RE: medical misadministration to child

[Carol Marcus <csmarcus@ucla.edu>]

At 04:29 PM 4/7/03 -0400, North, David wrote:

>Speaking as someone who has been involved in clinical nuclear medicine for

>almost 30 years, I can state that it is indeed common to image the thyroids

>of euthyroid adults (rare for children). It is really the only way to

>evaluate the functional status of one or more nodules which may have been

>picked up by palpation or ultrasound. A prescribed activity of 4 uCi of

>I-131 is quite reasonable for an uptake study on a child, given that 10 uCi

>would be a common dose for an I-131 uptake on an adult. One question would

>be why I-131 was used rather than I-123, but I think the latter is

>contraindicated if part of the thyroid gland is suspected to be substernal.

>Sodium pertechnetate-Tc99m can be used for thyroid imaging and uptakes, but

>it is only trapped by the thyroid, not organified into hormone, and released

>unchanged. So, you don't get a complete picture of the function.

>

>David L. North, Sc.M., DABR

>Medical Physics

>Main Bldg Rm 317

>Rhode Island Hospital

>593 Eddy St.

>Providence, RI 02903

>(401)444-5961

>dnorth@lifespan.org

>

>

> > ----------

> > From:         John Jacobus

> > Sent:         4, April 2003 16:08

> > To:   William V Lipton; North, David

> > Cc:   radsafe@list.vanderbilt.edu

> > Subject:      Re: medical misadministration to child

> >

> > An even better question is why was the scan being done.  It is not common

> > to perform a diagnostic scan on a euthyroid patient, adult or child.  If

> > the suspision is that the thyriod is cancerous, the thyroid would have

> > been surgically removed and I-131 given to destroy any remenants that

> > remained.

> >

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Dear Radsafers:



There is a medical myth that is decades old that you need the higher energy 

photon of I-131 to image substernal thyroid, and that I-123 cannot be used 

because its gamma is just a tad higher than that of Tc-99m, which indeed 

cannot be used to find substernal thyroid.  However, the reason why Tc-99m 

fails is not the strength of its gamma, but its low target/background 

ratio.  That is, there is so much relative pertechnetate in blood pool that 

the relatively low uptake of the substernal thyroid tissue makes it very 

difficult or impossible to differentiate from the mass of other tissue 

containing pertechnetate.  I-123, which is both trapped and organified, 

gives lovely images of substernal thyroid, much nicer than the I-131 ones, 

by the way.



For uptakes and scans, I do not see any reason to use I-131.  For 

metastatic surveys, however, I prefer it because some cancer tissue 

concentrates the iodine so slowly that you need to wait days to a week to 

get good information on the images, and the halflife of I-123 is too 

short.  I have seen false negatives with I-123.  These are, of course, 

patients who have had their thyroid glands removed.



As far as this case goes, the 400 uCi quantity of I-131 is strange.  Such a 

dose of NaI-131 is not used for anything, except perhaps for a metastatic 

survey where the physician is a fervent believer in thyroid stunning (I'm 

not).  However, my adult dose of I-123 for uptake and scan is 400 uCi, and 

I wonder whether the problem was a mix-up involving the two 

radionuclides.  IMHO, there isn't enough given about this case to say much 

about it.



Ciao, Carol



Carol S. Marcus, Ph.D., M.D.

<csmarcus@ucla.edu>



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