ctDNA is a part of circulating free DNA (cfDNA), which is released by tumor cell necrosis, apoptosis, micrometastasis, or the cleavage of CTCs and proliferated tumor cells.ctDNA includes genetic information of tumor cells, such as mutation, recombination, and deletion (15). ctDNA was approved by the European Medicines Agency in 2014 and the National Comprehensive Cancer Network Guidelines for NSCLC in 2017 as a supplement sample to assess genetic variation when tumor tissue samples were difficult to obtain. Clinical studies have shown that ctDNA can effectively reflect tumor load, malignant degree, metastasis ability, and real-time information of genetic mutation, which has a certain correlation with the genetic information of tumor tissues. Moreover, the content of cfDNA in patients with cancer was significantly higher than that in healthy people. Also, ctDNA has been used for detecting tumor load, monitoring tumor recurrence, and assessing minimal residual disease in a number of cancer types. Some studies have suggested that ctDNA can be used to estimate tumor recurrence, even before imaging. Therefore, ctDNA detection has an important value in the clinical application of lung cancer diagnosis, drug efficacy, surgical effect evaluation, recurrence monitoring, prognosis judgment, medication guidance, and molecular classification differentiation.