Different cases exist in the measurement of thyroid radiobioassays owing to the individual characteristics of the subjects, especially the potential variation in the counting efficiency. An
The efficiency calibration of a portable high-purity germanium (HPGe) detector was performed using ISOCS software. In contrast, the conventional efficiency calibration, which needed a radioactive material, was applied to a scintillator-based thyroid monitor. Four radioiodine samples that contained 125I and 131I in both aqueous solution and gel forms were measured to evaluate radioactivity in the thyroid. ANSI/HPS N13.30 performance criteria, which included the relative bias, relative precision, and root-mean-squared error, were applied to evaluate the performance of the measurement system.
The portable HPGe detector could measure both radioiodines with ISOCS but the thyroid monitor could not measure 125I because of the limited energy resolution of the NaI(Tl) scintillator. The 131I results from both detectors agreed to within 5% with the certified results. Moreover, the 125I results from the portable HPGe detector agreed to within 10% with the certified results. All measurement results complied with the ANSI/HPS N13.30 performance criteria.
The results of the intercomparison program indicated the feasibility of applying ISOCS software to direct thyroid radiobioassays. The portable HPGe detector with ISOCS software can provide the convenience of efficiency calibration and higher energy resolution for identifying photopeaks, compared with a conventional thyroid monitor with a NaI(Tl) scintillator. The application of ISOCS software in a radiation emergency can improve the response in terms of internal contamination monitoring.
Radioactive sources are widely used in nuclear power, nuclear medicine, radiopharmaceutical production, and research laboratories. However, their use can lead to health problems in occupational workers and the public via radiation or radionuclides entering the body through various routes such as inhalation, ingestion, skin absorption, and wound absorption [
Internal contamination in the thyroid gland is generally detected using a thyroid monitoring system. Typically, thyroid monitoring involves a type of gamma spectroscopy that measures gamma-emitting radionuclides in the sample [
Recently, semiconductor materials such as high-purity germanium (HPGe) have been used in
The Thyroid Radioiodine Intercomparison program (TRIP) arranged by the LLNL was performed to assess thyroid radiobioassays and validate measurement results in respect of the measured activity of radioiodine samples. This program was established to evaluate the performance of radioiodine measurements and to ensure regulation compliance. The TRIP also provided the International Atomic Energy Agency (IAEA)/American National Standards Institute (ANSI) neck calibration phantom [
The TRIP provided six radioiodine samples in 30 mL vials. As shown in
Two types of detection systems operated in NREMC were used in the current intercomparison program. One was a portable HPGe detector (Falcon 5000 Radionuclide Identifier, Canberra Industries, Meriden, CT) with a broad-energy germanium (BEGe) semiconductor sensor, and the other was a thyroid monitor (802, Canberra Industries, Meriden, CT) with a 2×2 in. NaI(Tl) scintillator. These systems were controlled by using the GENIE-2000 software (Canberra Industries, Meriden, CT). BEGe, a type of the HPGe, provides enhanced efficiency and energy resolution at the low-gamma-energy range.
The measurement geometry is shown in
The energy calibration procedure was performed using a mixed gamma ray reference material in 22 mL vials. The calibrated energies for the portable HPGe detector and thyroid monitor ranged from 59.5 to 1,836.0 keV. The low-energy calibration below 59.5 keV was performed by using a correlation equation of energies with the detector channel.
The conventional efficiency calibration was implemented in the thyroid monitor. By using the thyroid monitor, the 131I sample in the 20 mL vial could be measured, but samples D and E could not be measured owing to the limitation on the measurable energy range of the thyroid monitor. The gel-type sample could not be measured directly because of the geometrical discrepancy with the reference radioactive material, but 131I gel type sample was measured by applying an efficiency correction factor calculated by Monte Carlo simulation. MCNPX software was implemented to obtain the factor for correcting the geometrical discrepancy.
Efficiency calibration of the portable HPGe detector was performed using the ISOCS software. The geometry was composed and the efficiency calibration data were calculated by considering the calibration condition of each sample. The ISOCS was developed by Canberra Industries for efficiency calibration of the detection systems. The calibration was performed by using the Geometry Composer feature and the Monte Carlo radiation transport method. Geometry Composer is a support program in the ISOCS software for imitating various types of measurement samples. Based on the simulation geometry replicated by the geometry composer software, the counting efficiency could be calculated by using the ISOCS software. Thus, ISOCS provides convenient efficiency calibration with the capability to measure radioactive samples that have different shapes, sizes, chemical compositions, material fill-height, and wall materials. Radioactive sources in arbitrary containers can be measured by using the ISOCS modeling technique, without any calibration standard or radioactive reference materials. In conventional calibration, it is necessary to perform efficiency calibration with standardized measurement geometry and radioactive reference materials. Therefore, the ISOCS software provides a cost-effective method for radionuclide verification.
The number of net counts measured was converted to activity in accordance with IAEA TECDOC-1401 [
where A is the activity (Bq) of the sample,
and
where
The comparison of efficiency calibration curves between the thyroid monitor and portable HPGe detector with the ISOCS for the 20 mL aqueous solution of 131I is shown in
The difference between the activities of the 125I samples (A and D) was greater than that corresponding to 131I samples (B and E). When measuring the activity of the 125I sample, it was difficult to calibrate the detection system and measure the sample activity because of its low gamma emission energy and emission rate; 125I emitted 35.4 keV gamma rays with a 6.68% branching ratio.
In general, the efficiency is measured by using the reference radioactive material, which has an energy range between 59.5 (241Am) and 1,836.0 (88Y) keV. However, it is difficult to measure the efficiencies in low energy (below about 45 keV) using a source-based calibration, because of the lack of calibration sources or X-ray radiation emitted from the shield [
More details on the measured values and performance evaluation of the portable HPGe measurement system are given in
The measured activities of the 131I radionuclide in samples B and E were 12,615±972 Bq (relative expanded uncertainty: 7.7%) and 8,488±663 Bq (relative expanded uncertainty: 7.8%), respectively. The relative bias and relative precision of samples B and E were less than half those of samples A and D. The reason for such differences of performance evaluation factors is the gamma emission rate and photo-peak energy of 125I (35.4 keV, 6.68% branching ratio) and 131I (364 keV, 81.2% branching ratio) radionuclides. Samples B and E had RMSE values of 0.028 and 0.039, respectively. As shown in the performance evaluation results, the values measured by the portable HPGe were generally in agreement with the reference values with respect to the relative bias and RMSE. Percentages of relative bias in all samples indicated less than 10% difference. The RMSE values of each sample were lower than 0.25 and thus met the performance criteria of ANSI/HPS 13.30-2011. The sample activities measured by the portable HPGe detector fulfilled the performance criteria recommended by ANSI/HPS 13.30-1996.
The relative bias of the portable HPGe detector was under 4% and 9% in 131I and 125I, respectively. In the case of the thyroid monitor, the relative bias was under 3% in 131I with an efficiency correction by the Monte Carlo method (under 6% without correction). Considering the relative bias of the portable HPGe detector and thyroid monitor, all the measurement results showed good agreement with the internationally certified value (less than 10% of the relative bias). Moreover, all of the measurement data represented acceptable results in accordance with the ANSI/HPS reports. This demonstrates that measurements with the ISOCS software are technically feasible for thyroid radiobioassays.
This study presents the intercomparison results from participation in the TRIP. The data measured by the portable HPGe detector and NaI(Tl) thyroid monitor agreed well with the certified result, but the portable HPGe detector with ISOCS software provided convenient efficiency calibration and the ability to detect radionuclides over a wide range of gamma energies for thyroid monitoring. Based on the evaluation results of the thyroid monitoring, we concluded that the portable HPGe detector with ISOCS software would be applicable for the estimation of radioiodine intake. Although this study only provided results related to thyroid monitoring, further research on applicable measurement techniques with the ISOCS software would be desirable for use in radiation emergency-response situations.
This study was supported by a grant of the Korea Institute of Radiological and Medical Sciences (KIRAMS), funded by Ministry of Science, ICT and Future Planning, Republic of Korea (1711031804/50445-2016). The authors would like to appreciate the Lawrence Livermore National Laboratory (LLNL) for providing the opportunity to participate in the intercomparison program on thyroid radiobioassay.
Radioiodine samples for the intercomparison program (A) Samples provided by the LLNL, (B) Samples modified for measurement.
Schematic of the geometry for measuring radioiodine samples (A) Aqueous solution samples, (B) Gel samples.
Efficiency calibration curves of the thyroid monitor and portable HPGe detector with the ISOCS (131I) for 20 mL aqueous solution samples.
Comparison of the sample activity results obtained with the portable HPGe detector and thyroid monitor.
Activities of Radioiodine Samples and Performance Evaluation Results Obtained with the Portable HPGe Detector
Sample ID | Nuclide | Certified Value | Reported Value | Performance Evaluation | ||||||
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Activity (Bq) | Relative expanded uncertainty (%, k=3) | Averaged activity (Bq, N=5) | Relative expanded uncertainty (%, k=3) | Relative bias | Relative precision | RMSE | Meets ANSI |
Meets ANSI | ||
A | I-125 | 14,000 | 2.0 | 14,683 | 34.9 | 0.049 | 0.037 | 0.061 | yes | yes |
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B | I-131 | 12,300 | 1.4 | 12,615 | 7.7 | 0.026 | 0.012 | 0.028 | yes | yes |
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D | I-125 | 9,330 | 2.0 | 10,171 | 35.2 | 0.090 | 0.037 | 0.097 | yes | yes |
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E | I-131 | 8,200 | 1.4 | 8,488 | 7.8 | 0.035 | 0.018 | 0.039 | yes | yes |
Activities of Radioiodine Samples and Performance Evaluation Results Obtained with the Thyroid Monitor
Sample ID | Nuclide | Certified Value | Reported Value | Performance Evaluation | ||||||
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Activity (Bq) | Relative expanded uncertainty (%, k=3) | Averaged activity (Bq, N=5) | Relative expanded uncertainty (%, k=3) | Relative bias | Relative precision | RMSE | Meets ANSI |
Meets ANSI | ||
B | I-131 | 12,300 | 1.4 | 12,587 | 7.1 | 0.023 | 0.027 | 0.036 | yes | yes |
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E | I-131 | 8,200 | 1.4 | 8,266 | 7.7 | 0.008 | 0.016 | 0.018 | yes | yes |
7,706 |
7.7 | 0.060 | 0.068 | 0.091 | yes | yes |
There were results applied to no efficiency correction depending on the difference of sample geometry.