Measurable residual disease in acute lymphoblastic leukemia: How low is low enough?

Quantification of measurable residual disease (MRD) in acute lymphoblastic leukemia (ALL) is a well-established clinical tool used to risk stratify patients during the course of chemotherapy, immunotherapy, and/or transplant therapy. As technologies evolve, the sensitivity for quantifying exceptionally low disease burden using either next generation sequencing (NGS) or next generation flow cytometry (NGF) has improved. It is now possible to detect MRD and quantify it precisely in patients who would previously have been deemed MRD negative by older, lower sensitivity methods. Persistence or recurrence of ALL disease burden above 10^-4 (0.01%) is accepted as the minimum threshold for making clinical decisions, but with NGS and NGF, clinicians now confront decision-making with disease burdens sometimes quantified to as low as 10^-6 (0.0001%, or one leukemia cell in a million leukocytes). Emerging data suggest these higher sensitivity methods are superior for identifying patients at lowest risk for relapse, but it remains controversial whether to institute therapies such as blinatumomab or chimeric antigen receptor (CAR)-T cells or move patients to allogeneic hematopoietic cell transplant (alloHCT) when they have quantifiable disease burden less than 10^-4. With additional evidence to facilitate integration of highly sensitive MRD quantification into clinical care and to contextualize MRD within the genotype of individual patients, it will likely be increasingly possible to identify patients able to avoid alloHCT and potentially even de-escalate therapy.