Prognostic impact of the N2 subclassification and stage migration in the ninth edition of the TNM classification in surgically resected lung cancer

Objectives: In the ninth edition of the TNM classification of lung cancer, N2 is subdivided into single-station (N2a) and multiple-station involvement (N2b), and some stage changes are made to stage II-III. This study aimed to validate the new classification and determine the effect of stage migration and vice versa on the prognosis of each pathological stage due to these changes.

Materials and methods: A total of 1,754 patients with surgically resected lung cancer were included. Clinical characteristics of patients at each new pathological N (pN) status were compared. Overall survival (OS) was evaluated according to the new pN status and pathological stages.

Results: The OS of pN2a patients tended to be superior to that of pN2b patients without significance (p = 0.083). Several patient characteristics, including clinical N status, were not significantly different between pN2a and pN2b. The OS of the downstaged patients (T1N1M0 from IIB to IIA, T1N2aM0 from IIIA to IIB, and T3N2aM0 from IIIB to IIIA) was not significantly different from that of other patients in the same stages (p = 0.376, p = 0.630, and p = 0.970, respectively). The OS of the upstaged patients (T3N2bM0 from IIIA to IIIB) was significantly better than that of other patients in the same stage (p < 0.001). The 5-year OS rates of stages IIIA and IIIB were 63.3 % and 49.4 % in the eighth edition and 58.9 % and 54.3 % in the ninth edition, respectively.

Conclusion: Although the N2 subclassification had some impact on survival stratification, it was difficult to predict the subdivided pN2 status, preoperatively. The OS difference in stage III was smaller in the ninth edition than in the eighth edition, due to the complex effects of stage migration and vice versa.