No Adverse Affect in Clinical Outcome Using Low Preablation Diagnostic 131I Activity in Differentiated Thyroid Cancer: Refuting Thyroid-Stunning Effect

Context: Published studies of thyroid stunning due to preablation I-131 scanning in the treatment of differentiated thyroid cancer after thyroidectomy had shown inconsistent clinical impact. Objective: The objective of the study was to evaluate the clinical outcome in patients who were given a low diagnostic I-131 activity (1.1 mCi or 40 MBq) 6 days prior to radioiodine ablation (RAI). Design/Setting: Two cohorts of patients were treated in a cancer referral center in 2004-2011. The eligibility criteria were as follows: 1) diagnosis of differentiated thyroid cancer; 2) total or near total thyroidectomy; 3) no distant metastasis; and 4) receiving 82.4 mCi or greater (3050 MBq) therapeutic I-131 activity. Patients/Interventions: Three hundred five consecutive patients treated in 2004-2008 (group A) had a diagnostic activity 1.1 mCi of I-131 prior to RAI. The second cohort treated in 2009-2011 (group B) consisted of 237 patients who did not undergo diagnostic I-131 scanning prior to RAI. Main Outcome Measures: The tumor recurrence rate at 3 years and quantitative assessment using diagnostic whole-body radioiodine scans and TSH-stimulated thyroglobulin levels at 3-12 months after RAI were measured. Results: The 3-year recurrence-free survival rates were 96.4% in both groups, with 4.3% in group A and 3.4% in group B having tumor recurrence (P = .91). The ablation success rates measured by diagnostic whole-body radioiodine scans were 97.6% and 100% and by stimulated thyroglobulin were 85.3% and 85.8% in group A and B, respectively (P = .62). Conclusions: The use of low diagnostic I-131 activity (1.1 mCi) given 6 days prior to RAI was safe and convenient without adversely affecting the long-term clinical outcome.