2.1. Study population and data collection

The study was based on a consecutive case series of 99 NSCLC patients, who were all found to have pathological tissue masses in our previous study and treated surgically at Tongji Hospital between 2007 and 2011. All the tissue samples analyzed came from paraffin-embedded tumor masses. The Karnofsky fitness score (KPS) of all patients was above 70, and all had a life expectancy exceeding six months. Epidemiological data were collected with questionnaires before enrollment and included information on demographics, smoking history, alcohol consumption, medical history, family history of cancer, and occupational exposure to potential carcinogens. Clinical and follow-up data on treatment regimens, disease stage, pretreatment performance status, and survival status at the time of analysis were obtained from patients' medical records. Please refer to our previous study for the definitions of disease staging, smoking status, diagnosis of brain metastasis, time to brain metastasis, and follow-up time [11]. The study was approved by the Ethics Committee of Tongji Medical College. Written informed consent was obtained from all patients before interview.