T. Michael Martin, L. Vasudevan, S.S. Chirayath,
"Correlation between Exposure Rate and Residual Activity in Felines undergoing I-131 Thyroid Ablation Therapy,"
Radioiodine thyroid ablation therapy is a common method for
treatment of felines exhibiting hyperthyroidism. Due to the high
gamma-ray emission rate of radioiodine (I), patients following
treatment must be held in isolation for several days before release
to prevent unnecessary dose to owners and members of the public.
Dose rate measurement on the external surface of the patient of ≤
20 μSv h is maintained as the patient release criterion without
regard to residual activity. However, the Texas Department of State
Health Services regulatory guide recommends a release limit of 3.7
MBq to households with non-pregnant women and children over the age
of 18 y, and a limit of 925 kBq to households of pregnant women and
children who can be supervised. In this paper, Monte Carlo
computational radiation transport techniques are employed to
predict and standardize the patient isolation time at the clinic by
correlating the thyroid burden and surface dose rates of felines.
Measurements of patient dose rate as a function of time are used to
determine the patient-specific effective half-life experimentally
and to validate the model results. Results show that an average
holding time of 8 to 9 d is sufficient to reduce the residual
activity to 3.7 MBq levels. Additionally, contact dose rate
measurements of 20 μSv h or less correlate to residual activity
levels of approximately 925 kBq. Based on the model and
measurements, a protocol was developed for clinical use at Texas
A&M University Veterinary Medical Teaching Hospital to allow
estimation of residual activity following injection. This in turn
confirms that the surface dose rates used as the release criteria
follow the release limits recommended in the regulatory guide.