Thyroid stunning in post RA131 treatment in scan

Rustifox said:

Hi, Denyse
Stunning will sometimes occur if a big dose of I-131 is used for scans immediately prior to RAI treatment with I-131 - but it does NOT occur with I 123... I 123 is a much, much lighter dose of iodine - and radiation - than that found in I-131.

I had I123 followed by I-131 for scanning purposes - and the I 123 didn't even obliterate the views for the I-131. (I-123 is less commonly used these days - it's quite expensive, and doesn't give as good images as those found with I-131 for scanning purpose, but it does have lower radiation impacts, too.)

The "stunning' effect is generally only seen when a bigger dose of I-131 only (ie significantly more than 4mCi's - some doctors will use up to 10mCi's for scanning purposes - treatment doses of I-131 normally range from 50 to 150mCi's, by comparison) is used for a scan, then treatment follows within a few days.

The idea here is that the thyroid tissues will only 'suck up' so much iodine (and/or radiation) before they aren't 'hungry' for it - the more starved the tissues, the better the uptake; the hunger for iodine will also 'regrow' rapidly after a dose, particularly a small scanning dose - again, of I 131... if there is ever a question as to whether iodine uptake will be sufficient, a 24 hour iodine uptake test (measuring urine for iodine over a 24 hour period) will tell whether the uptake is sufficient or not... some centers will also use this to determine if a CT scan dye is interfering with uptake, too.

CT dye can also keep our bodies from 'sucking up' the I 131 for anywhere from 3 months to as long as a year, as an example. Even too much dietary iodine can potentially prevent us from 'sucking it up' - thus the reason behind doing a low iodine diet before our scans and treatment doses.

But it is a very, very rare occurance with even I-131 scans, (and literally unheard of with I-123, as I understand it), particularly if you are being cared for a by a team familiar with thyroid cancer... most radiation oncologists and/or endocrinologist's will always take care to scan with very low doses of I-131 to begin with, if treatment is contemplated to follow a scan, or use the (much safer/lower doses) I-123 instead.

But - don't worry about the I-123 at all... that's one of the reasons it is used, as it is purportedly such a benign affect on the thyroid tissues and/or thyroid cancer tissues to begin with, that stunning is not an issue. Hope this helps a bit - all the best with your scans and/or treatment.