Minimal or measurable residual disease (MRD) is a term that refers to the submicroscopic tumor disease persisting after therapy. Sensitive immunophenotypic and molecular techniques are used to detect the small amount of residual tumor cells, conferring a detection capacity clearly more sensitive of common cytomorphologic techniques. MRD evaluation now represents an important tool in the study of solid tumors and of hematological malignancies. Concerning hematological malignancies, MRD evaluation was particularly developed in the study of multiple myeloma and acute myeloid leukemia,
representing in these diseases a precious biomarker to quantify response to treatment, to evaluate the chemosensitivity/chemoresistance of the disease and to have a prognostic prediction on disease outcome. The finding that MRD evaluation may have a prognostic value, predicting the risk of relapse, stimulated interest in the introduction of MRD in clinical trials, either as a clinical endpoint or as a tool to guide treatment decisions. However, the clinical use of MRD requires a standardization of the techniques used for its detection, the use of multiple techniques and the development of a consistent accuracy and reproducibility. Finally, prospective clinical trials are required to assess the
real clinical benefit potentially deriving from the introduction of MRD evaluation into clinical studies.