Prognostic impact of M descriptors of the 8th edition of TNM classification of lung cancer

Background: The 8th edition of the tumor, node and metastasis (TNM) classification of lung cancer will be enacted in January 2017. The aim of this study was to analyze the survival differences among the three new categories of metastatic disease: intrathoracic metastasis (M1a), single extrathoracic metastasis (M1b) and multiple extrathoracic metastases (M1c) in our cohort of patients with non- small cell lung cancer (NSCLC). Methods: This is a retrospective single-center study including NSCLC patients with metastatic disease at diagnosis. Patients were divided into three groups (M1a, M1b, M1c). Overall survival (OS) within and between these subgroups was calculated using the Kaplan-Meier method. Results: A total of 288 patients were included (112 M1a, 28 M1b and 148 M1c). Median OS of M1c was significantly worse than M1a or M1b tumors (P<0.001). No significant differences were found among the M1a descriptors (pleural/ pericardial nodules/ effusion, bilateral tumor nodules or both descriptors) (P=0.722) and between M1a and M1b tumors (P=0.517). OS of patients with one metastasis in a single organ was not significantly different from OS of patients with two metastases in a single organ (P=0.180). Among M1c tumors, OS was significantly better in patients with multiple metastases in a single organ than in patients with multiple metastases in multiple organs (P=0.001). Conclusions: Our results support the proposal to keep the M1a category unchanged in the 8th edition as well as the proposed restructuring of the M1b in the new M1b and M1c categories. However, our results raise questions about the definition of oligometastatic disease and, consequently, the criteria of M1b and M1c category.