RADIOIODINE ABLATION OF RESIDUAL TISSUE IN THYROID-CANCER - RELATIONSHIP BETWEEN ADMINISTERED ACTIVITY, NECK UPTAKE AND OUTCOME

A retrospective review was performed to evaluate the effectiveness of radioiodine in the ablation of residual thyroid tissue after surgery f or differentiated thyroid cancer. 121 patients were treated at the Pri ncess Margaret Hospital, Toronto, Canada between 1977 and 1989, with t he activity of radioiodine determined empirically. Ablation of residua l thyroid was determined by I-131 nuclear scans, using absent visible uptake (compared with background) as the criterion for successful abla tion. 97 patients (80%) had successful ablation of residual thyroid ti ssue after the first administration of radioiodine. Patients with high er iodine uptake in the neck had a tendency to receive higher activiti es of I-131. There were no statistically significant differences in ag e, sex, type of surgery, initial iodine uptake in the neck and adminis tered radioiodine activity between those successfully ablated and thos e that were not. Patients receiving less than 3.7 GBq (100 mCi) tended to have lower iodine uptake (<5% at 24 or 48 h), but their rate of th yroid ablation was just as high as those given 3.7 GBq or more. In ord er to take advantage of this, we have formalized our approach by deriv ing guidelines to the empiric determination of radioiodine activity ba sed on the iodine uptake in the neck. Among the 11 patients treated wi th lobectomy only, the ablation rate was 64%. Although this was not si gnificantly lower than for other forms of surgery, we continue to reco mmend completion thyroidectomy for this group of patients, if the goal of treatment is to ablate all thyroid tissue.