No adverse affect in clinical outcome using low preablation diagnostic (131)i activity in differentiated thyroid cancer: refuting thyroid-stunning effect

Published studies of thyroid stunning due to preablation (131)I scanning in the treatment of differentiated thyroid cancer after thyroidectomy had shown inconsistent clinical impact. The objective of the study was to evaluate the clinical outcome in patients who were given a low diagnostic (131)I activity (1.1 mCi or 40 MBq) 6 days prior to radioiodine ablation (RAI). 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 (131)I activity. Three hundred five consecutive patients treated in 2004-2008 (group A) had a diagnostic activity 1.1 mCi of (131)I prior to RAI. The second cohort treated in 2009-2011 (group B) consisted of 237 patients who did not undergo diagnostic (131)I scanning prior to RAI. 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. 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). The use of low diagnostic (131)I activity (1.1 mCi) given 6 days prior to RAI was safe and convenient without adversely affecting the long-term clinical outcome.