Supplementary Material for: Minimal clonal plasma cell contamination of peripheral stem cell grafts have an adverse prognostic impact in patients with multiple myeloma undergoing autologous transplantation

Introduction
The significance of autografts' contamination by clonal plasma cells on clinical outcome in newly diagnosed multiple myeloma remains controversial.
Methods
We retrospectively reviewed the clinical and laboratory data of newly diagnosed multiple myeloma (NDMM) patients who underwent autologous stem cell transplantation (ASCT) and had received graft minimal residual disease (gMRD) examination by multi-color flow cytometry.
Results
From January 2011 to December 2022, 250 NDMM patients with complete cytogenetic information, gMRD information, and who received autologous stem cell transplantation (ASCT) as consolidation were enrolled. Multi-flow cytometry can achieve a median detection sensitivity of 0.004%, and gMRD positivity was 12.4% at a median level of 0.0160% (IQR, 0.0049%, 0.05394%). Its presence was correlated with response to induction treatment, with percentages of 2.65%, 12.94%, 28.89%, and 57.14% of patients achieving complete response, very good partial response, partial response, and minimal response/stable disease, respectively. gMRD (+) patients had a higher risk of not achieving bone marrow MRD negativity post-ASCT. After a median follow-up of 33.5 months for the whole cohort, patients in the gMRD (+) group had significantly worse PFS than those in the gMRD (-) group did (34.8 vs. 65.0 months, P = 0.001). Multivariable analysis revealed that gMRD (-) was independently predictive of better PFS (HR 0.464, 95%CI: 0.274-0.785, P = 0.004). We found the significance of gMRD on PFS was in high-risk subgroups and in patients who achieved ≤ partial response prior to ASCT.
Conclusions
In conclusion, gMRD (+) was an independent risk factor for inferior progression-free survival, with the impact primarily affecting high-risk groups and patients who achieved ≤ partial response before ASCT.