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

Abstract

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.

Recombinant Human Thyrotropin Administration Enhances Thyroid Uptake of Radioactive Iodine in Hyperthyroid Cats

“…This is usually the case at a distance of 1 m, 1 week after treatment in cats, depending on the injected activity. Nonetheless, radioactivity is still found in urine until 21 days after treatment in amounts that require labeling as radioactive material, 9,11,23 and removable radioactivity that can even exceed the maximum acceptable activity for a noncontrolled area is present on the cat during the 1st week after treatment 11,24,25 . The amount of radioiodine administered is the only factor determining the remaining radioactivity and duration of isolation; therefore, administration of the lowest possible activity is beneficial 10 …”

“…This issue was addressed earlier in human medicine when the hazards caused by the excreta and direct radiation from the cat were emphasized 8 . A lower efficacious dose will reduce the surface dose emission rate, radioactivity found in urine and on the hair and paws, and duration of isolation for cats treated with 131 I, thereby conforming to the ALARA principle 9–11 …”

“…8 A lower efficacious dose will reduce the surface dose emission rate, radioactivity found in urine and on the hair and paws, and duration of isolation for cats treated with 131 I, thereby conforming to the ALARA principle. [9][10][11] In humans with nodular goiter, the administration of recombinant human thyroid-stimulating hormone (rhTSH) has been adopted because administration of rhTSH increases radioactive iodine uptake (RAIU) in the thyroid and changes the regional distribution of 131 I in humans with nodular goiter or subclinical hyperthyroidism. [12][13][14][15] This results in lower therapeutic doses needed and less irradiation to extrathyroidal tissue.…”

Feline Hyperthyroidism

“…As such, 131 I-treated cats are a potential source of hazardous radiation to humans and to other animals due to gamma radiation released from 131 I trapped in the thyroid gland of the cat, as well as surface contamination of the cat's coat and paws by urine and feces containing radioactive iodine (so called "removable activity"; Chalmers, 2006). Surface emissions are correlated with urine concentrations of 131 I (Feeney, 2003). Isolation of treated cats in an approved facility is required for a variable period that depends upon the dose administered and state regulations.…”

Correlation Between Exposure Rate and Residual Activity in Felines Undergoing 131I Thyroid Ablation Therapy

“…A large fraction of administered activity is absorbed into thyroid tissues shortly after intake. Preferential uptake into hyperplastic and neoplastic thyroid cells allows efficient administration of dose, while limiting dose to normal (lessactive) tissue (Feeney et al 2003). An additional benefit is that radioactive iodine will also be taken up by any ectopic thyroid tissue (Forrest et al 1996;Peterson 2006).…”