Classifying circulating tumor cells to monitor cancer progression

Introduction: The term liquid biopsy' refers to molecular analysis of a tumor's genetic features based on circulating genetic material in the peripheral blood derived from circulating tumor cells (CTCs), circulating tumor DNA (ctDNA) and circulating miRNAs, and has emerged as a minimally invasive tool in early cancer diagnosis and disease monitoring. CTCs are believed to originate from the primary tumor and obtain genetic heterogeneity during evolution.Areas covered: The presence of CTCs has been associated with poor clinical outcome in patients with metastatic breast cancer, lung cancer, colorectal cancer and prostate cancer. In addition, the detection of CTCs in patients with early breast cancer has been shown to represent an independent prognostic factor associated with an unfavorable clinical outcome. Moreover, the longitudinal evaluation of CTCs in patients with early breast cancer may reveal the presence of chemo- and hormone-therapy resistant CTCs which are associated with an increased risk for disease relapse and disease-related death.Expert commentary: The molecular characterization of CTCs may provide an important tool for the monitoring and the evaluation of treatment efficacy in patients with different tumor types such as breast, prostate, colon, and non-small cell lung cancer.