Response Evaluation Criteria in Solid Tumors version 1.1 is currently considered the indicator to assess the curative effect of cancer. However, radiographic measurements often fail to detect tiny lesions or changes in the tumor burden, while tumor biomarkers possess low sensitivity and stability. Circulating tumor DNA (ctDNA) is a potential noninvasive approach that can be used to make an earlier diagnosis, monitor disease progress, and determine treatment efficacy. Previous studies of non-small cell lung cancer (NSCLC) have not clearly determined the optimum time to monitor ctDNA. Hence, the appropriate time to evaluate ctDNA to determine the curative effects of treatment in advanced NSCLC compared to conventional imaging and tumor markers should be explored in order to prevent unnecessary side effects and to avoid continuing ineffective therapies. This protocol outlines a prospective clinical trial in which advanced NSCLC patients will be recruited and longitudinal changes in ctDNA levels with changes in radiographic tumor size or tumor biomarkers will be assessed. ctDNA will be quantified by determining the allele fraction of cancer-associated somatic mutations in plasma using multigene next-generation sequencing assay. Conclusions will be drawn from data collection and analysis.