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Saturday, February 09, 2008
There is no difference in thyroid uptake results using with or without standard dose activity. A/Razak H. Nurre, CNMT Abstract: The purpose of this study is to assess the difference, in the results of the thyroid uptake measurements, between using a single dose activity with or without retaining a standard dose for comparison of the administered activity in the patients’ thyroid gland. Six patients were randomly used for the study. Each patient swallowed a single dose activity of Iodine 123 (235uCi to 305uCi). One single standard dose (298uCi) was retained for comparison the activity in the patients’ neck. Two different studies were performed using two different formulas. The same six patients involved the two studies. In this research, we found that there is no difference for the thyroid uptake measurements between the two methods. We recommended in the conclusion of this document that using a single dose method is the best choice to use for thyroid uptake and scan to save time and money. Background Thyroid nuclear medicine uptake and scan refers to a test that is used to study the structure and function of the thyroid gland. This test is usually performed to evaluate hyperthyroidism, hypothyroidism, detection and localization of metastases from thyroid cancer, differentiation of benign from malignant nodules, and detection and localization of benign ectopic thyroid tissue. The radioactivity used for this test is called Iodine-123; it is produced in an accelerator. It decays by electron capture with a physical half-life of 13.2 hours. The activity (I-123) is excreted by the kidney, and the risk of toxic reactions to this isotope may be greater in patients with impaired renal faction. Patient is given one capsule or two of I-123 (200-500uCi) activity with a cup of water---then patient is instructed to return 4-6 hour and 24 hour for thyroid uptake and scan using thyroid uptake probe system and gamma camera respectively. Thyroid uptake determination is the measurement of the fraction of an administered amount of radioactivity iodine that accumulates in the thyroid at selected times following ingestion (David V Becker, 1999). The primary purpose of this study is to determine the difference of the thyroid uptake results between using a single dose method without retaining standard dose and using a single dose activity while retaining a standard dose to compare the administered activity in the patient’s neck. Introduction Several sources of inaccuracy and inconsistence have been noted in some research papers. A literature has shown that the thyroid gland depth affects the accuracy of uptake measurements of thyroid scan studies. The research in a group of 40 patients, the phantom method was found to underestimate the mean gland depth by about 1 cm, thereby causing an average uptake error of 23% (Peter M. Martin, 1977). Another example in the same research indicates that radionuclide purity has been shown to affect the estimation of uptake. Though this research paper discussed the errors caused by the depth of thyroid gland and the radionuclide purity and their correction methods for thyroid uptake measurements, it did not present any other sources that can affect the accuracy of thyroid uptake studies. Another source of inaccuracy may result from “seemingly unimportant” variations in technique. A stable high voltage adjustment, careful selection of energy window, and the use of standardized neck phantom with generally accepted attenuation characteristics are absolutely essential if radioiodine uptake values are to be compared and appropriate therapeutic dose are to be administered on these measurements” (Shanta, Chervu, 1982). Contrary to the previous research paper, this literature focused the affect of improper use of the thyroid probe system. However, it did not include the source of inaccuracy of the thyroid uptake measurement the role of the radioactivity itself can play, particularly when using multiple activities. In the uptake measurement portion of his research paper, Floyd did not give more details when counting the second dosage capsule—the standard capsule. He explained that the second dosage capsule was counted at each uptake interval and used for decay correction (John L. Floyd, 1985). The decay correction that the author used at each uptake intervals did not express any solution to the different counts the two activities could produce. Another factor that can affect the accuracy of thyroid uptake measurement is the distance from the face of the probe crystal to the anterior aspect of the neck, 25 to 30 cm. (Mettler, 1998). The author’s explanation of comparing the standard activity to that of the patient’s neck was not enough to convince the reader that the difference of the two counts do not affect the accuracy of the result. This study is important for several reasons including discovering the affects from the standard dose counts on the results of thyroid uptake measurements and investigating the difference in the percentage of the thyroid uptake measurements when using a single dose activity of Iodine-123 with and without standard dose. The purpose of the study is to explore the differences in the percentage of thyroid uptake measurements when patients are given one dose of Iodine-123 activity (200-400uCi) while retaining a standard dose (200-400 fuci) to compare the activity in the patient’s thyroid gland and when they are given the same dose (Iodine-123 activity) without retaining the standard dose. In this research paper, the independent variables are the two doses of I-123 that one (patient’s dose) was given to all of the six different patients—each at a time—while the other dose (standard dose) was retained for comparison the activity in the patients’ thyroid glands. The nuclear medicine technologists in our facility measured each dose before giving the patients using a dose calibrator—a device that measures the amounts of radioactivity in unites of curies. The technologists also measured the standard doses using the same device. The dependent variables are the percentages of the thyroid uptake measurements that were calculated when used Sodium iodine (NaI) crystal uptake probe with suitable lead shielding and a flat field collimator. This system is used to detect and count the activity in the patients’ thyroid glands and the activity in the standard dose. The administered counts and the standard dose counts were obtained by counting the activity administered to the patients and by counting the standard dose activity in a neck phantom. The uptake measurements of the thyroid glands were performed 24 hrs post ingestion of the activity. Operationally, we compared how the results of the thyroid uptake measurements were affected by the two independent variables (the single dose of Iodine-123 activity that each patient swallowed and the standard dose activity that was retained to compare the activity in each patient’s neck) This research used posttest only design and the patients were randomly selected. Method Six patients from both sexes referred to our nuclear medicine facility at different times to evaluate the structure and the function of their thyroid glands. They were orally given a small amount (235uCi-305uCi) of Iodine 123 activity for both thyroid uptake and scan by the nuclear medicine technologists in our department. A standard dose (298uCi) was retained for comparison the activity in the patients, neck after 24hrs. Patient preparation was done ahead of the study date. Each patient was identified, doctors’ orders were verified, and the procedure was explained to the patients. Patients were instructed by physicians to discontinue thyroid medications, contrast material or any other interfering medicine weeks before they were scheduled for the test. Patients were asked prior to the thyroid uptake doses same questions about their history of cancer, thyroid disease, thyroid disease medications—if they take any—,chemical and radiation therapy on their neck and chest, any previous thyroid uptake and scan, their weight, appetite, and other voluble patient history that would be helpful for the test results. The contraindications for the test, such as any allergy to iodine, pregnancy, continuation of the thyroid medication, or other interfering medications were also checked prior to the patients’ swallow the isotopes. The nuclear medicine technologists in our facility measured all the six doses of the iodine-123 before giving the patients using a dose calibrator—a device that measures the amounts of radioactivity in unites of curies (235uCi to 305uCi). The patients swallowed the doses after their counts were obtained and recorded three times using the thyroid probe system. The room background counts (47CPM) were also obtained and recorded three times in the first day. The technologists also measured the standard dose activity of iodine-123 using the same device (298uCi). These patients swallowed the doses and were asked to come back the next day for thyroid uptake measurements. Study 1 The next morning, the administered counts (Patients’ doses) and the standard dose counts (Retained dose, 82874CPM) were obtained by counting the activity in the patients’ neck, 24 hr post ingestion and by counting the standard dose activity in neck phantom. The body background counts (thigh) were counted also using a thyroid probe system. Each measurement was counted and recorded three times. Then each patient was scanned using gamma camera and images were saved. The percentage of each patient’s thyroid uptake for study 1 was then calculated using this formula: Formula 1 [Patient thyroid CPM-patient background (thigh) CPM] % uptake = -------------------------------------------------------------------- X100 Standard count CPM – original background (room) Study 2 For study 2, no standard doses counts were obtained and recorded for comparison to the activity in the patients, neck. 24 hr post ingestion, administered activity in the patients’ neck and the patients’ body back ground (thigh) were counted and recorded three times using the same thyroid probe system. Also the same, previous room back ground counts used in study 1 were used again in this study too. The percentage of each patient’s thyroid uptake was then calculated using this formula: Formula 2 [Patient thyroid CPM – Patient background (thigh) CPM % uptake = ------------------------------------------------------------------------X 100
Original capsule
CPM x decay factor – original background Results Six patients from both sexes referred to our nuclear medicine facility at different times to evaluate the structure and the function of their thyroid glands. They were orally given a small amount (235uCi-305uCi) of Iodine 123 activity for both thyroid uptake and scan by the nuclear medicine technologists in our department. A standard dose activity (298uCi) was retained for comparison the activity in the patients’ neck after 24 hrs. All the activities administered to the patients are in Table 1 Table 1
Uptake measurements Thyroid uptake measurements of Study 1 and 2 are summarized in Table 2. This data includes percent of 24 hr uptake of study 1 that was calculated with using standard dose counts (82874 CPM). It also includes percent 24 hr uptake of study 2 that was calculated without using standard dose activity. Table 2
Study 1 The number of patients involved in study 1 is six patients. Standard dose was used for this study to compare the administered activity in the patients’ neck. Formula 1 was used to obtain the percentage thyroid uptake of the patients .The mean percentage uptake after 24 hr of this study was 27.9%. The median percentage uptake after 24 hrs for this study was 25.2%. Standard Deviation of this study was 17.5. (See Table 3) Table 3
Study 2 The same patients that were involved in study one were also used for this study. Standard dose activity was not used for this study. Formula 2 was used to calculate the percentage 24 hr thyroid uptake. The mean percentage thyroid uptake after 24 hr calculated 28.9 without using standard dose. The median percent uptake was 28. The standard deviation was 14.8. (See Table 4) Table 4
Difference in percentage of thyroid uptake measurements of study 1 and 2 The difference in percentage for thyroid uptake measurements between the two studies is in Table 5. The mean difference in percentage of the thyroid uptake of the two studies is 0.967. The Medan is -1.90. The Standard Deviation is 5.50. The result of paired T-test calculated was 0.430. The P value associated with this t-test (P<0.05) was smaller. Thus, the T-test value is within the range and the null hypothesis is not rejected. This means there is no difference in thyroid uptake results using with or without standard dose activity. Table 5
Discussion When we compared the two methods used to evaluate the function of the thyroid gland, we found that there is no difference between them regarding the results of the thyroid uptake measurements that were obtained in our research. The findings of this study are consistent with other similar studies that had been previously performed. This study had at least three important limitations, the most important of which was that the number of the patients used for the study was very small. The other limitation for the study was that we used only one standard dose to compare the administered activity in the patients’ neck. In addition, only one room background counts was used for the calculation of the thyroid uptake measurements for both studies. Although, our study was limited to very small number of patients, we believe that it is still useful and provided voluble data to proof that both of the methods can be used to evaluate the thyroid uptake measurements. Conclusion Our study evaluated the difference between the two methods that are used in nuclear medicine labs to evaluate the function of the thyroid gland. Each of the system has its formula that can be used to calculate the percentage of the thyroid uptake measurements. Within the limitations of our study, we did not find any significant differences between the results of the two methods. However, using a single dose method is the best choice to save time and money. References
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