Comparison of the distribution of diagnostic and thyroablative I-131 in the evaluation of differentiated thyroid cancers.

In 206 patients with differentiated thyroid cancer, the distribution of iodine-131 were compared after diagnostic (200-500 microCi) and thyroblative (approximately 100 mCi) doses. In the diagnostic scans, only normal thyroid tissue could be seen, whereas in one-fourth of the patients the therapeutic scans showed tumor tissue as well, usually in lymphnode metastases. In 16% of patients, the therapeutic scan was the only way to demonstrate the presence of tumor tissue, since no further uptake was achievable. In patients in whom all tumor was believed to have been removed by surgery alone, a "preventive" I-131 ablation was used, and in 16 of these 97 patients tumor was revealed in the therapeutic scan. In ten more, tumor was found in subsequent followup scans, its functioning having been induced by destruction of postsurgical remnants of normal thyroid. Some possible explanations for the differences in scans are proposed, and the importance of therapeutic scans for correct staging of thyroid cancer is stressed.