M1b Disease in the 8th Edition of TNM Staging of Lung Cancer: Pattern of Single Extrathoracic Metastasis and Clinical Outcome

BACKGROUND. The 8th edition of TNM staging of non‐small cell lung cancer (NSCLC) has revised M classification and defined M1b disease with single extrathoracic metastasis, which is distinguished from M1c with multiple extrathoracic metastases. We investigated the prevalence, characteristics, and overall survival (OS) of M1b disease in patients with stage IV NSCLC. METHODS. The study reviewed the medical records and imaging studies of 567 patients with stage IV NSCLC to determine M stage using the 8th edition of TNM staging. Clinical characteristics and OS were compared according to M stages. RESULTS. Among 567 patients, 57 patients (10%) had M1b disease, whereas 119 patients (21%) had M1a disease and 391 patients (69%) had M1c disease. Squamous histology was more common in M1b (16%) than in M1a (6%) and M1c (6%; p = .03). The median OS of patients with M1b disease was 14.8 months, compared with 22.6 months for patients with M1a and 13.4 months for those with M1c disease (p < .0001). Significant OS differences of M1b compared with single‐organ M1c and multiorgan M1c groups were noted (single‐organ M1c vs. M1b: hazard ratio [HR], 1.49; p = .02; multiorgan M1c vs. M1b: HR, 1.57; p = .01) in multivariable analyses adjusting for smoking and systemic therapy types. Among patients with M1b disease, the brain was the most common site of single metastasis (28/57; 49%), followed by bone (16/57; 28%). Single brain metastasis was more frequently treated with local treatment (p < .0001). CONCLUSION. M1b disease was noted in 10% of patients with stage IV NSCLC. Squamous histology was more common in M1b group than others. The brain was the most common site of single metastasis and was often treated locally. IMPLICATIONS FOR PRACTICE. The newly defined group of M stage consists of a unique subset among patients with stage IV non‐small cell lung cancer that can be studied further to optimize treatment approaches.