The clinical effects of thyroid stunning after diagnostic whole-body scanning with 185 MBq 131I

Thyroid stunning from diagnostic iodine-131 imaging prior to ablative therapy with I-131 for well-differentiated thyroid carcinoma has been well reported, but documentation of the effect on clinical outcome is sparse. The purpose of this retrospective study was to investigate the clinical effects of stunning. The outcome of I-131 ablative therapy in a group of patients (n=36) who had diagnostic scans using 185 MBq (5 mCi) of I-131 was compared with that in a group (n=36) who had diagnostic scans using 740 MBq (20 mCi) of I-123. Patients were imaged at least 4 weeks after near-total thyroidectomy, prior to their first I-131 ablative therapy. Follow-up imaging was performed every 3-6 months, and further I-131 treatment administered when indicated. A group of patients (n=36) who proceeded directly to their first therapy dose without a diagnostic scan and were followed up with I-123 was compared with the group who did have a I-123 diagnostic scan prior to the first ablative therapy. The efficacy of therapy was evaluated using ablation of the thyroid, evidenced by absence of uptake in the thyroid bed on the diagnostic scan, as the endpoint. Only 47% of patients in the I-131 diagnostic group had the thyroid gland ablated after a single administration of I-131 therapy, compared with 86% in the I-123 diagnostic group (P<0.005). Patients who had I-131 diagnostic scans required higher total I-131 therapeutic activity (6.7 GBq or 180 mCi) to ablate the thyroid gland than those in the I-123 diagnostic group (4.4 GBq or 119 mCi). There was no difference in outcome between the group who did and the group who did not have a diagnostic study with I-123 prior to their first ablative therapy. The difference in outcome between the I-131 and the I-123 diagnostic groups demonstrates that the efficacy of I-131 therapy is reduced subsequent to the use of 185 MBq of I-131 for diagnostic imaging. This indicates that the phenomenon of stunning is clinically significant and affects the outcome of therapy.