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1496
Letters to the Editor
Standardization of flow cytometric minimal residual disease assessment in international clinical trials. A feasibility study from the European Myeloma Network
For many decades, international collaborative efforts have driven therapeutic advances in multiple myeloma (MM). The establishment of uniform response criteria by the International Myeloma Working Group (IMWG) has been pivotal for this progress, as adherence to strict defi- nitions ensures data comparability between trials. An essential prerequisite for the use of uniform criteria is the application of standardized methods. Of particular inter- est herein is the assessment of minimal residual disease (MRD) by multiparametric flow cytometry (MFC). This has been incorporated into the IMWG response criteria since 2011 to enable better risk stratification of a growing number of patients reaching a complete remission and has the promise to be used both as a surrogate marker for overall and progression-free survival and to inform treat- ment decisions.1,2 However, in contrast to most routine diagnostic tests for response assessment in MM, this assay has until recently suffered from large interlaborato- ry variations in terms of sample processing and data acquisition, resulting in highly heterogeneous sensitivi- ties.3 To enable uniform and sensitive MFC MRD assess- ment between laboratories, EuroFlow has developed standardized operating procedures.4-6 Their next-genera- tion flow method has been incorporated as the gold stan-
dard for MFC MRD measurements in the latest IMWG response criteria, which is expected to greatly improve data validity and comparability.7
Even though the establishment of standardized proto- cols has been an important step towards achieving uni- form MFC MRD assessment in MM, the usefulness of these protocols depends heavily on their successful implementation in a wide range of laboratories. This is of particular relevance for international clinical trials that depend on a collaborative effort of multiple reference lab- oratories for timely MRD assessment, irrespective of geo- graphical location of sampling. Nevertheless, it remains largely unknown whether fully standardized multi-labo- ratory MM MFC MRD assessment can be achieved in such a setting.8 To investigate this, a novel quality assur- ance (QA) program was established in 2016 within the framework of the European Myeloma Network (EMN): the EMN MRD QA program. This program aims to assess the validity and comparability of MFC MRD measure- ments within and between EMN trials by distributing fresh MM bone marrow and peripheral blood samples and complements existing QA programs led by EuroFlow using peripheral blood samples from healthy donors9 or raw data files from MM MFC MRD measurements. Data obtained within the EMN MRD QA program show that it is feasible to fully standardize MFC MRD assessment between laboratories, resulting in a high concordance over the entire range of detectable MRD levels. Participation in QA programs is essential to ensure com- plete interlaboratory standardization without compro-
A
B
Figure 1. Sample characteristics and logistics for quality assurance rounds 1-4 of the European Myeloma Network minimal residual disease quality assurance program. A total of 20 fresh samples from patients with multiple myeloma (MM) were used in quality assurance (QA) rounds 1-4, resulting in the performance of 67 multiparameter flow cytometry (MFC) minimal residual disease (MRD) assessments in four participating laboratories in Europe. (A) In total, 17 bone mar- row and three peripheral blood samples were collected from MM patients with variable disease burden, with six patients receiving daratumumab treatment at the time of sampling. Sample volumes ranged from 2-6 mL, whereas sample white blood cell counts ranged from <5 x 109/L to >25 x 109/L. Response status was determined according to the International Myeloma Working Group (IMWG) 2016 criteria.7 (B) Between March 15, 2016 and December 17, 2019, MM sam- ples were collected and distributed from hospitals in Rotterdam, the Netherlands and Turin, Italy. In QA rounds 1-2, second-generation flow protocols (EuroFlow) were used by all laboratories, whereas next-generation flow protocols (EuroFlow) were used in QA rounds 3-4. All laboratories participated in the full European Myeloma Network (EMN) MRD QA program, except for laboratory 1. In 2018 and 2019, this laboratory did not serve as a reference laboratory for any EMN trials requiring the use of next-generation flow protocols and therefore decided to not join QA rounds 3-4. In general, laboratories were able to process 86-100% of received samples within the IMWG recommended timeframe of 24-48 hours after sampling. CR: complete response; ID: identifier; NDMM: newly diagnosed mul- tiple myeloma; PD: progressive disease; PR: partial response; QA: quality assurance; sCR: stringent complete response; TBSSA: time between sampling and sam- ple arrival; TBSSP: time between sampling and sample processing; VGPR: very good partial response; WBC: white blood cell.
haematologica | 2021; 106(5)