Combination of flow cytometry and functional imaging for monitoring of residual disease in myeloma

Abstract

The iliac crest is the sampling site for minimal residual disease (MRD) monitoring in multiple myeloma (MM). However, the disease distribution is often heterogeneous, and imaging can be used to complement MRD detection at a single site. We have investigated patients in complete remission (CR) during first-line or salvage therapy for whom MRD flow cytometry and the two imaging modalities positron emission tomography (PET) and diffusion-weighted magnetic resonance imaging (DW-MRI) were performed at the onset of CR. Residual focal lesions (FLs), detectable in 24% of first-line patients, were associated with short progression-free survival (PFS), with DW-MRI detecting disease in more patients. In some patients, FLs were only PET positive, indicating that the two approaches are complementary. Combining MRD and imaging improved prediction of outcome, with double-negative and double-positive features defining groups with excellent and dismal PFS, respectively. FLs were a rare event (12%) in first-line MRD-negative CR patients. In contrast, patients achieving an MRD-negative CR during salvage therapy frequently had FLs (50%). Multi-region sequencing and imaging in an MRD-negative patient showed persistence of spatially separated clones. In conclusion, we show that DW-MRI is a promising tool for monitoring residual disease that complements PET and should be combined with MRD.

Figure 1:. Patients and study design.

In (A) the study design is shown. (B) Simultaneous DW-MRI and PET scans are shown for a representative patient. A focal lesion in the left distal femur is indicated by arrows. TT, total therapy; DW-MRI, Diffusion-weighted magnetic resonance imaging; PET, positron-emission-tomography; Flow, 8-color flow cytometry with a limit of detection of ~1×10⁻⁵; FL, focal lesion; * MRD data was available for 83 first-line and all relapsed patients.

Figure 2.. The impact of residual focal lesions on outcome.

In (A) the impact of residual FL on DW-MRI is depicted stratified by restriction status of these lesions on EADC maps. (B) Prognostic value of residual FLs detectable using DW-MRI only and PET, respectively. The prognostic impact of residual FLs on outcome stratified by the GEP70 risk classifier or the International staging system is shown in (C) and (D). The log-rank test was used to perform the group comparison.

Figure 3:. Prognostic impact of MRD and its combination with functional imaging.

Progression free survival from onset of CR for newly diagnosed MM patients stratified by (A) MRD results, and the combination of MRD and functional imaging (B). The log-rank test was used to calculate p values.

Figure 4:. The impact of spatial heterogeneity on MRD diagnostics.

In (A) the phylogenetic relationship between clones from different regions at baseline and relapse and the corresponding PET scans are presented for one patient. The location of samples is marked in the PET scans using the color code that was assigned to clones. (B) Follow-up DW-MRI scans of this patient together with MRD results from the iliac crest. The MRD level is shown by the indicated color code.

Figure 5:. The iceberg metaphor for disease burden in multiple myeloma adjusted for spatial heterogeneity.

The tip of these icebergs, which correspond to FLs, may be visualized in medical imaging, depending on the size of disease foci and the imaging method used. The foundation of the iceberg can be assessed using highly sensitive approaches such as flow cytometry but false negative results will result during deep responses, if the iliac crest region is iceberg (disease)-free.