Validation of the 9th Edition of the TNM Classification in Patients with NSCLC and Lymph Node Involvement: A Retrospective, Multicentric, Observational Study

Background: The ninth edition of the TNM classification introduced refinements in nodal staging, subdividing mediastinal N2 disease into N2a (single-station) and N2b (multi-station) involvement, alongside several stage group adjustments. The aim of this study is to validate the new TNM in patients with nodal involvement who underwent surgery. Methods: This is a multicentric, retrospective study including NSCLC patients with pathological N1 or N2 involvement who underwent anatomical pulmonary resection between January 2020 and December 2023. Clinical, surgical, and pathological data were collected, including tumor characteristics, lymphadenectomy details, and adjuvant therapy. Patients were reclassified according to the ninth TNM groups: stage IIB, including T1N1, T2N1, and T3N0; IIIA, including T1N2b, T2-3N2a, T3N1, T4N0, and T4N1; and IIIB, including T2-3N2b, T4N2a, and T4N2b. Overall survival (OS) and disease-free survival (DFS) were analyzed using Kaplan–Meier curves. Results: The final analysis involved 291 patients. The three- and five-year OS rates were 82% and 71% for stage IIB, and 75% and 58% for stage IIIA, respectively. At stage IIB, a significant difference was found only for DFS comparing T2N1 and T1N2a, whereas stage IIIA showed no significant differences in either OS or DFS, confirming its prognostic homogeneity. Conversely, stage IIIB exhibited significant heterogeneity in survival (OS p = 0.031; DFS p < 0.0001), with T4N2b subgroups showing the worst outcomes compared to T2-3N2b and T4N2a. Conclusions: Our validation of the ninth edition of lung cancer staging shows improved prognostic granularity: IIB shows non-homogeneous DFS, IIIA is homogeneous, and IIIB shows major OS/DFS substage differences, warranting analyses with larger samples for refined stratification.