[ RadSafe ] thyroid uptakes

[JGarrett at mbhs.org]

The Nuc Med Department recently approached me about an issue they were
having with some of their uptake scans. The situation occurs when a patient
has had a Thyroid ablation performed with I-131 and returns 2 months later
for a 6 hour uptake. Prior to the uptake dose, ~ 12 uCi, thyroid and
background readings are performed. The I-131 is then administered and the
patient returns after 6 hours for his/her uptake reading. The problem they
are having is that the uptake reading is lower than the pre-dose reading
for the Thyroid and higher for the background.

The instrumentation used is a Capintec Captus 500. Since the unit does not
have a feature for calculating the uptake in this situation the Nuc Med
techs were given a formula to compute the uptake. However, the problem is
still the readings. Are we seeing just statistical fluctuations? Or is this
a case of the I-131 did its job and there is no uptake? Is the Thyroid able
to remove I-131 more efficiently?

Another curious thing is the formula provided by the vendor: Correct Uptake
=  [(Net Neck Counts Post  -  Net Neck Counts Pre) / Decay Factor] / Pill
Count. Why would you divide the Pre and Post counts by the decay factor? It
seems like you would only correct one or the other for decay?

Any help would be appreciated. Thanks in advance.

Jeffrey A. Garrett, MS, DABR
Chief Physicist
Mississippi Baptist Medical Center
1225 North State Street
Jackson, MS 39202

Office: 601-968-1725
Cancer Center: 601-968-1416 or 1420
Fax: 601-960-3317