 mGy []. In another study, the
average dose absorbed by the fetus was 1.2 mGy in thorax
CT scans, whereas it was 11.1 mGy in abdominal CT scans.[] More importantly, Xie et al.[] pointed
out the importance of CTDIvol and mAs on embryo/
fetal absorbed dose through a Monte Carlo simulation
(MCS) study. Accordingly, the mean embryo/fetal dose
per mAs was found to be 22.60 mGy/100 mAs, and the
calculated embryo/fetal dose ranged between 0.013 and
0.026 mGy in the first trimester.[] Globally, the ICRP
has published average embryo/fetal doses for different
radiological studies. The approximate mean embryo/fetal
dose obtained from thorax CT has been reported as
0.06 mGy for single detector sequential helix CT.[] On
CT scanners with automatic exposure control, mean
mA is used to calculate CTDIvol for embryo/fetal dose
determination. The CTDIvol values reported at the end
of the CT examinations of the patients were 40% higher
than the values displayed in the study panel.[]
When the fetus is exposed to a dose of 1.04-3.65
mGy, the risk of childhood cancer is 1 in 12500 and 1
in 3300, the hereditary risk is 1 in 200000 and 1 in 5000
based on long-term data. In addition, after CCT with an
embryo/fetal dose of 0.02 mGy, the fetus is expected to
have a childhood cancer risk of 1 in 500,000 and a hereditary risk of 1 in 1000,000, and if the dose of the fetus
is increased 10 times, the risk increases 1000 times.[]
On the other hand, the American College of Radiology
announced that the accreditation reference CTDIvol
value in abdominal CT was 25 mGy and increased to 35
mGy for a 70 kg patient. It has also been reported that
the upper embryo/fetal dose limit is 100 mGy, which
cannot be exceeded in most CT examinations. However,
it is stated that it may be possible for pregnant patients
to use a direct measurement technique (or equivalent)
to estimate embryo/fetus doses. As mentioned above,
embryo/fetal absorbed doses from thorax CT scans range from 0.02 mGy to 2.0 mGy. The current Phantom
study showed that the mean embryo/fetal dose was
0.173 ±0.018 mGy. However, the Phantom doses appear
to be considerably lower than the calculated doses
(1.54 ±0.71 mGy) for Covid-19 patients with thorax CT
only. The mean fetal dose was calculated as 5.31 ±1.67
mGy, and the mean embryo/fetal dose of the phantom
was 5.05 ±0.18 mGy in patients who underwent thorax
+ abdominal CT. Using the same calculation software,
Ozbayrak et al.[] the mean embryo/fetal dose was 1.53
mGy (range: 0.4-2.3 mGy), which is 3.47 times less than
the embryo/fetal doses calculated for patients in the current
study. MCS study showed that normalized fetal dose
estimates ranged from 7.3 to 14.3 mGy/100 mAs, with a
mean of 10.8 mGy/100 mAs.[] In contrast, there was
one study that reported the mean percent difference
between TLD measurements and MCS as -4.9% and
a standard deviation of 8.7%. MCS using digital voxelized
phantoms instead of stylized phantoms is the most
accurate dose estimation method, but the computation
time and complexity of MCS slow down to produce
patient-specific estimates.[] The risk of childhood
cancer due to embryo/fetal radiation exposure from
thorax CT is 1 in 200,000 and the risk of disease is 1 in
500,000.[] In addition, 1 in 33 healthy women (3.3%)
had birth defects without specific or family history, and
1 in 7 women (15%) had a potential risk of miscarriage
following CCT.[] It is clear that embryo/fetal dose
values are quite low to increase the risks of deterministic
effects in thorax CT scans. However, the occurrence
of the risk of many stochastic effects increases with increasing
doses absorbed. Overall, the risk of infection
was extremely high during the ongoing coronavirus era.
Therefore, the study team in the radiology department
faced a lot of stress and rapid performance overhead by
wearing protective coveralls, which likely caused nonconformal
collimation and abdominal involvement on
CCT scans and consequently higher embryo/fetal doses.
Conclusion
Embryo/fetus absorbed dose of pregnant women increased
significantly from thorax CT scans when the
abdomen was frequently included. This increase in embryo/
fetal dose is still small to cause a determinant effect,
but thorax CT scans should be limited to the lung
region to reduce dose and related stochastic effects.
Peer-review: Externally peer-reviewed.
Conflict of Interest: All authors declared no conflict of interest.
Ethics Committee Approval: The study was approved by
the Istanbul University-Cerrahpasa, Cerrahpasa Faculty of
Medicine Ethics Committee (no: 219488, date: 09/07/2015).
Financial Support: None declared.
Authorship contributions: Concept - M.D.; Design
- N.İ.I.; Supervision - M.D.; Funding - N.İ.I.; Materials -
N.İ.I.; Data collection and/or processing - M.D.; Data analysis
and/or interpretation - N.İ.I.; Literature search - N.İ.I.;
Writing - M.D., N.İ.I.; Critical review - N.İ.I.
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