The eighth TNM classification system for lung cancer: A consideration based on the degree of pleural invasion and involved neighboring structures

Objectives: The eighth tumor-node-metastasis (TNM) classification system for lung cancer has been used since January 2017 and must be applied to an individual institution's database. Methods: We analyzed pathological stage data of 2756 patients who underwent resection of non-small-cell lung cancer, particularly in terms of the degree of visceral pleural invasion and involved neighboring structures. Results: Few patients had stage IIA disease (103, 4%); stratification between stages IB and IIA was insufficient (p = 0.129). When T2a tumors were divided into PL1 and PL2 subgroups based on the degree of pleural invasion, there was a significant prognostic difference between the subgroups (p < 0.001). By incorporating T2a tumors with PL2 (T2a-PL2) into the T2b category, modified stages IB, IIA (234, 8%), and IIB were well stratified (IB vs. IIA, p < 0.001; IIA vs. IIB, p = 0.011). Focusing on T3 tumors with PL3 (T3-PL3) invading neighboring structures, multivariate analysis for surveying pT3N0-2M0 tumors revealed that completeness of resection (p = 0.002), implementation of any postoperative therapies (p = 0.003), and subcategorization of whether only the pleura was infiltrated or other deeper structures were also invaded (p = 0.024) were significant and crucial predictors. N2 disease showed worse outcome than NO-1 diseases, with marginal difference (p = 0.054). Conclusion: T2a-PL2 tumors could be categorized into a worse prognostic T2b category. For T3-PL3 tumors involving resectable neighboring organs, subcategorization of whether there is only pleura infiltration (T3a) or other deeper structure invasion (T3b) could be a practical consideration.