Circulating tumor plasma cells quantification in newly diagnosed transplant ineligible real world multiple myeloma patients: Comparison of single‐platform flow cytometry versus next generation flow

: Circulating tumor plasma cells (CTCs) at diagnosis are independent prognostic markers in newly diagnosed multiple myeloma (NDMM). Flow cytometry (FC) enables sensitive and cost-effective CTC quantification, but prognostic cut-offs vary across studies. The methodological approach used for quantification may affect the determination of the CTC percentage. In this study we compare head-to-head two FC techniques [single-platform FC versus Next Generation Flow (NGF)] to measure CTC levels in NDMM patients. Methodological differences between the two techniques can be found in sample processing (NGF includes fixation and permeabilization steps that are not required for CTC quantification by single-platform FC) and analytical sensitivity, which is higher for NGF than for single-platform FC. The percentage of CTCs was simultaneously assessed with both techniques in the peripheral blood (PB) of 34 patients enrolled in the REAL MM trial (NCT03829371) at screening. Single-platform FC detected CTCs in 29/34 patients (85.3%) patients, while NGF detected CTCs in 31/34 patients (91.2%), including 2 cases that were negative by single-platform FC. The CTC percentage measured using the NGF technique (median 0.0049%, range 0%-0.377%) was significantly lower than that obtained using single-platform FC (median 0.009%, range 0%-0.59%) (p < 0.0001). Indeed, quantifying CTCs with FC after fixation and permeabilization (comparably to NGF) led to results that were similar to NGF. In conclusion, NGF demonstrated higher sensitivity than single-platform FC to detect CTCs at diagnosis in MM patients. Employing standardized techniques like NGF may avoid variability across different laboratories and centers.