RADIATION EXPOSURE OF THE INVESTIGATOR DURING NAVIGATED FUSION OF 124IODINE PET IMAGING AND ULTRASOUND

RADIATION EXPOSURE OF THE INVESTIGATOR DURING NAVIGATED FUSION OF 124IODINE PET IMAGING AND... Abstract To assess the radiation exposure of the investigator during navigated fusion of nuclear medicine images with ultrasound after application of I-124. Dosimetry with two different types of thermoluminescent detectors (TLD) was performed in 25 patients. The dose rate at the patient’s neck was measured with a calibrated dose rate meter (DRM) and served as the standard of reference. The average exposure per investigation at the patient’s neck measured by LiF:Mg,Cu,P TLDs (cumulative: 212 μSv), LiF:Mg,Ti TLDs (cumulative: 112 μSv) and DRM (cumulative: 150.3 μSv). The radiation exposure of the hand during navigated fusion of nuclear medicine imaging with 124I and ultrasound with a mean duration of 13 min is low and comparable between different methods. Yearly examinations are not expected to add a relevant cumulative risk. INTRODUCTION Ultrasound (US) imaging is an established part of a diagnostic workup in nuclear medicine. Due to technical innovations, it is now not only used as a standalone tool, but also for hybrid imaging in a variety of settings(1–10). Casuistic experience in thyroid diagnostic is reported for 124I positron emission tomography (PET)/US, which allows the unambiguous correlation of sonographic and metabolic findings(8, 11). It is conceivable that the staff is exposed to radiation by these examinations and therefore, this aspect has to be considered and evaluated before undergoing clinical studies and introduction in clinical routine. The positron-emitting tracer 124I is not only used for staging and dosimetry in thyroid cancer(12, 13) but also for thyroid uptake measurements and characterization of thyroidal nodules(11, 14, 15). Because of the higher gamma energy of 124I, the radiation exposure due to these examinations may be significantly higher compared with 99mTc(16). This prospective study was undertaken to determine the radiation exposure of an investigator, particularly of his hands, during a navigated 124I nuclear imaging/US fusion examination, and to estimate the potential cumulative exposure per year. MATERIALS AND METHODS The study was performed as part of a larger project evaluating different aspects of US fusion. Written informed consent was obtained from each patient after the nature of the procedure had been fully explained. In total, 25 patients were included in the study (Table 1). The mean 124I activity was 1.07 ± 0.41 MBq (median: 0.99 MBq, range: 0.92–3.01 MBq). The PET/computed tomography (CT) examination using a standard PET/CT scanner (Siemens Biograph mCT40, Erlangen, Germany) was started 27.81 ± 2.38 h (median 28.5 h, range: 21.72–33.1 h) after oral administration and one bed position comprising the neck was scanned for 10 min. Table 1. Demographic and technical data. Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Table 1. Demographic and technical data. Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 The US examination of the thyroid gland was performed after 28.41 ± 1.97 h (median 29.08 h, range: 22.88–30.57 h) and consisted of two steps: - B-mode US - 124I PET/US fusion Using an electromagnetically tracked US probe (VNav, Logiq E9, General Electric, Milwaukee, WI, USA) the images were co-registered in real time, producing spatially correlated images of preacquired 124I PET and US. These images were displayed in a transparent overlay mode or side-by-side on the screen of the US machine in real time, i.e. if the US transducer is moved, the 124I PET dataset tracks in real time. This 124I-PET/US fusion examination allows for the exact assessment of a thyroid nodules function even in the case of multiple adjacent nodules. Three different kinds of detectors were used: Standard ringlet thermoluminescent detectors (TLDs) (beta/photon extremity dosemeter DXT-RAD 707 H, Thermo Fisher Scientific, Waltham, MA, USA) consisting of a Lithium Fluoride, LiF (7Li isotope) substrate doped with Magnesium, Mg, Copper, Cu and Phosphorus, P (LiF:Mg,Cu,P; abbreviated MCP; grammage 7 mg/cm2) (Figure 1, green arrow). The MCP-TLD was located under a polycarbonate cap (thickness: 27 μm; grammage: 3.3 mg/cm2). MCP is characterized by a decision threshold of 33 μSv and a detection limit 50 μSv. Chipstrate TLDs (photon single chipstrate extremity dosemeter EXT-RAD XD 700, Thermo Fisher Scientific) also consist of Lithium Fluoride, LiF (7Li isotope), which is doped with Magnesium, Mg and Titanium, Ti (LiF:Mg,Ti; abbreviated MT; grammage 100 mg/cm2). This MT-TLD was packed in a plastic pouch (thickness: 48 μm; grammage; 7 mg/cm2) (Figure 1, orange arrow). MT is characterized by a decision threshold of 20μSv and a detection limit of 30μSv. A calibrated Geiger counter dose rate meter (DRM) with gamma probe (Automess 6150AD6/E und 6150AD-18/E, Automation und Messtechnik GmbH, Ladenburg, Germany) was used concurrently (Figure 2C). The DRM had a range of measurement of 0.5–9.99 mSv/h and an energy range of 65 keV to 1.3 MeV. It was calibrated to Cs-137 (662 keV) and no correction factor was applied to the I-124 energy (603 keV). Dose rate measurement results at the skin of the patients’ neck and at the investigator’s chest were expressed as ambient dose equivalent H*(10). Figure 1. View largeDownload slide TLD measurement setup. Dosimetry setup with different types of thermoluminescent detectors (TLD) applied during PET/US fusion imaging procedure. The TLDs comprise MCP (green arrows), MT (orange arrows) and the personal dosemeter (red arrows) for the whole measurement arrangement. TLDs at the volume navigation tracking bracket attached to the ultrasound probe with translucent spacers (A, blue arrow); background TLDs (B); dosemeters placed on the investigator’s chest (C); TLDs positioned on the investigator’s hand (D). Figure 1. View largeDownload slide TLD measurement setup. Dosimetry setup with different types of thermoluminescent detectors (TLD) applied during PET/US fusion imaging procedure. The TLDs comprise MCP (green arrows), MT (orange arrows) and the personal dosemeter (red arrows) for the whole measurement arrangement. TLDs at the volume navigation tracking bracket attached to the ultrasound probe with translucent spacers (A, blue arrow); background TLDs (B); dosemeters placed on the investigator’s chest (C); TLDs positioned on the investigator’s hand (D). Figure 2. View largeDownload slide Fusion imaging procedure setup. Setup during the magnetic sensor navigated PET/US fusion imaging procedure. Two magnetic sensors (A, B, yellow arrow) are connected to the ultrasound transducer. The position of the magnetic sensors and therefore of the ultrasound transducer is recognized within a magnetic field generated by a transmitter (B, purple arrow). The PET/US images are shown in a semi-transparent fashion on the screen of the ultrasound device in real time (A). The relation of investigator, patient, and TLDs during the examination is nearly constant (B). Dose rate measurements at patient’s neck was performed at the beginning of the procedure (C). Figure 2. View largeDownload slide Fusion imaging procedure setup. Setup during the magnetic sensor navigated PET/US fusion imaging procedure. Two magnetic sensors (A, B, yellow arrow) are connected to the ultrasound transducer. The position of the magnetic sensors and therefore of the ultrasound transducer is recognized within a magnetic field generated by a transmitter (B, purple arrow). The PET/US images are shown in a semi-transparent fashion on the screen of the ultrasound device in real time (A). The relation of investigator, patient, and TLDs during the examination is nearly constant (B). Dose rate measurements at patient’s neck was performed at the beginning of the procedure (C). Due to their wide energy detection windows, both TLD types are suitable for 124I detection (MCP dosemeters: 12 keV–1.25 MeV, MT dosemeters: 15 keV–1.4 MeV). The measured radiation exposure was expressed as superficial person dose Hp(0.07). Background radiation was measured using two additional MCP and MT-TLDs which were stored in the examination room during PET/US procedure and stored in the same lead-shielded container when not in use. The readout process was performed off-site as per German regulations using a standard glow curve readout. To standardize the positions of the TLDs attached to the US probe and to reduce interobserver variability, custom made acrylic sockets were mounted to the Volume Navigation Tracking Bracket (ML 6–15, Civco medical solutions, Kalona, Iowa, USA) (Figure 1A—blue arrow, translucent spacers). Another ringlet MCP TLD was positioned on the ring finger facing the palm to obtain information about the dose at a position ringlet dosemeters are usually worn and to ensure comparability with dose equivalent limits. MT TLDs were not used on a finger as they were too bold to allow for a nonrestrictive movement of the investigator’s hand. Furthermore, one MCP and MT TLD each were positioned at the investigator’s chest. RESULTS In all 25 patients, US examinations consisting of the two steps (B-mode US, 124I PET/US fusion) were performed without technical difficulties or equipment failures. The mean examination time was 13 ± 5 min (median: 11 min; range: 6–25 min); the cumulative examination time was 324 min. Durations of respective examination steps are shown in Table 1. The evaluation of the MCP dosemeters resulted, after consideration of the background of 393 μSv, in a cumulative patient’s neck dose of 212 μSv, investigator’s hand dose of 157 μSv and investigator’s chest dose of 20 μSv. After dividing the cumulative dose by the number of examinations (n = 25), average patient’s neck, investigator’s hand and investigator’s chest doses per procedure were 8.5, 6.3 and 0.8 μSv, respectively (Table 2). Table 2. Dosimetric results. Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Table 2. Dosimetric results. Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 The evaluation of the MT dosemeters resulted, after consideration of the background of 224 μSv, in a cumulative patient’s neck dose of 112 μSv and investigator’s chest dose of 10 μSv. Average patient’s neck and investigator’s chest doses per procedure were 4.5 and 0.4 μSv, respectively (Table 2). The evaluation by DRM showed a calculated cumulative patient’s neck dose of 150.3 μSv and investigator’s chest dose of 2.7 μSv. A mean patient’s neck dose rate of 27.7 ± 8.5 μSv/h and investigator’s chest dose rate of 0.5 ± 0.02 μSv/h was measured. The calculated mean dose per procedure was 6.0 ± 3.3 μSv at the patient’s neck and 0.1 ± 0.04 μSv at the investigator’s chest (Table 2). DISCUSSION Current guidelines propose radiation exposure limits for nuclear medicine staff: An effective dose to the whole body of 20 mSv per annum (for 5 subsequent years, 100 mSv), and a dose equivalent to the hands and extremities of 500 mSv per annum(17, 18). In nuclear medicine departments, the hands of doctors, technologists, pharmacists and nurses are regularly exposed to radiation due to frequent handling of radiopharmaceuticals and contact to patients. Fortunately, the radiation sensitivity of the hands is relatively low(19). During US examinations of the thyroid gland, the hand of the investigator is close the neck of the patient for a prolonged time (Figure 2B). If these examinations are performed frequently in patients who previously received radiating iodine isotopes, the question arises if the cumulative radiation exposure may be harmful. With the positron emitter 124I needed for PET/US fusion imaging, the 511 keV annihilation photons and further 603 keV photons have to be considered(20). Compared with fhSPECT/US examinations with Tc-99m tracers, a higher radiation exposure is possible, despite a low administered activity(16). Examinations of the thyroid gland are common in nuclear medicine departments. An US examination with B-mode and duplex visualization takes up to 5 min. Despite different examination protocols, the average examination time of 13 ± 5 min in this study is lower than the examination time of fhSPECT/US (19 ± 7 min)(16). The superior information about thyroid metabolism of 124I PET compared to Tc-99m SPECT led to a more elaborate and more detailed US fusion examination. Because of detection thresholds of MCP and MT dosemeters, a cumulative evaluation of the study duration of a total of 6 months was necessary. Unfortunately, measurement of a patient cohort over a longer period of time with the same TLD has disadvantages: - an evaluation per examination is not possible; - a time-related fading of the measurements may occur; and - the dosimetry results of TL-crystals are obtained using slowly increasing temperatures, producing a so-called glow curve. Therefore, this represents a rather long time for readout and does not allow multiple analysis of the same results. Still, TLDs are more feasible for the measurement of a cumulative radiation dose and subsequent calculation of an average dose than performing the measurements with a DRM, which has also been used in this study. TLDs can be selected based on the intended usage and calibrated to superficial person dose Hp(0.07) or deep person dose Hp(10). TLDs are small, do not impede an examination, and are easy to place at several areas of interest. The DRM can only measure the ambient dose equivalent H*(10), and handling during an examination may be difficult. In this study, MCP and MT TLDs were used. Both TLDs are different considering detection limit and geometry, mainly attributable to the different grammage. Although MT-MCPs are characterized by a lower detection limit, the dose values measured with MT TLDs (cumulative 112 μSv; 4.5 μSv per examination) are approximately half of that measured with MCP TLDs (cumulative 212 μSv; 8.5 μSv per examination). The probable cause is the relation between the radiation characteristics of 124I (β+1 = 1535 keV, β+2 = 2138 keV, γ1 = 603 keV, γ2 = 1691 keV, γ3 = 723 keV) and the different grammages and thicknesses of the cover material(20). The grammage and the thickness of the MT plastic pouch is approximately twice the grammage and thickness of the window of the MCP disc-like polycarbonate lens, as described above. It is conceivable that not only gamma rays are emitted from the patients’ skin but also high-energy beta rays due to slim soft tissue between the thyroid and the TLDs. Radiation with a low penetrating power, including alpha rays, beta rays with an energy below 2 MeV, and gamma rays with an energy of <15 keV may cause a relevant dose equivalent to the investigator’s skin. Beta rays emitted from 124I might be absorbed by the thicker plastic pouch encasing the MT-TLD, therefore, leading to lower dose values. Dose rate measurements were performed under the premise that dose rates remained roughly constant during the course of the examinations. 124I has a radioactive half-life of 4.2 days, which allows omitting the decay through the short examination time of ~13 min. By multiplication of dose rate and examination time, the result of 6.0 ± 3.3 μSv is nearly equal to the average of the TLD measurements (patient’s neck) (MCP: 8.5 μSv; MT: 4.5 μSv). Based on these findings, DRM measurements can be seen as a convenient and straightforward way to estimate radiation exposure at specific locations. As the MCP TLD at the investigator’s hand was positioned at a greater distance from the patient’s neck as compared to the MCP TLD at the US probe, the calculated mean dose of 6.3 μSv was lower than the calculated mean dose of 8.5 μSv at the US probe (Figure 2B and Table 2). Our study is limited by the measurement setup. The long interval of TLD of 6 months usage may have caused an imprecision of the radiation exposure measurements. During the study, TLDs were placed at the US probe, closer to the thyroid gland and therefore receiving more radiation, and not at the investigator’s hand. This setup is not completely representative for the hand(21), but ensures a reproducible TLD positioning and eliminates observer-dependent variability. For a detailed evaluation, a higher number of patients has to be investigated, with dose measurements at several areas at the investigator’s body, including fingers, hands, chest and eyes. Evaluation of different types of dosemeters would help to determine the most feasible and exact option for clinical practice, like optically stimulated luminescence dosemeters based on beryllium oxide (BeO), which do not show fading over time and can be readout several times(22). CONCLUSION This study evaluates and quantifies the potential radiation exposure of an investigator, who performs US examinations in patients who previously received a radioactive tracer. The positron-emitting isotope 124I accumulates in the thyroid gland and facilitates PET/US fusion imaging. The results showed that radiation exposure of the hand of the investigator holding the probe to the neck of the patient (measured in the range from 4.5 to 8.5 μSv, depending on the type of dosemeter) and also exposure of the investigator’s body is very low. In clinical routine, a lower radiation exposure can be expected: Because of the extensive examination protocol with a mean duration of 13 min and positioning of the detection devices very close to the patient, our study setup presumably overestimated this exposure. Even with a high number of examinations, it is nearly impossible to exceed or even to reach regulatory radiation limits. Despite this, if PET/US examinations are performed frequently, basic radiation protection principles should be always observed to avoid unnecessary risk by radiation exposure. The examinations need to be justified and actions have to be taken to optimize the work flow and reduce radiation exposure. In staff surveillance, the differing detecting capabilities of different types of radiation detection devices (dosemeters, DRM) have to be taken into account. ACKNOWLEDGEMENTS GE Healthcare, Milwaukee, WI, USA is gratefully acknowledged for providing hardware (LOGIQ E9) for this study. FUNDING This study was funded exclusively with intramural grants from the Jena University Hospital. REFERENCES 1 Freesmeyer , M. , Opfermann , T. and Winkens , T. Hybrid integration of real-time US and freehand SPECT: proof of concept in patients with thyroid diseases . Radiology 271 ( 3 ), 856 – 861 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 2 Otte , A. and Hoppe , H. Hybrid SPECT/US . 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Radiation exposure of hands in radiopharmacies: monitoring of doses and optimisation of protection. Radiation Dose to Patients From Radiopharmaceuticals Addendum 3 to ICRP Publication 53, ICRP Publication 106 Ann ICRP 38(1–2): Elsevier; 2008 . 22 Sommer , M. and Henniger , J. Investigation of a BeO-based optically stimulated luminescence dosemeter . Radiat. Prot. Dosim. 119 ( 1–4 ), 394 – 397 ( 2006 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

RADIATION EXPOSURE OF THE INVESTIGATOR DURING NAVIGATED FUSION OF 124IODINE PET IMAGING AND ULTRASOUND

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Abstract

Abstract To assess the radiation exposure of the investigator during navigated fusion of nuclear medicine images with ultrasound after application of I-124. Dosimetry with two different types of thermoluminescent detectors (TLD) was performed in 25 patients. The dose rate at the patient’s neck was measured with a calibrated dose rate meter (DRM) and served as the standard of reference. The average exposure per investigation at the patient’s neck measured by LiF:Mg,Cu,P TLDs (cumulative: 212 μSv), LiF:Mg,Ti TLDs (cumulative: 112 μSv) and DRM (cumulative: 150.3 μSv). The radiation exposure of the hand during navigated fusion of nuclear medicine imaging with 124I and ultrasound with a mean duration of 13 min is low and comparable between different methods. Yearly examinations are not expected to add a relevant cumulative risk. INTRODUCTION Ultrasound (US) imaging is an established part of a diagnostic workup in nuclear medicine. Due to technical innovations, it is now not only used as a standalone tool, but also for hybrid imaging in a variety of settings(1–10). Casuistic experience in thyroid diagnostic is reported for 124I positron emission tomography (PET)/US, which allows the unambiguous correlation of sonographic and metabolic findings(8, 11). It is conceivable that the staff is exposed to radiation by these examinations and therefore, this aspect has to be considered and evaluated before undergoing clinical studies and introduction in clinical routine. The positron-emitting tracer 124I is not only used for staging and dosimetry in thyroid cancer(12, 13) but also for thyroid uptake measurements and characterization of thyroidal nodules(11, 14, 15). Because of the higher gamma energy of 124I, the radiation exposure due to these examinations may be significantly higher compared with 99mTc(16). This prospective study was undertaken to determine the radiation exposure of an investigator, particularly of his hands, during a navigated 124I nuclear imaging/US fusion examination, and to estimate the potential cumulative exposure per year. MATERIALS AND METHODS The study was performed as part of a larger project evaluating different aspects of US fusion. Written informed consent was obtained from each patient after the nature of the procedure had been fully explained. In total, 25 patients were included in the study (Table 1). The mean 124I activity was 1.07 ± 0.41 MBq (median: 0.99 MBq, range: 0.92–3.01 MBq). The PET/computed tomography (CT) examination using a standard PET/CT scanner (Siemens Biograph mCT40, Erlangen, Germany) was started 27.81 ± 2.38 h (median 28.5 h, range: 21.72–33.1 h) after oral administration and one bed position comprising the neck was scanned for 10 min. Table 1. Demographic and technical data. Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Table 1. Demographic and technical data. Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 Characteristic Patient group Age, years  Median (range) 60 (29–84)  Mean ± SD 59.1 ± 11 Patients, n (%)  All 25  Female 22 (88%)  Male 3 (12%) Total examination time, min  Median (range) 11 (6–25)  Mean ± SD 13 ± 5  Cumulative 324 B-scan duration, min  Median (range) 2 (1–12)  Mean ± SD 3 ± 2  Cumulative 79 I-124 PET/US duration, min  Median (range) 8 (4–18)  Mean ± SD 10 ± 4  Cumulative 245 The US examination of the thyroid gland was performed after 28.41 ± 1.97 h (median 29.08 h, range: 22.88–30.57 h) and consisted of two steps: - B-mode US - 124I PET/US fusion Using an electromagnetically tracked US probe (VNav, Logiq E9, General Electric, Milwaukee, WI, USA) the images were co-registered in real time, producing spatially correlated images of preacquired 124I PET and US. These images were displayed in a transparent overlay mode or side-by-side on the screen of the US machine in real time, i.e. if the US transducer is moved, the 124I PET dataset tracks in real time. This 124I-PET/US fusion examination allows for the exact assessment of a thyroid nodules function even in the case of multiple adjacent nodules. Three different kinds of detectors were used: Standard ringlet thermoluminescent detectors (TLDs) (beta/photon extremity dosemeter DXT-RAD 707 H, Thermo Fisher Scientific, Waltham, MA, USA) consisting of a Lithium Fluoride, LiF (7Li isotope) substrate doped with Magnesium, Mg, Copper, Cu and Phosphorus, P (LiF:Mg,Cu,P; abbreviated MCP; grammage 7 mg/cm2) (Figure 1, green arrow). The MCP-TLD was located under a polycarbonate cap (thickness: 27 μm; grammage: 3.3 mg/cm2). MCP is characterized by a decision threshold of 33 μSv and a detection limit 50 μSv. Chipstrate TLDs (photon single chipstrate extremity dosemeter EXT-RAD XD 700, Thermo Fisher Scientific) also consist of Lithium Fluoride, LiF (7Li isotope), which is doped with Magnesium, Mg and Titanium, Ti (LiF:Mg,Ti; abbreviated MT; grammage 100 mg/cm2). This MT-TLD was packed in a plastic pouch (thickness: 48 μm; grammage; 7 mg/cm2) (Figure 1, orange arrow). MT is characterized by a decision threshold of 20μSv and a detection limit of 30μSv. A calibrated Geiger counter dose rate meter (DRM) with gamma probe (Automess 6150AD6/E und 6150AD-18/E, Automation und Messtechnik GmbH, Ladenburg, Germany) was used concurrently (Figure 2C). The DRM had a range of measurement of 0.5–9.99 mSv/h and an energy range of 65 keV to 1.3 MeV. It was calibrated to Cs-137 (662 keV) and no correction factor was applied to the I-124 energy (603 keV). Dose rate measurement results at the skin of the patients’ neck and at the investigator’s chest were expressed as ambient dose equivalent H*(10). Figure 1. View largeDownload slide TLD measurement setup. Dosimetry setup with different types of thermoluminescent detectors (TLD) applied during PET/US fusion imaging procedure. The TLDs comprise MCP (green arrows), MT (orange arrows) and the personal dosemeter (red arrows) for the whole measurement arrangement. TLDs at the volume navigation tracking bracket attached to the ultrasound probe with translucent spacers (A, blue arrow); background TLDs (B); dosemeters placed on the investigator’s chest (C); TLDs positioned on the investigator’s hand (D). Figure 1. View largeDownload slide TLD measurement setup. Dosimetry setup with different types of thermoluminescent detectors (TLD) applied during PET/US fusion imaging procedure. The TLDs comprise MCP (green arrows), MT (orange arrows) and the personal dosemeter (red arrows) for the whole measurement arrangement. TLDs at the volume navigation tracking bracket attached to the ultrasound probe with translucent spacers (A, blue arrow); background TLDs (B); dosemeters placed on the investigator’s chest (C); TLDs positioned on the investigator’s hand (D). Figure 2. View largeDownload slide Fusion imaging procedure setup. Setup during the magnetic sensor navigated PET/US fusion imaging procedure. Two magnetic sensors (A, B, yellow arrow) are connected to the ultrasound transducer. The position of the magnetic sensors and therefore of the ultrasound transducer is recognized within a magnetic field generated by a transmitter (B, purple arrow). The PET/US images are shown in a semi-transparent fashion on the screen of the ultrasound device in real time (A). The relation of investigator, patient, and TLDs during the examination is nearly constant (B). Dose rate measurements at patient’s neck was performed at the beginning of the procedure (C). Figure 2. View largeDownload slide Fusion imaging procedure setup. Setup during the magnetic sensor navigated PET/US fusion imaging procedure. Two magnetic sensors (A, B, yellow arrow) are connected to the ultrasound transducer. The position of the magnetic sensors and therefore of the ultrasound transducer is recognized within a magnetic field generated by a transmitter (B, purple arrow). The PET/US images are shown in a semi-transparent fashion on the screen of the ultrasound device in real time (A). The relation of investigator, patient, and TLDs during the examination is nearly constant (B). Dose rate measurements at patient’s neck was performed at the beginning of the procedure (C). Due to their wide energy detection windows, both TLD types are suitable for 124I detection (MCP dosemeters: 12 keV–1.25 MeV, MT dosemeters: 15 keV–1.4 MeV). The measured radiation exposure was expressed as superficial person dose Hp(0.07). Background radiation was measured using two additional MCP and MT-TLDs which were stored in the examination room during PET/US procedure and stored in the same lead-shielded container when not in use. The readout process was performed off-site as per German regulations using a standard glow curve readout. To standardize the positions of the TLDs attached to the US probe and to reduce interobserver variability, custom made acrylic sockets were mounted to the Volume Navigation Tracking Bracket (ML 6–15, Civco medical solutions, Kalona, Iowa, USA) (Figure 1A—blue arrow, translucent spacers). Another ringlet MCP TLD was positioned on the ring finger facing the palm to obtain information about the dose at a position ringlet dosemeters are usually worn and to ensure comparability with dose equivalent limits. MT TLDs were not used on a finger as they were too bold to allow for a nonrestrictive movement of the investigator’s hand. Furthermore, one MCP and MT TLD each were positioned at the investigator’s chest. RESULTS In all 25 patients, US examinations consisting of the two steps (B-mode US, 124I PET/US fusion) were performed without technical difficulties or equipment failures. The mean examination time was 13 ± 5 min (median: 11 min; range: 6–25 min); the cumulative examination time was 324 min. Durations of respective examination steps are shown in Table 1. The evaluation of the MCP dosemeters resulted, after consideration of the background of 393 μSv, in a cumulative patient’s neck dose of 212 μSv, investigator’s hand dose of 157 μSv and investigator’s chest dose of 20 μSv. After dividing the cumulative dose by the number of examinations (n = 25), average patient’s neck, investigator’s hand and investigator’s chest doses per procedure were 8.5, 6.3 and 0.8 μSv, respectively (Table 2). Table 2. Dosimetric results. Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Table 2. Dosimetric results. Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 Dosemeter and location Dose Thermoluminescent detectors (MCP), μSv, Hp(0,07)  Background 393  Cumulative dose (patient’s neck) 212   Calculated mean dose 8.5  Cumulative dose (investigator’s hand) 157   Calculated mean dose 6.3  Cumulative dose (investigator’s chest) 20   Calculated mean dose 0.8 Thermoluminescent detectors (MT), μSv, Hp(0,07)  Background 224  Cumulative dose (patient’s neck) 112   Calculated mean dose 4.5  Cumulative dose (investigator’s chest) 10   Calculated mean dose 0.4 Dose rate meter (DRM)  Measured dose rate, μSv/h, H*(10)   Patient’s neck    Median (range) 24.5 (17–54)    Mean ± SD 27.7 ± 8.53   Investigator’s chest    Median (range) 0.5 (0.5–0.6)    Mean ± SD 0.5 ± 0.02  Calculated dose (dose rate × time), μSv, H*(10)   Patient’s neck    Cumulative dose 150.3    Median (range) 5.33 (3.1–17.1)    Mean ± SD 6 ± 3.3   Investigator’s chest    Cumulative dose 2.7    Median (range) 0.1 (0.1–0.2)    Mean ± SD 0.1 ± 0.04 The evaluation of the MT dosemeters resulted, after consideration of the background of 224 μSv, in a cumulative patient’s neck dose of 112 μSv and investigator’s chest dose of 10 μSv. Average patient’s neck and investigator’s chest doses per procedure were 4.5 and 0.4 μSv, respectively (Table 2). The evaluation by DRM showed a calculated cumulative patient’s neck dose of 150.3 μSv and investigator’s chest dose of 2.7 μSv. A mean patient’s neck dose rate of 27.7 ± 8.5 μSv/h and investigator’s chest dose rate of 0.5 ± 0.02 μSv/h was measured. The calculated mean dose per procedure was 6.0 ± 3.3 μSv at the patient’s neck and 0.1 ± 0.04 μSv at the investigator’s chest (Table 2). DISCUSSION Current guidelines propose radiation exposure limits for nuclear medicine staff: An effective dose to the whole body of 20 mSv per annum (for 5 subsequent years, 100 mSv), and a dose equivalent to the hands and extremities of 500 mSv per annum(17, 18). In nuclear medicine departments, the hands of doctors, technologists, pharmacists and nurses are regularly exposed to radiation due to frequent handling of radiopharmaceuticals and contact to patients. Fortunately, the radiation sensitivity of the hands is relatively low(19). During US examinations of the thyroid gland, the hand of the investigator is close the neck of the patient for a prolonged time (Figure 2B). If these examinations are performed frequently in patients who previously received radiating iodine isotopes, the question arises if the cumulative radiation exposure may be harmful. With the positron emitter 124I needed for PET/US fusion imaging, the 511 keV annihilation photons and further 603 keV photons have to be considered(20). Compared with fhSPECT/US examinations with Tc-99m tracers, a higher radiation exposure is possible, despite a low administered activity(16). Examinations of the thyroid gland are common in nuclear medicine departments. An US examination with B-mode and duplex visualization takes up to 5 min. Despite different examination protocols, the average examination time of 13 ± 5 min in this study is lower than the examination time of fhSPECT/US (19 ± 7 min)(16). The superior information about thyroid metabolism of 124I PET compared to Tc-99m SPECT led to a more elaborate and more detailed US fusion examination. Because of detection thresholds of MCP and MT dosemeters, a cumulative evaluation of the study duration of a total of 6 months was necessary. Unfortunately, measurement of a patient cohort over a longer period of time with the same TLD has disadvantages: - an evaluation per examination is not possible; - a time-related fading of the measurements may occur; and - the dosimetry results of TL-crystals are obtained using slowly increasing temperatures, producing a so-called glow curve. Therefore, this represents a rather long time for readout and does not allow multiple analysis of the same results. Still, TLDs are more feasible for the measurement of a cumulative radiation dose and subsequent calculation of an average dose than performing the measurements with a DRM, which has also been used in this study. TLDs can be selected based on the intended usage and calibrated to superficial person dose Hp(0.07) or deep person dose Hp(10). TLDs are small, do not impede an examination, and are easy to place at several areas of interest. The DRM can only measure the ambient dose equivalent H*(10), and handling during an examination may be difficult. In this study, MCP and MT TLDs were used. Both TLDs are different considering detection limit and geometry, mainly attributable to the different grammage. Although MT-MCPs are characterized by a lower detection limit, the dose values measured with MT TLDs (cumulative 112 μSv; 4.5 μSv per examination) are approximately half of that measured with MCP TLDs (cumulative 212 μSv; 8.5 μSv per examination). The probable cause is the relation between the radiation characteristics of 124I (β+1 = 1535 keV, β+2 = 2138 keV, γ1 = 603 keV, γ2 = 1691 keV, γ3 = 723 keV) and the different grammages and thicknesses of the cover material(20). The grammage and the thickness of the MT plastic pouch is approximately twice the grammage and thickness of the window of the MCP disc-like polycarbonate lens, as described above. It is conceivable that not only gamma rays are emitted from the patients’ skin but also high-energy beta rays due to slim soft tissue between the thyroid and the TLDs. Radiation with a low penetrating power, including alpha rays, beta rays with an energy below 2 MeV, and gamma rays with an energy of <15 keV may cause a relevant dose equivalent to the investigator’s skin. Beta rays emitted from 124I might be absorbed by the thicker plastic pouch encasing the MT-TLD, therefore, leading to lower dose values. Dose rate measurements were performed under the premise that dose rates remained roughly constant during the course of the examinations. 124I has a radioactive half-life of 4.2 days, which allows omitting the decay through the short examination time of ~13 min. By multiplication of dose rate and examination time, the result of 6.0 ± 3.3 μSv is nearly equal to the average of the TLD measurements (patient’s neck) (MCP: 8.5 μSv; MT: 4.5 μSv). Based on these findings, DRM measurements can be seen as a convenient and straightforward way to estimate radiation exposure at specific locations. As the MCP TLD at the investigator’s hand was positioned at a greater distance from the patient’s neck as compared to the MCP TLD at the US probe, the calculated mean dose of 6.3 μSv was lower than the calculated mean dose of 8.5 μSv at the US probe (Figure 2B and Table 2). Our study is limited by the measurement setup. The long interval of TLD of 6 months usage may have caused an imprecision of the radiation exposure measurements. During the study, TLDs were placed at the US probe, closer to the thyroid gland and therefore receiving more radiation, and not at the investigator’s hand. This setup is not completely representative for the hand(21), but ensures a reproducible TLD positioning and eliminates observer-dependent variability. For a detailed evaluation, a higher number of patients has to be investigated, with dose measurements at several areas at the investigator’s body, including fingers, hands, chest and eyes. Evaluation of different types of dosemeters would help to determine the most feasible and exact option for clinical practice, like optically stimulated luminescence dosemeters based on beryllium oxide (BeO), which do not show fading over time and can be readout several times(22). CONCLUSION This study evaluates and quantifies the potential radiation exposure of an investigator, who performs US examinations in patients who previously received a radioactive tracer. The positron-emitting isotope 124I accumulates in the thyroid gland and facilitates PET/US fusion imaging. The results showed that radiation exposure of the hand of the investigator holding the probe to the neck of the patient (measured in the range from 4.5 to 8.5 μSv, depending on the type of dosemeter) and also exposure of the investigator’s body is very low. In clinical routine, a lower radiation exposure can be expected: Because of the extensive examination protocol with a mean duration of 13 min and positioning of the detection devices very close to the patient, our study setup presumably overestimated this exposure. Even with a high number of examinations, it is nearly impossible to exceed or even to reach regulatory radiation limits. Despite this, if PET/US examinations are performed frequently, basic radiation protection principles should be always observed to avoid unnecessary risk by radiation exposure. The examinations need to be justified and actions have to be taken to optimize the work flow and reduce radiation exposure. In staff surveillance, the differing detecting capabilities of different types of radiation detection devices (dosemeters, DRM) have to be taken into account. ACKNOWLEDGEMENTS GE Healthcare, Milwaukee, WI, USA is gratefully acknowledged for providing hardware (LOGIQ E9) for this study. FUNDING This study was funded exclusively with intramural grants from the Jena University Hospital. REFERENCES 1 Freesmeyer , M. , Opfermann , T. and Winkens , T. Hybrid integration of real-time US and freehand SPECT: proof of concept in patients with thyroid diseases . Radiology 271 ( 3 ), 856 – 861 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 2 Otte , A. and Hoppe , H. Hybrid SPECT/US . 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Radiation Protection DosimetryOxford University Press

Published: Mar 2, 2018

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