Circulating tumor-derived biomarkers in lung cancer

The NIH Biomarkers Definition Working Group defined a biomarker as “a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacological responses to a therapeutic intervention” (1). In lung cancer, an enormous research effort is underway related to biomarkers in airway epithelial cells, sputum, breath, urine and blood for early diagnosis or prediction of high risk. Here, we will focus on blood as the source of biomarker research and in particular comment on circulating tumor cells (CTCs) and circulating nucleic acids.