Validation of prognostic impact of number of extrathoracic metastases according to the eighth TNM classification: a single-institution retrospective study in Japan

Background In the eighth edition of the TNM classification of lung cancer, the M1b and M1c descriptors are newly defined by the number of extrathoracic metastases. To verify the prognostic value of these descriptors in Japan, we reclassified our cases and re-evaluated prognosis in M1b and M1c patients. Methods All non-small cell lung cancer (NSCLC) patients with extrathoracic metastases who visited Saitama Medical Center from 2010 to 2016 were evaluated, divided according to the eighth edition of the TNM classification criteria into two groups (M1b, patients with single extrathoracic metastasis, and M1c, patients with multiple extrathoracic metastases), and followed up until December 31, 2017. Survival time analysis was performed using the Kaplan–Meier method, and between-group differences in overall survival time (OS) were evaluated by the log-rank test. Results A total of 231 NSCLC patients were divided into 57 patients with M1b and 174 with M1c. Median OS was 15.2 months (95% confidence interval [CI]: 9.3–19.9) and 7.3 months (95% CI 5.7–10.7) for M1b and M1c, respectively, with no significant between-group difference ( P = 0.239). However, after excluding patients with epidermal growth factor receptor (EGFR) mutation or echinoderm microtubule-associated protein-like 4 and anaplastic lymphoma kinase (EML4–ALK) fusion gene, median OS was 12.9 months (95% CI 7.2–19.9) for M1b and 5.4 months (95% CI 3.8–6.3) for M1c, respectively, showing a significant difference ( P = 0.029). Conclusions The effect of therapy directed toward EGFR mutation or EML4–ALK fusion gene might obscure the significant prognostic difference between M1b and M1c.