Influence of Diagnostic Radioiodines on the Uptake of Ablative Dose of Iodine-131

The uptakes of thyroablative doses of 131 I by postoperative thyroid remnants and/or thyroid carcinoma metastases following diagnostic surveys with 131 I or 123 I were retrospectively compared by visual inspection. Only those patients with a diagnostic scan demonstrating functioning tissue, remnant, and/or metastasis, following thyroidectomy for differentiated thyroid carcinoma, were evaluated. The 131 I survey group ( n = 26) had received a diagnostic dose of 3 to 10 mCi of 131 I. The 123 I group ( n = 14) had received the usual diagnostic dose of 300 μCi of 123 I. The age, sex, and tumor type in the two groups were not statistically different. The uptake of the ensuing thyroablative dose of 131 I appeared, by visual inspection, to be impaired in 20 of 26 patients in the 131 I group and in none of the 14 patients in the 123 I group ( p < 0.00003). In the 131 I group there was suggestion of a dose-response, that is, the higher the administered activity of 131 I for the diagnostic scan, the more reduced was the subsequent apparent uptake of the thyroablative dose ( p = 0.0007). Thyroid remnants or cervical lymph node metastases appeared to be affected more frequently than were the distant (pulmonary or skeletal) metastases ( p = 0.004). This study suggests that iodine uptake function may be suppressed by the absorbed radiation from the 3 to 10 mCi "diagnostic" scanning dose of 131 I. In this regard, 123 I may be a better initial diagnostic agent to be used prior to radioablation therapy.