Circulating Tumor Cells in Breast Cancer Patients

Breast cancer is the most commonly diagnosed cancer among women, resulting in an estimated 40,000 deaths in 2014.1 Metastasis, a complex, multi-step process, remains the primary cause of death for these patients. Although the mechanisms involved in metastasis have not been fully elucidated, considerable evidence suggests that metastatic spread is mediated by rare cells within the heterogeneous primary tumor that acquire the ability to invade into the bloodstream. In the bloodstream, they can travel to distant sites, sometimes remaining undetected and in a quiescent state for an extended period of time before they establish distant metastases in the bone, lung, liver, or brain. These occult micrometastatic cells (circulating tumor cells, CTCs) are rare, yet their prognostic significance has been demonstrated in both metastatic and non-metastatic breast cancer patients. Because repeated tumor tissue collection is typically not feasible and peripheral blood draws are minimally invasive, serial CTC enumeration might provide "real-time liquid biopsy" snapshots that could be used to identify early-stage breast cancer patients with micrometastatic disease who are at risk for disease progression and monitor treatment response in patients with advanced disease. In addition, characterizing CTCs might aid in the development of novel, personalized therapies aimed at eliminating micrometastases. This review describes current CTC isolation, detection, and characterization strategies in operable breast cancer.