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Relationship between orally administered dose, surface emission rate for gamma radiation, and urine radioactivity in radioiodine-treated hyperthyroid cats

Daniel A. FeeneyDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Carl R. JessenDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Ralph C. WeichselbaumVeterinary Imaging Consultants, 2932 108th Ln NW, Coon Rapids, MN 55433.

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Daniel E. CronkDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Kari L. AndersonDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Abstract

Objective—To determine the relationship between surface emission rate of gamma radiation and urine concentration of I131 (urine radioactivity) during the period 7 to 21 days after oral or SC administration of I131 to hyperthyroid cats.

Animals—47 hyperthyroid cats administered I131 PO and 24 hyperthyroid cats administered I131 SC.

Procedure—A dose of I131 (1.78 to 2.04 X 102 MBq [4.8 to 5.5 mCi]) was administered orally. Surface emission at the skin adjacent to the thyroid gland on days 7, 10, 14, 18, and 21 and number of counts/30 s in a urine sample (1 mL, obtained via cystocentesis) on days 7, 14, and 21 after oral administration were measured. Effective half-life (T1/2E) was derived for each point. Surface emission thresholds for maximum urine radioactivity values were established. A dose of I131 (1.48 X 102 MBq [4.0 mCi]) was administered SC. Urine radioactivity and surface emission rates for SC administration were compared with values for oral administration.

Results—The T1/2E for surface emissions and urine radioactivity progressively increased toward values for physical T1/2 over time. The T1/2E for surface emissions was 2.19 to 4.70 days, and T1/2E for urine radioactivity was 2.16 to 3.67 days. Surface emission rates had a clinically useful threshold relationship to maximum urine concentrations of I131.

Conclusions and Clinical Relevance—Surface emission rates for cats administered I131 appeared useful in determining upper limits (threshold) of urine radioactivity and are a valid method to assess the time at which cats can be discharged after I131 administration. (Am J Vet Res 2003;64:1242–1247)

Abstract

Objective—To determine the relationship between surface emission rate of gamma radiation and urine concentration of I131 (urine radioactivity) during the period 7 to 21 days after oral or SC administration of I131 to hyperthyroid cats.

Animals—47 hyperthyroid cats administered I131 PO and 24 hyperthyroid cats administered I131 SC.

Procedure—A dose of I131 (1.78 to 2.04 X 102 MBq [4.8 to 5.5 mCi]) was administered orally. Surface emission at the skin adjacent to the thyroid gland on days 7, 10, 14, 18, and 21 and number of counts/30 s in a urine sample (1 mL, obtained via cystocentesis) on days 7, 14, and 21 after oral administration were measured. Effective half-life (T1/2E) was derived for each point. Surface emission thresholds for maximum urine radioactivity values were established. A dose of I131 (1.48 X 102 MBq [4.0 mCi]) was administered SC. Urine radioactivity and surface emission rates for SC administration were compared with values for oral administration.

Results—The T1/2E for surface emissions and urine radioactivity progressively increased toward values for physical T1/2 over time. The T1/2E for surface emissions was 2.19 to 4.70 days, and T1/2E for urine radioactivity was 2.16 to 3.67 days. Surface emission rates had a clinically useful threshold relationship to maximum urine concentrations of I131.

Conclusions and Clinical Relevance—Surface emission rates for cats administered I131 appeared useful in determining upper limits (threshold) of urine radioactivity and are a valid method to assess the time at which cats can be discharged after I131 administration. (Am J Vet Res 2003;64:1242–1247)