Diagnosis
Iodine-131 whole body images from the 3 and 7 day diagnostic study and the 7 day post therapy study have been combined in composite anterior (54k bytes) and posterior (59k bytes) image sets. The 3 and 7 day images from the diagnostic study show two regions of focal uptake in the neck and mediastinum (arrows, 63k bytes). The stomach (thin arrow) and the bladder (arrow head) are seen on the 3 day images. The post therapy study shows the same two regions, but in addition, more extensive metastatic disease with numerous regions of increased uptake in the neck mediastinum and lungs. Faint liver activity is seen on the post therapy study. (Liver activity is usually associated with metabolism of the thyroid hormone produced by the functioning metastases.)
Post therapy imaging frequently shows more extensive disease than is seen on diagnostic imaging. There are several differences between diagnostic and post therapy images which might account for the increased sensitivity of post therapy imaging. The dose for diagnostic imaging is 2-10 mCi (5 mCi in this patient); therapeutic doses are often in the range of 30-200 mCi (400 mCi in this patient). Diagnostic imaging is usually performed 2 or 3 days after administration of iodine (3 and 7 days in this patient); post therapy imaging is typically performed 7-14 days after administration (7 days in this patient). Therapy usually follows diagnostic administration allowing for more stimulation by TSH (8 days in this patient). Of these factors, the most important in this patient may have been the large difference in dose 5 versus 400 mCi.
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J. Anthony Parker, MD PhD, Tony_Parker@bih.harvard.edu