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


METHODS
47 hyperthyroid cats administered I131 PO and 24 hyperthyroid cats administered I131 SC.


METHODS
A dose of I131 (1.78 to 2.04 X 10(2) 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 10(2) 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
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.
Feeney
Daniel A
DA
Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108, USA.
Jessen
Carl R
CR
Weichselbaum
Ralph C
RC
Cronk
Daniel E
DE
Anderson
Kari L
KL
D000818:Animals; D002371:Cat Diseases / Q000532:radiotherapy*; D002415:Cats; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays; D006980:Hyperthyroidism / Q000532:radiotherapy / Q000662:veterinary*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics* / Q000627:therapeutic use* / Q000652:urine; D008657:Metabolic Clearance Rate; D011879:Radiotherapy Dosage
D007457:Iodine Radioisotopes