Evaluating Feline Release Criteria Following 131 I Therapy for Hyperthyroidism

Hyperthyroidism is a very common endocrine disorder in both humans and cats. Radioactive iodine ablation therapy is the preferred treatment option because of its high success rate; however, the patient becomes a radiation source and poses a potential radiological risk to others. The regulations governing release criteria differ between human medicine and veterinary medicine. A human patient can receive up to 1,200 MBq of 131 I and be discharged the same day; yet a cat can receive a dose as low as 74 MBq and require weeks of hospitalization. This discrepancy has not been satisfactorily addressed; and overly restrictive release criteria can place a burden on the veterinary staff, the cat, and the owner. In this study, administered activities and exposure rates are measured for hyperthyroid cats undergoing treatment at Louisiana State University Veterinary Teaching Hospital to determine if current criteria are too restrictive for releasing cats to their owners. Additionally, radioassays are performed on the surface of the cat, its excreta, and its environment to characterize the potential exposures to the cat owners. Annual total effective dose equivalents to the cat owners are calculated using the Nuclear Regulatory Commission's equation from Regulatory Guide 8.39 Revision 1. The results of the assays show that minimal radioactivity is present post-treatment. The results of the total effective dose equivalent estimates indicate that most cats can be released the same day of injection and that the resulting total effective dose equivalent to the cat owner is unlikely to exceed 1 mSv, suggesting that current veterinary release criteria are overly conservative.