131I therapeutic efficacy is not influenced by stunning after diagnostic whole-body scanning

PURPOSE: To determine if stunning can be seen with a 185-MBq (5-mCi) dose of iodine (131) I-131 at diagnostic whole-body scanning and, if stunning is seen, determine if there is any I-131 therapeutic efficacy. MATERIALS AND METHODS: A retrospective review of findings involving 166 patients who underwent thyroidectomy for differentiated thyroid carcinoma was performed. Diagnostic I-131 scans were compared with postablation scans for evidence of stunning. Stunning was defined when the diagnostic scan showed activity that was subsequently decreased on the postablation scan. The sample population was divided into two groups: group NS, patients with no stunning, and group S, patients with stunning. Patients were considered successfully treated if no functioning thyroid tissue and/or metastases were seen on follow-up diagnostic scans. Fisher exact and Student t tests were used to evaluate the statistical significance of therapy success rates, clinical characteristics, and scanning parameters between the two groups. RESULTS: Group NS included 135 (81.3%) of 166 patients, with 36 (26.7%) of 135 lost to follow-up. Group S included 31 (18.7%) of 166 patients, with eight (26%) of 31 patients lost to follow-up. There was no significant difference (P = .61) in treatment success rates between group NS (87 of 99, 88%) and group S (21 of 23, 91%). The treatment success rates for thyroid remnants were 87% (48 of 55) for group NS and 91% (10 of 11) for group S (P = .63). Treatment success rates for metastases (mostly lymph nodes) were 89% (39 of 44) for group NS and 83% (10 of 12) for group S (P = .55). CONCLUSION: Thyroid stunning can occur with 185 MBq of I-131 diagnostic imaging. However, data did not show any effect of stunning on the efficacy of I-131 therapy for differentiated thyroid carcinoma. (C) RSNA, 2004.