Cellular residual disease (CRD) in early breast cancer –Expanding the concept of minimal residual disease monitoring?

Despite a significant evolution in treatment strategies for early breast cancer (EBC) patients, up to 30% of them experience recurrence due to occult micrometastasis. The minimal residual disease (minimal RD) in EBC patients after the treatment with curative intent cannot be easily detected by clinical examination and radiological imaging, as they are both burdened by limited sensitivity. A new frontier and promising approach to address this unmet need is the study of liquid biopsy (LB). The most studied tumor-derived analytes in the peripheral blood for minimal RD monitoring are currently: i) the circulating tumor DNA (ctDNA), for the detection of somatic DNA alterations, so referred to as molecular residual disease (MRD); ii) circulating tumor cells (CTCs), for the detection of cellular residual disease (CRD). MRD detection, while reaching a high specificity, is still presenting a number of limitations. On the other hand, CRD allows a real-time disease monitoring, detecting live cells, and possess the potential to provide an enormous amount of biological information. Indeed, CTCs can provide a multi-level portrait (i.e., DNA, RNA and proteins) of the tumor, longitudinally depicting its evolving landscape, and can be used for functional (in vitro/in vivo) characterization. Moreover, CRD goes beyond the association with the risk of recurrence: predictive biomarkers for treatment response can also be evaluated. Nevertheless, CTCs are less studied in this context, because of their need to be immediately processed and their limited detection in a small fraction of patients in the early and post-surgery setting. These limitations could however be overcome by the use of newly developed technologies that enable an increased CTC detection rate and retrospective studies. Here, we review the strengths and limitations of using MRD and CRD for minimal RD detection, focusing on the methodologies available for LB analysis in this setting, and on the main clinical studies investigating MRD and CRD in EBC. Considering the limits and the advantages of both MRD and CRD, we propose the integration of ctDNA and CTCs as complementary tools for minimal RD assessment to achieve a synergistic and novel approach for minimal RD analysis.