What is the role of 1100 MBq (< 30 mCi) radioiodine 131I in the treatment of patients with differentiated thyroid cancer?

Based on pre-therapy whole-body 131I scintiscans showing only residual thyroid, 64 patients with differentiated thyroid cancer were treated with 1100 MBq ( < 30 mCi) 131I as out-patients. A follow-up whole-body scan with uptake measurements was made 6-12 months later. An uptake of <or= 0.3% and a scan showing no abnormal uptake outside of the region of the thyroid bed was accepted as a successful outcome. Four patients were subsequently excluded, three of whom had no repeat uptakes and one developed a local recurrence before the repeat 131I scan could be completed. When uptake of 131I before treatment was restricted to the region of the thyroid and the percentage retained was < or = 10%, the probability of successful ablation was 92%. When the uptake was > 10%, this fell to 59%. A serum TSH > 10 microU ml-1 at the time of the first scan was associated with ablation (i.e. an uptake of < or = 0.3% on the follow-up scan) in 93% of patients. We recommend measurement of TSH and a whole-body scan to exclude regional or distant metastases, plus calculation of uptake in the neck to determine whether out-patient therapy with 1100 MBq is appropriate.