Background: Success rate of thyroid remnant ablation in patients with low-risk differentiated thyroid cancer (DTC) is commonly based on measurement of serum thyroglobulin levels and 185 MBq (5 mCi) diagnostic 131I scanning or neck ultrasound, performed 6-9 months after ablation. In the present study, we report the rates of successful 131I ablation based on a 5550 MBq (150 mCi) post-therapy scan performed 6-9 months after ablation.
Methods: Retrospective cohort study of 77 adult patients with DTC, stage T1-T3, N0 or N1, M0, demonstrating thyroid remnant uptake one week after a 2775 MBq (75 mCi) ablation dose. Six to nine months later, all patients received a 5550 MBq dose of 131I, followed by a post- therapy scan after one week. Complete thyroid ablation was defined as no thyroid remnant uptake and a thyroglobulin level < 0.2 μg/l after thyroid hormone withdrawal.
Results: Thyroid ablation was complete in 20 patients (26%). Forty-eight patients (62%) demonstrated persistent remnant uptake. This was associated with thyroglobulin levels > 0.2 μg/l in 24/48, and positive thyroglobulin antibodies in 4/48 patients.
Conclusion: Thyroid remnant ablation success assessed by 5550 MBq post-therapy scanning was much lower than reported in studies evaluating ablation success based on 185 MBq diagnostic 131I scanning or neck ultrasound. The latter techniques may be too inaccurate to detect thyroid remnants and thus may not be sufficiently reliable to predict long-term disease outcome.