Fetal Dose From Emergency CT Examination: Fetal
Simulation and Dose Measurements
Christine
Lo, Martin Law, Francis Cho, Dominic Pang, Patricia Cheng, Lawrance
Yip, Wendy Lam
Christine Lo, Martin Law, Francis Cho, Dominic Pang,
Patricia Cheng, Lawrance Yip, Wendy Lam, Department of Radiology, Queen Mary
Hospital, Hong Kong, 102 Pokfulam Road, Hong Kong
Correspondence to: Martin Law, Department of Radiology, Queen Mary
Hospital, Hong Kong, 102 Pokfulam Road, Hong Kong
Email: lawwm@ha.org.hk
Telephone: +852-2255 4215
Fax: +852-
2255-3872
Received: November 3,
2014
Revised: December 16, 2014
Accepted: December 19, 2014
Published online: December 31, 2014
ABSTRACT
AIM: To experimentally simulate a late
stage fetus, to measure fetal irradiation dose and to estimate associated
childhood cancer risk resulting from an emergency thorax-abdominopelvic CT
imaging for a pregnant patient with suspected aortic dissection.
methods: To simulate the
fetal body, a physics CTDI cylindrical phantom was placed onto the abdomen of a
standard female humanoid phantom. Layers of beeswax were used to make the
phantom maternal dimension similar to that of the pregnant patient. A 10 cm
pencil chamber was used to measure the dose at each quadrant location and at
the center of the CTDI phantom and the fetal dose was obtained by averaging
these measured doses. Stochastic effect of childhood cancer risk induction
(0.4% per 10 mGy irradiation) was estimated to provide a reference value to
monitor the growth of the newborn child.
Results: The fetal dose was
measured as 20 mGy for a single acquisition according to the patient scanning
protocol. Since the patient had both pre- and post-contrast CT acquisition, the
total fetal dose was therefore 40 mGy. The lifetime risk of developing
childhood cancer was estimated as 1.6%.
Conclusion: By using readily
available equipment in a radiology department, fetal simulation and then dose
measurements can be efficiently performed with the moulding technique. Results
obtained are useful as reference value to monitor the growth of the newborn.
© 2014 ACT. All
rights reserved.
Key words: Emergency CT for pregnant patients; Fetal simulation and
dose measurements; Childhood cancer induction
Lo C, Law M, Cho
F, Pang D, Cheng P, Yip L, Lam W. Fetal Dose From Emergency CT
Examination: Fetal Simulation and Dose Measurements. International
Journal of Radiology 2014; 1(1): 4-6 Available from: URL:
http://www.ghrnet.org/index.php/ijr/article/view/909
INTRODUCTION
Pregnant patients may experience non-obstetrical emergencies over the
course of pregnancy, and CT remains essential in the emergent setting for
pregnant population[1]. For pregnant women undergoing abdominopelvic
CT, the concern of consequent cancer risk is particularly relevant because the
fetus, with a greater sensitivity to radiation than the mother, is also
directly exposed to radiation[1-3].
Most published
estimates of fetal dose from radiological examinations have relied on Monte
Carlo computation[3] or on measurements with thermoluminescent
dosimeters for specific clinical application[4] that requires
special facility and expertise[5]. We report a simple fetal
simulation technique to perform fetal dose measurements for a clinical case of
a 38 year old previously healthy pregnant woman who was referred to our
institution at 36 weeks gestation for acute chest pain. Investigations were
immediately performed to ascertain the cause of the patient’s pain. Troponin I
was increased to 1.12. ECG showed T-wave inversion in leads III and avF. Urgent
bedside echocardiogram showed suspected aortic dissection. CT imaging was
requested to confirm the echocardiogram findings and to delineate the extent of
dissection. Referring clinicians would then decide their therapeutic decision,
whether to treat with medical or surgical means, based on the level of
dissection from the CT findings. General knowledge of risks and benefits due to
the radiation dose delivered to the fetus was informed to the patient. Fetal
dose from a single acquisition abdominopelvic CT study has been reported to
have an average of 25 mGy[1]. An urgent CT scan of
thorax-abdominopelvis was then performed and a Type A aortic dissection
extending from the dilated aortic root down to the bifurcation was verified.
The patient underwent emergency Caesarean section, Bentall operation and
ascending aortic interposition graft repair. The child was delivered successfully
with normal Apgar scores and the mother
was recovered uneventfully. The child has been growing well with normal
parameters. Retrospective fetal irradiation dose was measured with the use of
phantom and estimated associated risks were estimated as information to the
mother and as reference to monitor the growth of the newborn child.
Method
A standard reference female humanoid of weight 70 kg (The Phantom Laboratory, New York, USA)
was used as the mother in this retrospective measurement study. To simulate a
fetus of gestational age of 36 weeks, we used a cylindrical phantom (a CTDI
head phantom commonly used for CT quality control work) of weight about 3 kg to
mimic the fetal body. The CTDI cylinder was then embedded onto the humanoid
abdominal region to closely match with the patient’s maternal parameters.
Layers of tissue-equivalent beeswax (available from most radiation oncology
department moulding laboratory) were moulded around the humanoid abdominal
region to simulate the maternal dimensions (Figure 1). The finished dimensions
were: maternal curvature 42 cm, maternal anteroposterior thickness 34 cm, total
maternal weight 79 kg, all of which were comparable to reported late stage
gestation dimensions[6].
The simulated fetal body had a series of holes of length 10 cm with each
hole located at each quadrant position and at the center of the CTDI phantom. A
calibrated 10 cm pencil chamber was then inserted into each hole for dose
measurement using the patient irradiation CT protocol. By averaging the doses
measured at these five positions, the average irradiated dose to the simulated
fetal body was obtained.
Figure 2 shows
the patient CT scout image (Toshiba Aquilion 320 rows CT, Otawara, Japan). CT
parameters were 120 kVp, automatic mA with manually adjusted tube current
within the fetal region[7]. The same CT beam parameters were used to
irradiate the simulated pregnant humanoid. For exposure to a newborn, the
lifetime attributable risk of childhood cancer induction was estimated with
0.4% per 10 mGy irradiation to the fetus[8].
Results
The pregnant patient had undergone 2 identical thorax-abdominopelvis CT
sets, namely pre-contrast CT scan for intramural hematoma and post-contrast CT
for dissection. Figure 3 shows the image of the post-contrast CT examination,
demonstrating the extensive dissection flap from dilated aortic root down to
the bifurcation and the direct CT exposure to the fetus. The average dose
delivered to the fetus was measured as 20 mGy for a single
thorax-abdominopelvic CT acquisition. Therefore, the total irradiation CT dose
to the fetus was 40 mGy because
pre- and post-contrast scans
were used.
The lifetime
attributable risk of childhood cancer induction was estimated as 1.6% due to
the CT irradiation.
Discussion
Osei and Darko recently
presented a multi-national study in that average fetal absorbed doses for
pelvis and abdomen under CT irradiation were 10.6 mGy (range: 1.3-17 mGy) and 2
mGy (range: 1.0-3.7 mGy) respectively without details in gestational age[9].
Goldberg-Stein et al retrospectively analyzed data from a 7-years
database and identified 86 pregnant patients from 180,000 abdominopelvic
examinations[2]. Based on each identified pregnant patient CT
parameters, the authors used Monte Carlo technique applied to a mathematical
phantom which was not specifically designed for pregnant woman in order to
calculate the fetal absorbed dose. Nevertheless, it was reported an average
fetal dose of 24.8 mGy (range: 6.7-56 mGy) for those identified pregnant
patients who had undergone a single acquisition abdominopelvis CT
examination. Lazarus et al
also reported a mean fetal dose of 17 mGy (range: 8-44 mGy) from a review of 10
years database[10]. Our result of 20 mGy per thorax-abdominopelvis
CT acquisition was in good accord with multi-national survey and with large
database reviews for abdominopelvic CT scan. It should be noted that our dose
measurement referred to
thorax-abodminopelvic region that would result in a slight increase in fetal absorbed
dose when compared with that from an abdominopelvic scan due to increase in
internal scatter dose from the thorax region irradiation.
The fetal
irradiated dose of 40 mGy, as resulted from the emergency CT to diagnose the
patient’s aortic dissection, was measured to be well below the safety limit of
100 mGy. Lethal effects will be very infrequent for doses under 100 mGy and
birth malformation fetal exposure well below 100 mGy is not expected[11].
Therefore, we would not expect any birth defects to the newborn. Stochastic
effect of childhood cancer risk induction (0.4% per 10 mGy irradiation) was the
main concern[8]. Exposure of 40 mGy increases the lifetime risk of
developing cancer to the newborn child by 1.6%. In other words, there is a
better than 98% likelihood that the child will be unaffected by the
irradiation. The cancer risk is low of the order <2% for emergency CT scan
through thorax-abdominopelvis region, indicating that such high dose
examination may be performed on pregnant patients who have medically necessary
indication. The radiologist performing the emergency CT must be aware of any
prior radiological examinations in the abdominopelvis region to the pregnant
patient in order to have an updated risk and benefit consideration.
CONCLUSION
By using readily
available equipment in a radiology department, fetal simulation and then dose measurements can
be efficiently performed with moulding technique. Such simulation and
measurement are efficient in simulating various clinical applications. In the
future, fetal dose can be prepared as a look-up table according to gestational
age and for common maternal dimensions as a preparedness to cope with the
increasing trend of emergency presentations in the pregnant patient population[10].
One also has to consider if the mother has had any high dose radiological
examinations prior to the emergency CT to provide an update in risk and benefit
analysis as information to the pregnant patient.
ACKNOWLEDGMENTS
The authors would like to thank the Department of Clinical Oncology of
Queen Mary Hospital, Hong Kong, for the use of the humanoid phantom and
laboratory facilities.
CONFLICT OF INTERESTS
All the authors of the manuscript entitled ‘Fetal dose from emergency CT
examination: fetal simulation and dose measurements’ declare that there is no
conflict of interest with regard to equipment, contrast, drug and other
materials described in the study.
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Peer reviewers: Bing Wang, MD, PhD, Team Leader, Senior Researcher, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan; Xin Liu, PhD, School of
Medical Instrument & Food Engineering, University of Shanghai for Science
and Technology, 516 Jun Gong Road, Shanghai, 200093, China.
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