Radioactive 131I treatment of feline hyperthyroidism: development of radiation protection guidelines for Dutch legislation based on external dose rate and excretion of 131I in 10 cats

Introduction– 131-Iodine (131I) is an effective treatment for feline hyperthyroidism with minimal morbidity and >95% cure rate. β-radiation of 131I (Eβmax=606 keV), responsible for its therapeutic efficacy, poses an internal radiation exposure risk after excretion with urine/feces. γ-radiation of 131I (Eγ=364 keV) poses an external exposure risk. Radiation protection regulations concerning time of discharge from hospital vary between countries. Tracing 131I excretion in time has only been performed with limited number of measurements of radioactivity levels in excreta and external dose-rate in time, while excreted 131I may be an important risk factor for unwanted public exposure. Methods– 10 cats with hyperthyroidism were treated with 200 MBq 131I and hospitalized for 14 days under constant monitoring of external dose-rate. All urine/feces were collected and radioactivity levels measured to trace total excreted radioactivity and radioactivity/gram. Data were analyzed and biological/effective t1/2 determined. Results– External dose-rate data could be reliably fitted according to a 1st order exponential decay formula with 0.95<R2<0.98 per individual cat and R2>0.99 for the average curve of all cats. Based on these results, an algorithm was developed to determine time of hospital discharge based on external dose rate or injected amount of 131I (MBq). User input parameters for the algorithm include: legal maximum accumulated dose limit for patient-owners (Hmax) and estimated parameters for contact hours/d and fractions of inhaled/ingested 131I from patient-excreta at home. Discussion/conclusion– Considering the current Hmax of 40 µSv, administered 200 MBq and strict instructions for housing and care of patients during the first 2 weeks after discharge, a minimum of 6 days hospitalization is needed to stay under Hmax. Furthermore, 20-22 MBq 131I will be excreted following discharge, 95% of which will occur during the first two weeks at home. Safety instructions have to be obeyed to prevent unwanted radiation exposure in the public domain.