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Measured I-125 Contamination in I-123

B Lofton

B Lofton*, G Gibbs , J Stickel , K Lofton , Colorado Associates in Medical Physics, Colorado Springs, CO


TU-C3-GePD-IT-3 (Tuesday, August 1, 2017) 10:30 AM - 11:00 AM Room: Imaging ePoster Theater

Purpose: Na I-123 is a cyclotron-produced radiopharmaceutical administered orally to determine thyroid function and/or morphology. Complete cessation for breastfeeding patients due to the potential for I-125 contamination is generally recommended to limit thyroid dose to nursing infants. I-125 contamination is commonly estimated to be 2.5% [1], although this is likely dependent on methods on I-123 production. This study evaluates the accuracy of this widely accepted contamination assumption.

Methods: Thirteen Na I-123 samples were decayed at minimum 80 half-lives and analyzed using Germanium gamma spectrometry using the EPA 901.1 method. The samples were obtained from unused capsules manufactured by Cardinal Health Radiopharmaceuticals. The contamination results were then decay-corrected to the original calibration date of the sample to determine contamination of I-125 as a ratio to I-123. Additionally, breastmilk samples were collected from a patient who had been administered 370 μCi of Na I-123 while breastfeeding and submitted for gamma spectrometry.

Results: Of the Na I-123 capsules submitted for analysis, the maximum ratio of I-125 μCi to I-123 μCi was 0.004. The highest decay-corrected activity of I-125 determined to be in the sample was 0.34 μCi. The estimated contamination level observed in the breastmilk sample was 1.2 pCi/ml. The total thyroid dose equivalent is estimated at 13 mSv over the course of a year, not accounting for additional biological clearance, and the annual effective dose estimate is 0.41 mSv. Using Bier VII risk estimates, this results in an increased risk of fatal cancer of 0.005%.

Conclusion: Both sets of results indicate that the previous I-125 contamination assumptions may be overly conservative. More research needs to be conducted to correlate I-125 contamination with manufacturing method, but these initial findings present some evidence that complete cessation of breastfeeding may not be required if the contamination level of I-125 can be estimated prior to administration.

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