Validation of the Ninth Edition TNM Classification for Thymic Carcinoma After Complete Resection in a High-Volume Center

Objective This study aimed to externally validate the 9th edition TNM staging system for thymic carcinoma (TC) in an independent Chinese cohort. Methods We retrospectively analyzed 245 patients with TC who underwent complete surgical resection between 2010 and 2023. Tumors were staged according to the 8th and 9th TNM editions. Overall survival (OS) and disease-free survival (DFS) were assessed using Kaplan–Meier analysis. Model performance was evaluated using the concordance index (C-index), Akaike Information Criterion (AIC), R 2 , and time-dependent area under the ROC curve (AUC). Results Compared with the 8th edition, the 9th edition showed improved discrimination for OS, with a higher C-index (0.724 vs. 0.701), lower AIC (565.81 vs. 568.34), higher R 2 (0.131 vs. 0.121), and superior 5-year AUC (0.693 vs. 0.674). Significant differences in OS and DFS were observed between most adjacent stage groups(all P<0.05), except between stages III and IV. Lung or phrenic nerve invasion was associated with more favorable DFS (5-year DFS: 47.3% vs. 27.1%, P=0.032), supporting reclassification from T3 to T2. Among patients with T1 disease, the Masaoka–Koga staging system provided additional prognostic stratification for DFS, distinguishing between stage I and stage ≥II (5-year DFS: 85.8% vs. 56.2%, P=0.021), whereas no significant difference was observed in OS. Conclusions The 9th TNM edition provides improved prognostic discrimination after complete resection, although differentiation between stages III and IV remains limited. The Masaoka–Koga system offers complementary value in early-stage disease, particularly for DFS.