Minimal residual disease and circulating tumor cells in breast cancer

Breast cancer (BC) is the most common cancer in women in Europe [1]. Despite surgery and adjuvant systemic therapy, many women with early BC still relapse and die of their disease. Minimal residual disease (MRD) after potentially curative surgery for BC is thought to contribute to disease relapse and to be the target of adjuvant treatment. MRD is defined as micrometastatic cells undetectable by conventional imaging and laboratory tests. Surrogates of MRD are tumor cells detected in the bone marrow (disseminated tumor cells (DTCs)) and peripheral blood (circulating tumor cells (CTCs)) [2]. The detection and characterization of DTCs/CTCs are expected to lead to personalized treatment strategies and accelerate the development of novel therapeutic agents for BC [2]. Furthermore, genotypic and phenotypic characterization of DTCs/CTCs at the single cell level may provide novel insights into the biology of tumor progression [3].The detection of DTCs/CTCs in BC is challenging since these cells are rare, occurring at a frequency of one tumor cell per 106 to 107 mononuclear cells. To isolate DTCs/ CTCs, enrichment techniques are therefore typically applied. These techniques are based either on the physical properties of the cells (for example, cell density by ficoll centrifugation or cell size by filtration) or on their immunological characteristics (for example, cell surface antigens of DTCs/CTCs by immunoenrichment or markers of hematopoietic cells by immunodepletion). Ficoll centrifugation was widely used in the initial clinical studies of bone marrow DTCs [4]. Currently, however, enrichment techniques incorporating immunomagnetically labeled monocolonal antibodies are more often used because they improve tumor cell recovery (recovery rates of >50% to 85%) [5,6] over ficoll enrichment (recovery rate of 40%) [7] in spiking experiments using cell lines. After the initial enrichment step, DTCs/CTCs have been detected using assays based on either antibodies (immunocytochemistr