The prognostic significance of tumor size in T3b differentiated thyroid cancer (DTC) remains debated and underexplored. This study aimed to examine the varying impact of T3b based on tumor size, analyzing disease-specific survival, disease-free survival, and overall survival. A retrospective review of 6282 DTC patients who underwent thyroid surgery at Seoul St. Mary’s Hospital from September 2000 to December 2017 was conducted. T3b was classified into three subcategories, T3b-1 (≤2 cm), T3b-2 (2–4 cm), and T3b-3 (>4 cm), using the same size criteria for T1, T2, and T3a. T3b-1 showed no significant difference in disease specific survival compared to T1, and both disease-free and disease-specific survival curves were sequentially ranked as T1, T3b-1, T2, T3a, T3b-2, and T3b-3. The modified T category, reclassifying T3b-1 as T1, demonstrated superior staging performance compared to the classic T category (c-index: 0.8961 vs. 0.8959 and AUC: 0.8573 vs. 0.8518). Tumors measuring 2 cm or less within the T3b category may require downstaging, and a modified T category could improve the precision of prognostic staging compared to the current T category.
肿瘤大小对T3b分化型甲状腺癌（DTC）的预后意义仍存在争议且研究不足。本研究旨在探讨基于肿瘤大小的T3b亚类对疾病特异性生存率、无病生存率和总生存率的影响差异。研究回顾性分析了2000年9月至2017年12月在首尔圣母医院接受甲状腺手术的6282例DTC患者。采用与T1、T2和T3a相同的肿瘤大小标准，将T3b细分为三个亚类：T3b-1（≤2 cm）、T3b-2（2-4 cm）和T3b-3（>4 cm）。T3b-1在疾病特异性生存率方面与T1无显著差异，且无病生存率与疾病特异性生存率曲线均呈现T1、T3b-1、T2、T3a、T3b-2、T3b-3的依次排序。将T3b-1重新归类为T1的改良T分期系统，相较于经典T分期展现出更优的预后分层效能（c指数：0.8961 vs. 0.8959；AUC：0.8573 vs. 0.8518）。T3b分期中肿瘤直径≤2 cm的病例可能需要降期处理，改良T分期系统较现行T分期能提升预后评估的精确性。
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