Circulating tumor cells in early breast cancer: Recent advances in our understanding of minimal residual disease

One-third of early breast cancer patients suffer from a relapse following adjuvant therapy. In this context, circulating tumor cells (CTCs) may be viewed as precursors of metastatic disease and serve as surrogate marker of minimal residual disease. Women with CTCs are at higher risk for distant relapse and breast cancer specific death than CTC-negative patients. Since CTCs differ from the primary tumor with regard to expression profiles, several study groups explored the question whether treatment choices should be based on receptor status of CTCs as well as the primary tumor’s. This review focuses on clinical relevance of CTCs and provides an overview of ongoing and completed trials on minimal residual disease in non-metastatic breast cancer.