INTRODUCTION The International Association for the Study of Lung Cancer recently proposed new M descriptors for the next edition of the TNM classification for NSCLC, subdividing the current M1b category into two subcategories: M1b, which indicates a solitary extrathoracic metastasis in a single organ, and M1c, which indicates multiple extrathoracic metastasis. The purpose of this study was to validate the prognostic value of the newly proposed M descriptors in an independent cohort with multivariate and subgroup analysis.

METHODS A total of 1024 patients in a consecutive lung cancer database who had stage IV NSCLC treated between 2011 and 2014 were analyzed. Newly proposed M staging was used for classification and comparison of survival. Adjustment for other clinical covariates and subgroup analysis was conducted.

RESULTS According to the newly proposed M descriptors, 262 patients (25.6%), 152 patients (14.8%), and 610 patients (59.6%) were classified into the subgroups M1a, M1b, and M1c, respectively. The median overall survival times were 22.5, 17.8, and 13.6 months for the M1a, M1b, and M1c groups, respectively (p < 0.001). After adjustment for other covariates, Cox proportional hazards regression revealed statistically significantly shorter overall survival for the M1b group than for the M1a group (hazard ratio = 1.30; 95% confidence interval: 1.03-1.65, p = 0.03) and for the M1c than the M1b group (hazard ratio = 1.57; 95% confidence interval: 1.28-1.93, p < 0.001). These differences showed a consistent tendency regardless of pathologic and molecular subtypes.

CONCLUSIONS The newly proposed M descriptors have prognostic value in patients with stage IV NSCLC.