Reference dataset for benchmarking fetal doses derived from Monte Carlo simulations of CT exams

Lifetime attributable risks (LARs) of cancer in the fetus associated with maternal radiography examinations

PURPOSE

For various reasons, pregnant women are occasionally exposed to ionizing radiation during radiology examinations. In these situations, it is essential to determine the radiation dose to the fetus and any associated risks. The present study attempts to calculate the mean dose for the fetus to estimate the possible cancer induction and cancer mortality risks resulting from maternal radiography exams.

MATERIAL AND METHODS

The GATE Monte Carlo platform and a standard voxelized pregnant phantom were employed to calculate fetal radiation dose during maternal radiography exams. The data published in Biological Effects of Ionizing Radiation VII were used to convert fetal dose to lifetime attributable risks (LARs) of cancer incidence and cancer-related mortality.

RESULTS

The fetal doses and LARs of cancer incidence and cancer-related mortality for the radiographs of the chest and skull were negligible. The maximum LAR values for the lateral view of the abdomen in computed and digital radiography are 5598.29 and 2238.95 per 100,000 individuals, respectively. The computed radiography of the lateral view of the abdomen revealed the highest LAR of cancer-related mortality (2074.30 deaths for every 100,000 people).

CONCLUSION

The radiation dose incurred by the fetus due to chest and skull radiographs was minimal and unlikely to cause any abnormalities in the fetus. The discernible elevation in the lifetime attributable risk associated with cancer incidence and mortality arising from lateral computed radiography examinations of the abdomen warrants careful consideration within the realm of maternal radiography examinations.

Patient-specific fetal radiation dosimetry for pregnant patients undergoing abdominal and pelvic CT imaging

BACKGROUND

Accurate estimation of fetal radiation dose is crucial for risk-benefit analysis of radiological imaging, while the radiation dosimetry studies based on individual pregnant patient are highly desired.

PURPOSE

To use Monte Carlo calculations for estimation of fetal radiation dose from abdominal and pelvic computed tomography (CT) examinations for a population of patients with a range of variations in patients' anatomy, abdominal circumference, gestational age (GA), fetal depth (FD), and fetal development.

METHODS

Forty-four patient-specific pregnant female models were constructed based on CT imaging data of pregnant patients, with gestational ages ranging from 8 to 35 weeks. The simulation of abdominal and pelvic helical CT examinations was performed on three validated commercial scanner systems to calculate organ-level fetal radiation dose.

RESULTS

The absorbed radiation dose to the fetus ranged between 0.97 and 2.24 mGy, with an average of 1.63 ± 0.33 mGy. The CTDI_vol -normalized fetal dose ranged between 0.56 and 1.30, with an average of 0.94 ± 0.25. The normalized fetal organ dose showed significant correlations with gestational age, maternal abdominal circumference (MAC), and fetal depth. The use of ATCM technique increased the fetal radiation dose in some patients.

CONCLUSION

A technique enabling the calculation of organ-level radiation dose to the fetus was developed from models of actual anatomy representing a range of gestational age, maternal size, and fetal position. The developed maternal and fetal models provide a basis for reliable and accurate radiation dose estimation to fetal organs.

Evaluating Size-Specific Dose Estimate (SSDE) as an estimate of organ doses from routine CT exams derived from Monte Carlo simulations

PURPOSE

Size-specific dose estimate (SSDE) is a metric that adjusts CTDI_vol to account for patient size. While not intended to be an estimate of organ dose, AAPM Report 204 notes the difference between the patient organ dose and SSDE is expected to be 10-20%. The purpose of this work was therefore to evaluate SSDE against estimates of organ dose obtained using Monte Carlo (MC) simulation techniques applied to routine exams across a wide range of patient sizes.

MATERIALS AND METHODS

Size-specific dose estimate was evaluated with respect to organ dose based on three routine protocols taken from Siemens scanners: (a) brain parenchyma dose in routine head exams, (b) lung and breast dose in routine chest exams, and (c) liver, kidney, and spleen dose in routine abdomen/pelvis exams. For each exam, voxelized phantom models were created from existing models or derived from clinical patient scans. For routine head exams, 15 patient models were used which consisted of 10 GSF/ICRP voxelized phantom models and five pediatric voxelized patient models created from CT image data. For all exams, the size metric used was water equivalent diameter (D_w ). For the routine chest exams, data from 161 patients were collected with a D_w range of ~16-44 cm. For the routine abdomen/pelvis exams, data from 107 patients were collected with a range of D_w from ~16 to 44 cm. Image data from these patients were segmented to generate voxelized patient models. For routine head exams, fixed tube current (FTC) was used while tube current modulation (TCM) data for body exams were extracted from raw projection data. The voxelized patient models and tube current information were used in detailed MC simulations for organ dose estimation. Organ doses from MC simulation were normalized by CTDI_vol and parameterized as a function of D_w . For each patient scan, the SSDE was obtained using D_w and CTDI_vol values of each scan, according to AAPM Report 220 for body scans and Report 293 for head scans. For each protocol and each patient, normalized organ doses were compared with SSDE. A one-sided tolerance limit covering 95% (P = 0.95) of the population with 95% confidence (α = 0.05) was used to assess the upper tolerance limit (T_U ) between SSDE and normalized organ dose.

RESULTS

For head exams, the T_U between SSDE and brain parenchyma dose was observed to be 12.5%. For routine chest exams, the T_U between SSDE and lung and breast dose was observed to be 35.6% and 68.3%, respectively. For routine abdomen/pelvis exams, the T_U between SSDE and liver, spleen, and kidney dose was observed to be 30.7%, 33.2%, and 33.0%, respectively.

CONCLUSIONS

The T_U of 20% between SSDE and organ dose was found to be insufficient to cover 95% of the sampled population with 95% confidence for all of the organs and protocols investigated, except for brain parenchyma dose. For the routine body exams, excluding the breasts, a wider threshold difference of ~30-36% would be needed. These results are, however, specific to Siemens scanners.

Fetal dose evaluation for body CT examinations of pregnant patients during all stages of pregnancy

PURPOSE

CTDI_vol-to-fetal-dose coefficients from Monte Carlo simulations are useful for fetal dose evaluations, but the available data is limited to the fetus being completely inside the abdominopelvic scan range. Whereas in a chest examination, the fetus is completely outside the scan range. In an abdominal examination, the fetus after 16 gestational weeks is partly in the scan region, and an earlier fetus is completely outside of it. This work proposes a practical approach to evaluate fetal dose for pregnant patients undergoing body CT examinations, without using Monte Carlo simulation.

METHODS

The proposed method was based on the z-axis dose profile computed for a CT examination, considering CTDI_vol, scan range, mA, and maternal WED (water equivalent diameter) at the fetus centroid. Fetal average dose was calculated over the fetus z-axis coverage. For validation, we considered a reference dataset of 24 pregnant patients, each underwent two abdominopelvic examinations (fixed mA, tube current modulation). WED was 30.1 ± 3.3 (25.3-35.6) cm [mean(range)]. Gestational age was <5 weeks for one patient, and 20.3 ± 9.1 (5-35.9) weeks for the others. Fetal depth (from the anterior skin surface to the most anterior part of fetus) was 6.1 ± 2.1 (2.5-10.9) cm. We further considered three whole-body models of a pregnant patient (gestational age, 3, 6, 9 months; weight, 62-73 kg) undergoing chest, abdominal, and abdominopelvic examinations (fixed mA). For the patients and models, profile-based fetal dose calculations were compared with the results of Monte Carlo simulations. Statistical software (R, version 3.5.1) was used to determine the mean and 95th percentile.

RESULTS

The fetal dose difference between profile-based evaluations and Monte Carlo simulations was (5.9 ± 3.8)% for 24 fixed-mA examinations, (5.8 ± 4.6)% for 24 tube current modulated examinations, and (8.8 ± 5.9)% for the whole-body models in three scan ranges.

CONCLUSIONS

Profile-based fetal dose calculations can be performed for patients in body CT, considering maternal size, fetus size and location, and whether fetus is completely inside, partly inside, or outside scan ranges.