Post-surgical ablation of thyroid remnants with high-dose I-131 in patients with differentiated thyroid carcinoma

The aims of this study were to evaluate the efficacy of an empirically dete rmined 'fixed' high ablative dose of radioiodine (I-131) therapy and to det ermine the utility of ultrasonography (US) in dose determination. A retrosp ective analysis was performed of 242 thyroid cancer cases treated with 'fix ed' high-dose I-131 for ablation of thyroid remnants without a pre-ablative I-131 diagnostic scintigraphy or radioiodine uptake study. Treatment doses ranged from 1850 MBq (50 mCi) to 7.4 GBq (200 mCi). The selection of the t reatment dose was based on the surgical and pathological findings as well a s the remnant thyroid volume calculated by US. A successful ablation was de fined as the absence of activity in the thyroid bed on subsequent imaging s tudies. Successful ablation was obtained in 218 of the 242 patients (90%). In 162 of the 218 patients (74.3%), successful ablation was achieved after a single I-131 treatment. The remnant thyroid volume calculated by US was s ignificantly different (P=0.04) between those who were successfully ablated and those who were not. The total I-131 dose needed for successful ablatio n was significantly higher in males (P=0.003). Patients with higher post-op erative thyroglobulin (Tgb) levels and patients with a higher stage of dise ase required higher doses (P=0.036 and P=0.021 respectively). Serum Tgb lev els were under 10 ng.ml(-1) in 220 of the 242 patients (90%) following radi oiodine ablation while not receiving L-thyroxine suppression. Nineteen pati ents (7.8%) showed metastases on post-therapy scan and successful treatment was achieved in 11 of 19 (57.8%). Four of the 19 patients with distant met astases (revealed on post-treatment scan) were found to have been given a t reatment dose of less than 200 mCi based on the proposed empirical approach . These results indicate that 'fixed' high-dose I-131 treatment is clinical ly feasible with an acceptable dose underestimation rate, and the utilizati on of US in the determination of the thyroid remnant volume provides more a ccurate and reproducible results. ((C) 2001 Lippincott Williams & Wilkins).