Radiation Absorbed Dose From 201 TI Thallous Chloride - Case Study Example

RUNNING HEAD:Radiation Absorbed Dose From Thallous chloride  Radiation-Absorbed Dose from 201Tl-Thallous Chloride John Q. Wright University Abstract 201 Thallous chloride, administered for post-exercise and resting myocardial scintigraphy, and for newer applications in oncology, was used to obtain data from 28 patients who underwent scintigraphy to measure radiation absorbed dose for various organs, to calculate the radiation absorbed by the testes; similar studies done before and used for calculation of dosimetry by the ICRP have tended to overestimate the values. The radiation absorbed by the testes was lower than in some other studies. Further, the absorbed dose for children was much higher when compared with adults, and with increasing shelf life, the effects of contaminants like 202 Tl were significantly enhanced. Radiation-Absorbed Dose from 201Tl-Thallous Chloride Thomas SR, Stabin MG Castronovo FP, Radiation-Absorbed Dose from 201Tl-Thallous Chloride. Journal of Nuclear Medicine Vol. 46 No. 3 502-508. 2005 by Society of Nuclear Medicine. 02-03-06. Radiation dosimetry for 201Tl-thallous chloride has been developed by several authors1. The International Commission on Radiation Protection (ICRP) dose estimates for 201Tl chloride have incorporated values of testicular uptake published by Gupta et al2 and Hosain and Hosain. 1. There is a problem when dosimetry is calculated in this way: the values for testicular uptake at 24h reported by these authors for adults are significantly higher than those reported by other authors e.g., Atkins et al, Krahwinkel et al., Rao et al. 2. The ICRP not only uses these estimates, it also extends the model to children with the prediction of correspondingly higher testes radiation dose at younger ages. 3. The error arising from these estimates are important when calculating the dose for adults as well as for children 4. Further, the authors estimated the possible contribution to radiation dose from several radioactive contaminants that are often present in 201 Tl preparations, and have evaluated the consequence of administration time relative to preparation. The hypotheses being tested are: 1. The uptake of 201Tl in the testes after intravenous administration of an isotonic solution is not as high as estimated in some studies, which have formed the basis for recommendation of radiation dosimetry by the ICRP. 2. Since these data have been used to estimate testes radiation dose in children, the corresponding values are also higher; the authors hypothesized that even these values need to be revised downwards. 3. When Thallous chloride is provided as an isotonic solution, the possible contaminants are 200Tl, 202Tl, and 203Pb. With longer shelf life, the contribution of 200Tl decreases whereas that of 202Tl increases. The equipment necessary to perform this research are: (1)Diagnostic exercise myocardial scintigraphy (2)Quantitative testicular scintigraphy (3)Lead vinyl shield (4)201Tl standard vial with saline (5)MIRDOSE software The material used for this particular study were patients in a clinical trial whose primary objective was to compare bolus injection of an experimental pharmaceutical clearance agent with that of a placebo on the identification of thallium redistribution in the myocardium. The experimental design: randomized, placebo-controlled, crossover protocol, with second injection occurring at 1-2 wk after the first. Statistical methods being employed: SigmaPlot 8.0. Chicago, IL: SPSS Science Marketing Department, SPSS Inc. –Standard regression software to calculate the uptake component. For testes parameters, averages of individual results obtained from quantitative scintigraphy, were used. For organs other than testes, Mean and SD of kinetic data were used. Residence times calculated from these methods were then used with the MIRD methodology to provide dose estimates. The effective dose was calculated according to ICRP publication 80. The results of this study are set out in Tables 1 - 5. Table 1 presents the biologic parameters derived for the source organs considered. The data for organs other than the testes were derived from Krahwinkel et al. For the testes, the data were derived from quantitative scintigraphy of 28 patients (56 studies). The fractional distribution functions for testes (αh1) = 0.00568/h. The biologic rate constants (λ h1/h) = 0.00445/h. Residence time for testes = 0.095 – 0.46 h; SD = 0.087 h. Table 2: The testicular uptake in % at 24 h, was 0.29 ± 0.09, and the testicular biologic half life was 429 h. The estimated radiation dose to testes was 0.77 rad/mCi or 0.21 mGy/MBq. Table 3: The estimated absorbed dose at time of calibration from iv administration along with percent contribution of each contaminant is given for each organ considered. Those for testes are as follows: mGy/MBq, total % contribution 201Tl 200 Tl 202 Tl 203Pb 2.09E-01 95.56 0.34 4.08 0.03 The values when allowing for shelf life, as given in Table 4 are as follows: 2.17E-01 90.82 9.16 0.02 Table 5 gives the radiation dose to testes as a function of age using data from this study in comparison with values provided by ICRP Publication 80: Radiation dose to testes This study ICRP 80 rad/mCi mGy/MBq mGy/MBq Newborn 27.8 7.5 1 y 19.0 5.1 13 5 y 14.3 3.9 9.6 10 y 12.1 3.3 8.3 15 y 1.6 0.43 1.1 Adult 0.77 0.21 0.45 The distribution data given in Table 1 was coupled with appropriate dose conversion factors described by Stabin MG 3. These were according to MIRD schema; In Tables 3 & 4, this calculation of absorbed dose estimates for various organs are given with inclusion of contributions from contaminants, like 200Tl, 202Tl, and 203 Pb. In Table 4, the contributions from these contaminants are given after consideration of the shelf life. The effective dose is 0.155 mSv/MBq (0.57 rem/mCi) when the contaminant level contribution is considered without considering the shelf life; when the shelf life is included into consideration, the contribution of 202Tl increases, and the effective dose becomes 0.162 mSv/MBq (0.60 rem/mCi). Radiation dose estimates to the testes of children were determined by the data generated for adults and extrapolated by applying these to formulations derived for various ages4. The findings in this study and the calculations performed by the authors using data and formulation generated in other studies support their three hypotheses: (1) In this study, the calculated radiation dose to adult testes 0.21 mGy/MBq per unit administered activity is approximately a factor of 2 less than the value of 0.45 mGy/MBq currently quoted within ICRP Publication 80. (2) The calculated radiation dose to the testes per unit administered radiation activity in children increases significantly as age decreases, and reaches a value of 7.5 mGy/MBq for a newborn. This supports their second hypothesis (3)The contribution to the calculated radiation dose to the testes per unit administered radiation activity from the contaminant 202Tl increases with the shelf life, and this is important in considering the radiation dose; this hypothesis is amply supported by the authors’ data. The authors made no attempts to investigate small-scale dosimetry considerations, but provided simply whole organ estimates. These have been described by Rao et al5. Application of this method to future studies can enhance the accuracy of data. The Relative Biologic Effectiveness (RBE) of the Auger counter was not taken into consideration in this study. When mice were administered 202Tl via intratesticular injection, Rao et al6 found a 2- to 4- fold increase in the effect on spermatogonia from electron emission measured by the Auger counter; this was greater than the effect from high-energy beta particles and contrary to expectations based on conventional dosimetry of such radionuclides. This effect was not studied by the current authors, and has the potential to influence the interpretation and application of physical dose estimates. This is another area worthy of further investigation. Other groups currently publishing their results in this same area of research are the following: (1) Nettleton JS, Lawson RS, Prescott MC, Morris ID. Uptake, localization, and dosimetry of In-111 and Tl-201 in human testes. J Nucl Med. 2004;45:138–146.[Abstract/Free Full Text]7Percentage uptake found to be actually lower (0.19%), but they included only 4 patients, who were to undergo orchidectomy as part of their treatment for prostate cancer, and post-orchidectomy the weights of their testes were on average lower than those of normal, non-orchidectomized patients in other studies. Sources 1. Atkins HL, Budinger TF, Lebowitz E, et al. Thallium-201 for medical use. Part 3. Human distribution and physical imaging properties. J Nucl Med. 1977;18:133–140.[Abstract]. 2. (Gupta SM, Herrera N, Spencer RP, et al. Testicular-scrotal content of Tl-201 and Ga-67 after intravenous administration. Int J Nucl Med Biol. 1981;8:211–213.[Medline] 3. Stabin MG. MIRDOSE: personal computer software for internal dose assessment in nuclear medicine. J Nucl Med. 1996; 37:538–546.[Medline]. 4. (Cristy M, Eckerman K. Specific Absorbed Fractions of Energy at Various Ages from Internal Photon Sources ORNL/TM-8381 V1–V7. Oak Ridge, TN: Oak Ridge National Laboratory; 1987). 5. Rao DV, Shepstone BJ, Wilkins HB, Howell RW. Kinetics and dosimetry of thallium-201 in human testes. J Nucl Med. 1995;36:607–609.[Abstract]. 6. Rao DV, Govelitz GF, Sastry KSR. Radiotoxicity of thallium-201 in mouse testes: inadequacy of conventional dosimetry. J Nucl Med 1983;24:145–153.[Abstract] 7. Nettleton JS, Lawson RS, Prescott MC, Morris ID. Uptake, localization, and dosimetry of In-111 and Tl-201 in human testes. J Nucl Med. 2004;45:138–146.[Abstract/Free Full Text] Read More