A User-Friendly Thyroid Monitor

Transcription

1 A User-Friendly Thyroid Monitor Herman Cember & Wei-Hsung Wang Purdue University, School of Health Sciences, 1338 Civil Engineering Building, West Lafayette, IN , USA ABSTRACT Good radiation safety practice includes routine thyroid monitoring for workers who are occupationally exposed to iodine-131. This work describes the design, construction, and calibration of a simple and reliable thyroid monitoring system. This system consists of a single channel analyzer and a sodium iodide detector mounted to assure a fixed and reproducible counting geometry. The system was calibrated using a mock thyroid into which a known iodine-131 activity was injected. The mock thyroid was made by inserting one thyroid-lobe shaped sponge into each of two glass vials. The two vials were capped and placed into a neck phantom. At a 5 cm skin to detector distance, the lower limit of detection was about 142 Bq (3.84 nano-curie). This detector system provides an easy, economic, and effective means to measure the internal radioiodine deposition in radiation workers and also meets both the regulatory standards and the good health physics practice. INTRODUCTION Iodine-131 (I-131) is widely used in the application of health physics, nuclear medicine, veterinary science, biology, and chemistry. It is also a significant fission product in a nuclear power plant. Because of its high volatility under room temperature, iodine can easily be inhaled by radiation researchers and workers due to an unknown or subtle release of radioiodine to the atmosphere. Healthy subjects accumulate about 30% of the inhaled iodine in the thyroid gland which is the critical organ for iodine (1). The thyroid gland is subject to radioiodine carcinogenesis (2). The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report suggested that a total risk of radiation induced thyroid cancer is per year per gray (3). Hence, the most efficient way to determine the internal deposition of iodine-131 is to perform in vivo measurement of the iodine-131 thyroid burden, and thus to prevent overexposure. According to the United States Nuclear Regulatory Commission's Title 10 Code of Federal Regulation (CFR) Part , it is required to monitor the occupational intake of radioactive material for radiation workers if they are likely to receive a yearly intake in excess of ten percent of the applicable annual limit on intake (ALI), as listed in Appendix B, 10 CFR 20. The inhalation ALI for I-131 is 50 µci and the corresponding derived air concentration (DAC) is 2 x 10-8 µci/ml. DAC means the concentration of a given radionuclide in air which, if breathed by the reference person for a working year of 2,000 hours under conditions of light work, results in an intake of one ALI. Kopp et al. (4) suggested using a high-purity germanium (HPGe) detector system for routine thyroid measurements of radioiodine in radiation workers. The cost of an HPGe detector system is much higher than a sodium iodide (NaI) scintillator monitoring system. The HPGe detector system has poorer counting efficiency than a NaI scintillator monitoring system. Besides, the HPGe detector must be operated at low temperature of 77 K (5). Tran (6) proposed to use two collimated 2" x 2" NaI detectors and a Canberra 1 System 100 Multi-Channel Analyzer to calibrate the I-131 uptake in the thyroid in a neck phantom provided by the Canadian Bureau of Radiation and Medical Devices. This detector system costs about $5,500 without the computer related expense. The report presented here describes a simpler user-friendly thyroid monitor. The monitor consists of an ORTEC 2 single channel analyzer (SCA) with a 1.25" diameter x 5/16" thick NaI detector that is calibrated for the assessment of I-131 uptake in the thyroid gland of radiation workers. This NaI scintillation counting system costs less and can also be applied to various other practices. This monitoring device provides an effective and economic way to demonstrate compliance with the regulatory limits. It also accomplishes good health physics practice to use radiation sources safely. MATERIAL AND METHODS COUNTING SYSTEM Scintillation counters have widespread uses in measuring gamma rays and low-energy beta rays. The NaI scintillator has a high sensitivity for the detection of gamma rays. When the analyzer of the scintillation counting system is in the differential mode, it operates as a single channel analyzer and allows only pulses of a certain size to be counted. The essentials of a single channel analysis system consist of a detector, a high voltage power supply, a linear amplifier, a pulse height analyzer, and a readout device (timer and counter). The main use of the SCA is to optimize the signal to noise ratio when measuring a low radioactivity source in the presence of a significant background (7). 1 Canberra Industries, 800 Research Parkway, Meriden, CT EG&G ORTEC, 100 Midland Road, Oak Ridge, TN

2 The sodium iodide counting system uses an ORTEC SCA to electronically process the signals from the iodine-131 radioactivity in the thyroid. A Harshaw " diameter x 5/16" thick NaI detector which can measure the radioactivity at levels well below legal limits was utilized. Iodine-131 emits several photons of different energies. In this case, we counted the signals from the 364 kev photons with a radiation abundance of 82% (8). The use of the SCA effectively reduced the background count rate to a very low level relative to the I- 131 activity in the thyroid at levels of interest to the health physicist. PREPARATION OF THE COUNTING SYSTEM 1. Determination of the Operating Voltage Most photomultiplier (PM) tubes commonly used with a NaI crystal operate well in the 800-volt to 1,100-volt range. The appropriate high voltage setting is obtained by plotting the integral count rate vs. high voltage with the analyzer in the integral mode. In the integral mode, all pulses greater than a given size are counted. As a good rule of thumb, the operating voltage should be selected relatively close to the threshold voltage (within the lower 25% of the plateau). Since the plateau curves for different gamma-emitting nuclides vary somewhat, the operating potential must be determined individually for each isotope used (9). In order to obtain a good voltage curve, at least 30 minutes should elapse between successive increases in voltage to allow the PM tube to stabilize. This stabilization period helps minimize the variations in output voltage and ambient temperature due to the voltage shifts in the PM tube. The operating voltage should be chosen to avoid the rapidly rising thermal noise level at the higher voltages and to help preserve the life of the PM tube. The operating voltage is determined on statistical grounds as that voltage where the ratio of the square of the gross sample count rate to the total background count rate is at maximum (10). Results from this procedure are shown in Table 1 and Figure 1. From the data on Table 1, 925-volt was selected as the operating voltage. Voltage (volts) Gross (cpm) BKG (cpm) Net (cpm) (Gross) 2 /BKG * Gross: gross sample counting rate in counts per minute ** BKG: background counting rate in counts per minute *** Net: net counting rate in counts per minute = (Gross - BKG) Table 1. Determination of the Operating Voltage 3 Harshaw Chemical Company, Cleveland, Ohio 2

3 counting rate (cpm) Net Counting voltage (volts) Figure 1. Plot of Integral Count Rate vs. High Voltage 2. Energy Calibration An SCA must be calibrated before performing any sorting of the pulse sizes produced in the analyzer. The 364 kev gamma photon from I-131 was chosen for the calibration. A window can be established using the base and upper discriminators of the analyzer. This window must be wide enough to allow a narrow range in pulse sizes to get through, but not too wide or the calibration will be inaccurate. A window whose width is about 5-10% of the full energy peak is usually acceptable. This window should be placed on the division scale of the base discriminator so that a division will correspond to a gamma ray with a given energy. For this experiment, the base discriminator was set at 354 and with a 20-division window and so the center was at 364. With this analyzer setup, pulses corresponding to energies between 354 and 374 kev would pass through this window. After the 364 kev photopeak had been defined, a calibration plot of I-131 was also established to provide the complete information on the shape and position of the photopeak and help confirm the discriminator settings. The amplifier's coarse and fine gains were adjusted to maximize the counting rates in the kev channel. The data and the calibration plot of I-131spectrum are shown in Table 2 and Figure 2. Base Gross BKG Net Base Gross BKG Net * Base: base discriminator setting ** All counting rate measurements are in counts per minute *** Instrument settings: high voltage = 925 volts; coarse gain = 1K; fine gain = 1.24 Table 2. Determination of Iodine-131 Spectrum 3

4 counting rate (cpm) Net Counting Rate base discriminator setting Figure 2. Iodine-131 Spectrum THYROID MODEL Standard radiation absorbed doses are calculated for the reference person. The thyroid gland of a reference person weighs 20 g (11). Each adult lateral lobe is 2 cm x 2 cm x 4 cm, elongated in the cephalocaudal dimensions, and opposed on either side of the trachea, with the isthmus connecting the two lobes (12). Each of two simulated thyroid lobes made of sponge were placed into 15 ml vials to represent the thyroid organ. The breadth of the isthmus is 1.25 cm (13). The three thyroid organ components were positioned into a 13 cm diameter water filled plastic bottle representing the neck phantom (14). The anterior surface of the three components is 1 cm behind the outside surface of the phantom (14). The two vials were fixed at a center to center separation distance of 3.3 cm between the "lobes". Since the trachea, esophagus, and pharynx lie behind the thyroid, no photon originating from the thyroid could pass through these regions and still deposit its full energy in the detector. SETUP AND CALIBRATION OF THE COUNTING SYSTEM Simulated in vivo measurements of the thyroid were made by positioning a sodium iodide detector 5 cm from the anterior surface of the neck, centered over the isthmus between the two thyroid lobes. The center of the thyroid was aligned with the center of the NaI detector. The geometrical arrangement of the cylinder, vials, and the detector used during the experiments is shown in Figure 3. The arrangement and geometry were maintained the same for all measurements. This counting system must be appropriately calibrated to infer the I-131 thyroid burden from the measurement. Calibration was performed using an I-131 source and the neck phantom as shown in Figure 3. The I-131 source was in the form of sodium iodide solution. 7.4 kbq (0.2 µci) in 10 ml aqueous solution was injected into each "lobe". The amount of 14.8 kbq (0.4 µci) represents sixty-three percent of the maximum allowable eight-hour uptake of I-131 in the thyroid based on an ALI of 50 µci. Both thyroid and background measurements were carried out for 1,000 seconds each for continuous seven weeks. All measurements were taken at the same time everyday. Because the half-life of I-131 is much longer than the sample counting time, no correction for the counting time is needed. A plot of the net count rate of I-131 on the ordinate versus the absolute activity of I-131 on the abscissa was generated to provide a calibration curve for the thyroid monitoring. Since I-131 activity in the thyroid was related to the count rate, one could easily determine the radioiodine deposition in the thyroid. After the monitoring system has been calibrated, the counting efficiency of the NaI detector should be determined. Counting efficiency can be calculated using the ratio of the net count rate to the absolute activity of a radionuclide. The geometry of the detector and source arrangements must be fixed in order to have a constant counting efficiency. The lower limit of detection (LLD) of the monitoring system is dependent on both the detector efficiency and the counting time. The LLD should be well below the activity level of concern in operational radiation protection. 4

5 Side View 13 cm water filled 1 cm SCA 15 cm 5 cm 2 cm NaI detector within lead shield neck surface Top View 2 cm neck surface 13 cm 3.3 cm SCA 3.5 cm 5 cm isthmus :simulated thyroid lobe :detector :lead shield Figure 3. Geometry Details of the Thyroid Organ Model within the Neck Phantom RESULTS AND DISCUSSION The data and plot of the calibration are shown in Table 3 and Figure 4. The radiation background is in the range of 6 to 10 counts per minute. The calibration curve indicates a perfect linear relationship between the calculated activity and the measured net count rate (r 2 = ). This straight line also suggests that this thyroid monitoring system performed stably. The counting efficiency of the NaI detector can be easily obtained from the estimated regression function, y(cpm) = x(µci), on the calibration plot in Figure 4. The slope of cpm/µci of the regression function represents the counting efficiency of the NaI detector at this specific setup. 5

6 Day Gross BKG Net Act calc Day Gross BKG Net Act calc * Gross sample counting rate and background counting rate are in counts per 1,000 seconds ** Net counting rate is in counts per minute *** Act calc : calculated activity of I-131 based on the original activity of 0.4 µci (14.8 kbq) using the radioactive decay equation Table 3. Calibration Data of Iodine-131 counting rate (cpm) Net Counting Rate y = x r 2 = calculated activity (uci) Figure 4. Calibration Curve of Iodine-131 Monitoring System When the sample counting time is 100 seconds and the background counting time is 1,000 seconds, the LLD, in units of counts, during the 100-second sample counting time is given by (7): 6

7 t g LLD = 3.29 rb t g (1 + ) + 3 t b where r b is the background counting rate t g is the sample counting time t b is the background counting time At a maximum background count of 166 in 1,000 seconds, LLD = 3.29 = 10.24cpm above background LLD = cpm 166 counts 100 seconds counts 100 seconds (1 + ) seconds 1000 seconds 100 seconds μci 1000 nci = 3.84 nci cpm μci 37 Bq LLD = 3.84 nci = 142 Bq nci Therefore, the most conservative estimate of the LLD of this thyroid monitoring system was 142 Bq (3.84 nci) above the background for a 100 second monitoring time. At equilibrium, the accumulation rate of I-131 in the thyroid is equal to the removal rate of I-131 from the thyroid. With an ALI of 50 µci, the steady state activity of I-131 in the thyroid for an eight-hour working day can be calculated as follows, 50 μci 0.3 hours 1 8 = Q μci λe 2000 hours day day where 0.3 represents that 30% of the inhaled iodine is deposited in the thyroid 2000 hours represent the total working hours per working year 8 hours/day represent the total working hours per working day Q µci represents the steady state activity of I-131 in the thyroid λ e day -1 represents the effective clearance constant of I-131 in the thyroid In order to obtain λ e, we need to know the effective half-life, T e, which can be computed from the following formula, Tr Tb Te = Tr + Tb where T r represents the radiological half-life of I-131 of 8.04 days T b represents the biological half-life of I-131 of 80 days (15) Substituting the appropriate values into equation and solving for T e yields: 60 seconds 1minute So the effective clearance constant λ e is 8.04 days 80 days = 8.04 days + 80 days Te = 7.3 days λe = = days day The solution for the steady state activity of I-131 in the thyroid is given by hours 50 μci day Q = = 0.63μCi hours day The value of 0.63 µci represents the maximum allowable eight-hour deposition of I-131 in the thyroid based on an ALI of 50 µci. The sensitivity of this detector system can also be computed as follows, In other words, this thyroid monitoring system is capable of measuring 0.61% of the maximum allowable eighthour thyroid burden. LLD 3.84 nci 1μCi = 100% = 0.61% Q 0.63μCi 1000 nci 7

8 MONITORING SYSTEM STEUP The elements of this thyroid monitoring system are shown in Figure 5. The radiation worker places his/her chin on the chin rest to keep a fixed position during the counting period of 100 seconds. There is a Styrofoam spacer between the worker's neck and the detector to maintain a constant and reproducible geometry. The NaI detector is within lead shield which is mounted to the bottom of the chin rest. NaI detector within lead shield Styrofoam spacer 5 cm SCA : chin rest : clamp : thyroid lobe Figure 5. Setup of the Thyroid Monitoring System CONCLUSION 1. This thyroid monitoring system has reproducible geometry and stable performance. 2. The background count rate was in the range of 6 to 10 cpm with a 20 kev wide window of the analyzer. 3. With a thyroid monitoring time of 100 seconds, the lower limit of detection for this monitoring system is about 142 Bq (3.84 nci) at a 10 cpm background with a background counting time of 1,000 seconds. 4. This monitoring system is able to measure 0.61% of the maximum allowable eight-hour thyroid burden. 5. This monitoring system provides an easy, effective, and economic means not only to measure the internal radioiodine deposition for the radiation workers but also to demonstrate compliance with the regulatory limits. ACKNOWLEDGMENT The radioiodine used in this project was provided by Syncor 4 International Corporation. The authors greatly appreciated the generous offer of Syncor. Thanks are extended to Ms. Kara Duncan of Syncor for her continuous assistance. REFERENCES 1. International Commission on Radiological Protection. Limits for intakes of radionuclides by workers. Oxford: Pergamon Press; ICRP; Publication 30; Syncor International Corporation, 7920 Georgetown Road, Suite 100, Indianapolis, IN

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