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To compare the new, 6th edition, UICC TNM staging system with the former edition, we updated TNM staging in patients with differentiated thyroid carcinoma.The new and old TNM classification systems for differentiated thyroid carcinoma were applied in a retrospective analysis of 169 patients who underwent therapy with radioiodine (131I) from 1975 through 2002 at the Department of Nuclear Medicine, Frankfurt.According to the new staging system, 83 patients (49%) were classified as T1 compared to 54 patients (32%) based on the former edition; 32 patients (19%) as T2 compared to 61 (36%) patients formerly. In 44 patients with minimal extrathyroid extension, formerly classified T4, the new TNM staging changed to T3, and no patient was classified T4. The one year relapse-free survival fraction under the former edition staging was 100% for T1 and 92.2% for T2, compared to 96.8% for new edition T1 and 93.3% for T2.The new TNM classification causes a significant change in staging. New T1 classified tumors had a slightly worse relapse-free survival fraction compared with the old T1 carcinomas. For patients treated at our department, the altered criteria for classifying extrathyroid extensions have had only a minor impact on disease management.