How VICSD’s 4/7 mCi Dosing Protocol Was Born

Dr. Seth Wallack recently presented VICSD’s original research on the thyroid to background ratio and how using this ratio to guide I-131 treatment has resulted in a 98% first-time success rate (one of the highest in the country) during the ACVR 2010 Annual Scientific Meeting in August. You can read the abstract on pages 58 and 59 in the ACVR 2010 Annual Scientific Meeting Program PDF.

Key points to I-131 Therapy:

• The thyroid to salivary ratio IS NOT a statistically accurate predictor for treatment dosage. Therefore, it should not be used to determine I-131 dosage.

• The thyroid to background ratio IS statistically proven to be an accurate predictor for treatment dosage. Therefore, it should be used to determine I-131 dosage.

• Using the thyroid to background ratio and a 4mCi/ 7mCi dosing protocol has resulted in a 98% success rate for first time I-131 treatment.

The inception of the thyroid to background ratio came early in our company’s history. When we began treating hyperthyroid cats in 2005, we followed the I-131 treatment protocol that nearly all other treatment facilities use, which was to use a standard 4 mCi I-131 dosage for all cats. We quickly recognized that some of the hyperthyroid cats were not being cured with this dosage. Veterinary literature discusses this problem and several attempts have been made to identify a solution. Some clinics and researchers have achieved success rates approaching 98% using a multivariable approach to treatment including T4, patient weight, methimazole dosage, and the thyroid to salivary ratio.

Interestingly, I-131 treatment failures also occur in people, the only other species with naturally occurring benign hyperthyroidism, and have been extensively studied. In humans, treatment failure has been associated with larger thyroid size and I-131 dosage. In cats, radiologists have used the thyroid to salivary ratio to identify hyperthyroidism for over 30 years. However, using the thyroid to salivary ratio to determine treatment dose is fraught with error due to inconsistent salivary activity. Salivary activity is affected by a cat’s saliva production (less if the cat is dehydrated and more if the cat is nauseous) and swallowing. The background activity made more sense for a ratio.

So, with that our thyroid to background ratio was born. The thyroid to background ratio was put to the test by evaluating the thyroid to background ratio as a predictor for which cats would fail I-131 therapy when a standard 4mCi of I-131 was used. Results showed without a doubt that the higher the thyroid to background ratio the higher the cat’s chance for failure. The results also showed that the thyroid to salivary ratio did not correlate to which cats would fail I-131 treatment using the standard 4mCi dosage. This finding blew the doors off of using the thyroid to salivary ratio to guide treatment (see key points above).

This information changed the way we performed I-131 therapy and resulted in our 4/7 mCi treatment protocol. This treatment protocol, developed right here in San Diego, has resulted in some of the best success I-131 treatment success in the country with 98% of our I-131 patients being cured after the first treatment. So what does that mean for your patients? It means fewer patients having to return for a second treatment. And while we strive for a 100% success rate, one thing we know for certain is that there is 100% owner displeasure when they have to return for another treatment, which is what we want to avoid.

Because of our research and success rate, we are the only I-131 treatment facility in the country to offer an I-131 Therapy Service Guarantee.To read more about our I-131 Therapy program and our Service Guarantee to pet owners, please click here.