Despite improved overall stratification, the 9th edition stage IIIA category remains prognostically heterogeneous: insights from a T3N2a subgroup analysis

The 9th edition TNM staging system refines N2 subclassification into single-station (N2a) and multi-station (N2b) metastases, leading to substantial stage migration within the previously heterogeneous stage IIIA category. While the 9th edition has demonstrated improved overall prognostic performance, it remains unclear whether the newly defined stage IIIA category is prognostically homogeneous. This study aimed to address this question in a real-world cohort of resected occult N2 NSCLC. Data of NSCLC patients who underwent surgery at the Department of Thoracic Surgery, Fourth Hospital of Hebei Medical University from January 2014 to December 2017 were retrospectively analyzed. Patients were staged according to the 8th and 9th editions of the AJCC TNM guidelines. Kaplan-Meier analysis was used to assess the impact of clinicopathological factors on survival, and Cox regression analysis was performed for multivariate analysis. The Receiver Operating Characteristic (ROC) curve was utilized to evaluate the predictive performance of the two TNM staging systems. A retrospective study analyzed 550 T1-4N2M0 NSCLC patients post-surgery. 262 (47.6%) had stage reclassifications: 93 downstaged from IIIA to IIB, 36 downstaged from IIIB to IIIA, and 133 upstaged from IIIA to IIIB. The 9th edition demonstrated significantly improved three-tier prognostic discrimination across stages IIB, IIIA, and IIIB ( p < 0.001). However, significant prognostic heterogeneity was observed within the newly defined stage IIIA category: patients with T3N2a disease (downstaged from 8th edition IIIB) had a 5-year OS of only 28.8%, compared to 51.2% for other stage IIIA patients ( p < 0.05). The 9th edition demonstrated superior prognostic performance, as reflected by a larger area under the ROC curve (AUC: 0.606 vs. 0.550 for 5-year OS). Multiple mediastinal lymph node metastases emerged as an independent risk factor for reduced DFS and OS. While the 9th edition TNM system improves overall prognostic stratification for resected occult N2 NSCLC, the stage IIIA category remains prognostically heterogeneous. T3N2a patients have outcomes more consistent with IIIB than IIIA, suggesting that further refinement of stage grouping may be warranted.