Is diagnostic iodine-131 scanning useful after total thyroid ablation for differentiated thyroid cancer?

A diagnostic iodine-131 (I-131) total body scan (TBS) is usually recommende d 6 to 12 months after thyroid ablation for differentiated thyroid carcinom a. Its usefulness was evaluated in 256 consecutive patients treated and fol lowed up at the Institut Gustave Roussy for papillary (n = 200), well diffe rentiated (n = 27), or poorly differentiated (n = 29) follicular thyroid ca rcinomas. All patients underwent a near total or total thyroidectomy and I-131 ablati on with 3.7 GBq (100 mCi). No TBS was performed before I-131 ablation. The TBS performed after the administration of I-131 to destroy the thyroid remn ants showed uptake (<2%) limited to the thyroid bed. A diagnostic I-131-TBS was obtained after withdrawal of T-4 treatment, with either 74 MBq (2 mCi; n = 82) or 185 MBq (5 mCi; n = 174), 6 to 12 months after initial treatmen t, with serum thyroglobulin (Tg) determination. No interference in the Tg a ssay was found in these 256 patients. Uptake in the thyroid bed was not detected (total ablation) in 236 patients , was visible but too low to be measured in 19 patients, and attained 1% in only 1 patient. No uptake was found outside the thyroid bed. The serum Tg level, once thyroid hormone treatment had been withdrawn, was below 1 ng/mL in 210 patients, ranged from 1-10 ng/mL in 31 patients, and was above 10 n g/mL in 15 patients. A I-131-TBS performed with 3.7 GBq in nine patients wi th a Tg level above 10 ng/mL, showed foci of uptake outside the thyroid bed in three patients; lung metastases were demonstrated by a CT scan in anoth er patient, and palpable lymph node metastases were found in one patient. In conclusion, a diagnostic I-131-TBS With 74-185 MBq performed 1 yr after thyroid ablation demonstrated no abnormal uptake; it did not correlate with results of Tg determination and only confirmed the completeness of thyroid ablation. The serum Tg level obtained after withdrawal of T-4 treatment pe rmits the selection of patients with a Tg level exceeding 10 ng/mL, for sca nning with 3.7 GBq (100 mCi).