Re: Repeated I-131 thyroid carcinoma therapies

[Jocelyn Towson <jtowson@nucmed.rpa.cs.nsw.gov.au>]

>|TAB|Our Nuc Med Chief has brought to my attention the case of
>a patient who has received six or seven I-131 therapies already,
>each 6 mos. - 1 yr. apart, and whom she is considering treating
>again tomorrow.  Her concern is she has found all his blood
>cell levels suppressed now for the first time, in particular
>the platelets.  A recent I-123 scan revealed no significant
>iodine receptive mets in the marrow itself.  Her obvious
>concern is further suppression of bc production, and is
>curious as to a ballpark marrow dose in such a patient per
>mCi I-131.

Several references indicate a bone marrow dose from RAI therapy in thyroid
cancer patients of around 0.08mGy/MBq. (eg Smith et al. Radiat Prot Dosim
1984;5:141-149; McEwan, Brit J Radiol 1977;50:329-331; Gunther et al.
Strahl. Onkol. 1987;163:185-189).  This is more than you would expect for a
euthyroid person, regardless of the amount of I131 accumulated in thyroid
tissue.  The patient is in a hypothyroid state, with a low rate of renal
iodide clearance, at the time of dose administration and for several days
afterwards.  This was confirmed in a study of chromosome aberrations in
peripheral blood samples from thyroid cancer patients, which indicated the
bone marrow dose might be 2-4 times higher than MIRD estimates (M'Kacher et
al. J Nucl Med 1996;37:1860-1864).
Regards
Jocelyn Towson

Jocelyn Towson, RSO
Dept of PET & Nuclear Medicine
Royal Prince Alfred Hospital
Camperdown, NSW 2050
Australia

tel [national]  02 9515 8011    [international] 61 2 9515 8011
fax [national]  02 9515 6381    [international] 61 2 9515 6381
email   jtowson@nucmed.rpa.cs.nsw.gov.au