Re: Thyroid radiation doses are much too high

[Carol Marcus <csmarcus@ucla.edu>]

At 07:36 AM 3/7/2004, Fred Dawson wrote:

>New Scientist reports 6 March that Thyroid radiation doses are much too high





Dear Radsafers:



I don't understand this thread at all, or maybe I'm missing 

something.  When Graves' disease is adequately treated with I-131 NaI, the 

thyroid gland is gone.  Burned away.  Otherwise, it is likely that the 

Graves' disease will recur.  You can't get cancer from tissue that is 

gone.  You can't get thyroid cancer from adequate treatment of Graves' disease.



Years ago, a friend of mine who is a leading nuclear medicine physician was 

asked what he would do if his service administered 20 mCi I-131 NaI to the 

wrong patient.  He immediately quipped, "I'd give him another 20 mCi!"  The 

idea is to wipe out the gland so that there would be no possibility of 

thyroid cancer in the future. In any case, I-131 is very inefficient at 

causing thyroid cancer.  It appears to differ from external beam radiation 

in this respect.  I-131 induced thyroid cancer is 0.05 cases/year/million 

persons per rem for adults (Maxon HR, Thomas SR, Saenger EL, et al.: Amer. 

Jour. Med. 63:967-978, 1977).



Soon after I-131 was introduced widely for treatment of hyperthyroidism 

(after WWII), concerns about radiation-induced cancer prompted a large 

study of patients treated for hyperthyroidism with I-131 vs. those treated 

with surgery.  The end point was of course not thyroid cancer, but 

leukemia.  There was no increase in leukemia in the group treated with 

I-131 vs. surgery.  (Thompkins E: Late effects of radioiodine therapy. AEC 

Symp 20, 431-440, 1970).



A large Swedish study and an FDA study on American children given I-131 for 

diagnostic purposes failed to show any increase in thyroid cancer (these 

patients had thyroid tissue).



Physicians who give calculated doses of I-131 for Graves' disease commonly 

have to give a second or a third dose to do the job.  Some (like me) 

"doctor up" the parameters for the "calculated dose".  Most patients who 

receive 29.9 mCi (the maximum for outpatient treatment determined by NRC's 

old scientifically unfounded "30 mCi rule") burn out their thyroids the 

first time.  It is because no one has ever scientifically credibly 

documented any deleterious effects from the large "one size fits all" doses 

of I-131 that there are still physicians on both sides of the issue.  I 

have used an initial large dose when it was important to get the patient 

hypothyroid as quickly as possible, and on synthroid at that point.  I 

think that both strategies are valid.  There are of course patients who 

need more than 29.9 mCi, sometimes much more, but that is another 

issue.  For these patients, calculated doses are the way to go.



So, I think that the "study" we are discussing is probably junk science.



Ciao, Carol



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