Determination of Release Criteria for I-131 Therapy Cats

Miles K.G., Simpson S.A., Zimmerman R.B., et al.

Conference Proceedings, (2004). American College of Veterinary Radiology, Montreal:


INTRODUCTION/PURPOSE: Radioactive iodine (I-131) therapy represents the current gold standard treatment option for feline hyperthyroidism. At present, release criteria for treated cats are non-uniform, poorly standardized, and largely extrapolated from experiences in treating humans with hyperthyroidism. At our institution, the duration of radiation isolation confinement has been determined by a < 2 mrem/hr (20uSv/hr) dose rate at patient surface resulting in an average total hospitalization period of 14 days. Hospitalization fees comprise 35% of the total cost of therapy and substantial owner anxiety may occur while they are separated from their pet. The objective of the present study was to prospectively collect feline patient dose rates and owner demographic data that could be analyzed to determine client exposure. Resulting data has been submitted to the Iowa Department of Public Health, Bureau of Radiological Health to support a change in our licensure and permit cat hospitalization time to be decreased.

METHODS: The owners of 30 hyperthyroid cats treated with radioactive iodine were given a 13-item questionnaire at the time of discharge to be completed on-site. This form evaluated a spectrum of demographic parameters including owner age (e.g., adults versus minors), the estimated time that the owner would spend daily in close physical contact with their cat (petting, grooming, holding), whether or not cats were allowed to sleep with the owners, if cats were confined entirely indoors or spent time outdoors, and whether indoor-confined cats were confined to a single room or allowed access to all parts of the house. Survey completion time was approximately 10 minutes. Owner responses were tabulated and statistically integrated with patient dose rate data obtained at patient surface, 1 foot and 3 foot distances.

RESULTS: Cats were treated with an average of 4 mCi (148 MBq) of I-131; range 3-4.5 mCi (111-167 MBq). The average effective half-life of I-131 in the cats was 2.3 days. The vast majority (> 96%) of households surveyed were comprised solely of adult (> 18 years of age) owners. Eighty-seven percent of owners planned to spend 6 or fewer hours daily in close physical contact with their pet. Fifty-seven percent planned to allow the cat to sleep with a member of the household. Only 13% of owners planned to spend 12 or more hours daily in close physical contact with their cat; most owners in this group stated that their cat slept on the bed. Lastly, the majority (77%) of indoor cats were not confined to a single room but were allowed to roam throughout the house. DISCUSSION/CONCLUSIONS: The final release criteria recommended following statistical analysis of the patient dose rate data with respect to owner contact time is a dose rate of 5 mrem/hr (50uSv/hr) at one foot (30 cm). These criteria are based on maintaining a total effective dose equivalent to any member of the general public to less than 100 mrem (1mSv). A minimum 3 day confinement in radiation isolation will be required, with the owners restricted from sleeping with the pet for seven days post- release. These revised guidelines will sharply reduce the length of hospitalization subsequent to 1-131 injection at our institution.