To the editor,

Multiple Myeloma (MM) is a malignant plasma cell disorder secreting a monoclonal immunoglobulin [1]. The detection of this M-protein is central to the diagnosis and monitoring of multiple myeloma [2]. In 2006, the International Myeloma Working Group (IMWG) defined complete response (CR) as a negative immunofixation (IFE) on the serum and urine, and less than 5% plasma cells (PCs) on bone marrow examination (BME) [3]. Achieving CR is an important aspect of response assessment, considering its association with better progression-free survival (PFS) and overall survival (OS) [4, 5].

Several years ago, Chee et al. showed that CR assessment with IFE without BME was associated with a significant proportion of false-positive assessments [6]. However, BME requires an uncomfortable invasive procedure for patients, resulting in noncompliance among patients and physicians.

Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) is a new and more sensitive assay to detect and monitor small M-proteins [7, 8]. Herein, we hypothesized that Mass-Fix could be an alternative to BME in the assessment of IMWG-defined complete response (CR) in MM patients.

This retrospective cohort study included consecutive patients seen at Mayo Clinic Rochester with newly diagnosed multiple myeloma (NDMM) between May 30th, 2017, and May 30th, 2020. Only patients who had a serum Mass-Fix assay with a concomitant BME and achieving at least very good partial response (VGPR) after initiation of therapy were included. The Mass-Fix and BME must have been performed within 60 days of each other. Patients with non-secretory disease or light-chain myeloma at diagnosis were excluded. Approval for this study was obtained from the Mayo Clinic IRB. The assessment of response was based on IMWG criteria and Mass-Fix was used as a surrogate of serum immunofixation (a patient achieving ≥CR must have a negative Mass-Fix status).

Patients with undetermined MRD or MALDI-TOF mass spectrometry results were considered as positive. MALDI-TOF techniques were already described in previously published papers [8]. The minimal residual disease was measured by flow cytometry (with threshold of 1 tumor cell/10−5 white blood cells).

The endpoints of the study included PFS, measured from the date of the Mass-Fix result to date of confirmed disease progression with censoring performed at the last follow-up date. The Kaplan Meier method with log-rank with semi-landmark analysis and Tukey’s multiple comparison test were used to determinate PFS.

We identified 402 NDMM patients achieving at least VGPR or better who had a mass spectrometry with concomitant BME. Patient demographics characteristics are presented in Table 1. Patients achieving positive (n = 249) or negative Mass-Fix (n = 153) status had a similar distribution of sex, ISS stage III, and high-risk cytogenetics.

Table 1 Patient characteristics.
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In a 1-year landmark analysis, patients achieving negative Mass-Fix had a better 12-month PFS than patients with positive Mass-Fix (90.1% vs. 79.5%, p = 0.001, Fig. S1). Among patients with negative Mass-Fix, 100% (153/153) had <5% of PCs on BME (positive predictive value-PPV), while 8% (20/249) of those with a positive Mass-Fix had ≥5% of PCs on BME (negative predictive value—NPV). It’s important to remember that NPV is based on a population achieving at least VGPR. Examining patients with ≥5% of PCs, 100% (20/20) had positive Mass- Fix (specificity). (Fig. 1a).

Fig. 1: Performance of serum Mass-Fix as compared to bone marrow examination.
figure 1

Performance of patients having mass spectrometry as compared to (a, b) bone marrow or (c) minimal residual disease (MRD).

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Patients achieving negative Mass-Fix status and <5% of PCs on BME had a better 24-months PFS than patients with positive Mass-Fix and ≥5% PCs or <5% PCs on BME (85.3% vs. 43.7% or 68.8%, p < 0.001, Fig. S2a).

Among patients with ≥1% of PCs on bone marrow morphology, 68% (190/281, specificity) had positive Mass-Fix. Forty-one % (62/153) of Mass-Fix negative patients had morphologically negative BME (PPV); 76% of patients (190/249) with positive Mass-Fix had ≥1% of PCs on BME (NPV) (Fig. 1b).

Patients achieving negative Mass-Fix and <1% of PCs on BME had a similar 24-month PFS than patients with negative mass spectrometry and ≥1% of PCs on BME (84.6% vs. 83.8%, p = 0.989, Fig. S2b). Achieving negative Mass-Fix with ≥1% of PCs was associated with a better 24-month PFS than patients with positive Mass-Fix and absence of PCs on BME (83.8% vs. 66.8%, p = 0.021, Fig. S2b).

Only 3% of patients achieving VGPR had a negative Mass-Fix status. According to the IMWG response criteria, at time when BME was performed 65%, 26%, and 10% of patients achieved VGPR, CR, and sCR respectively. Patients achieving CR had a better 24-month PFS than patient achieving VGPR (86.9% vs. 66.5%, p < 0.001). There was no significant PFS difference between patients achieving stringent complete response or complete response (85.3% vs. 86.9%, p = 0.277, Fig. S3a). However, patients achieving ≥CR and Mass-Fix negativity had a better 24-month PFS than patients achieving VGPR or Mass-Fix positivity (86.7% vs. 66.5%, p < 0.001, Fig. S3b).

Patients achieving normal serum free light-chain ratio with negative Mass-Fix and negative urine IFE with or without <1% of plasma cells on BME had a similar 3-year PFS (85.7% vs. 82.1%, p = 0.180, Fig. S4).

Finally, more than 68% (45/66, specificity) of patients with positive bone marrow-based-MRD by flow had a positive Mass-Fix . Among patients achieving Mass-Fix negativity, 85% (119/140, PPV) achieved absence of PCs by minimal residual disease assessment. In contrast, 56% (119/213, sensitivity) of patients with negative bone marrow based-MRD achieved Mass-Fix negativity (Fig. 1c).

Since 2006, monoclonal protein immunofixation and bone marrow examination has been the backbone of CR assessment in multiple myeloma. Serum immunofixation is increasingly controversial given its low sensitivity while mass spectrometry is emerging as a powerful predictor of PFS and OS [7, 9]. The IMWG also stated that Mass-Fix is a suitable replacement of IFE for monitoring multiple myeloma [10]. Additionally, noncompliance to BME is observed because this procedure is considered as cumbersome by some clinicians and uncomfortable for patients. False-negative results can also occur due to heterogenous involvement of the bone marrow in MM.

Several studies explored the predictive value of mass spectrometry and demonstrated that it can be used to detect residual disease when serum immunofixation or even bone marrow studies do not. In a recent study by Dispenzieri et al., Mass-Fix outperformed standard immunofixation in the STAMINA trial [9]. Similarly, we showed that every patient with a negative Mass-Fix had less than 5% PCs on the BME. Moreover, mostly all patients with negative Mass-Fix reached MRD negativity (85%). Some of the disparities observed between patients achieving MRD negativity with positive Mass-Fix could be related to longer half-life of immunoglobulins in comparison with bone marrow PCs [11]. Indeed, many patients included in this study had late mass spectrometry conversion (positive-> negative).

We showed that patients achieving negative Mass-Fix and negative urine IFE had a similar PFS in presence or absence of PCs in histopathology. Therefore, we demonstrated that Mass-Fix outperformed traditional bone marrow examination for CR assessment. Moreover, achieving CR or sCR with negative Mass-Fix status was associated to a similar outcome. This is consistent will several other studies demonstrating that MALDI-TOF mass spectrometry is a valuable and sensitive assay to detect residual disease while overperforming standard BME [9, 12,13,14,15]. However, it is unclear if reaching both Mass-Fix negativity and absence of PCs on morphological BME provides any additional PFS benefit.

Furthermore, we propose that bone marrow examination should only be done if a MRD examination is warranted, preferably when patients achieved Mass-Fix negativity (≥CR). Indeed, mostly all patients achieving VGPR in our cohort had positive Mass-Fix status, whereas many patients achieving Mass-Fix negativity had negative MRD. Additionally, we demonstrated that patients who are Mass-Fix negative (≥CR) had an improved PFS versus those who do not reached CR and were Mass-Fix positive. Thus, we think that BME is useless if use only for bone marrow plasma cells quantification (until a new definition of CR is validate).

Limitations of our study include the retrospective nature of this work. The median follow-up of 30 months limits the possibility to detect differences in OS and PFS. It is important to keep in mind that the bone marrow is the homing for the malignant clone, while the monoclonal protein is just a surrogate marker of the disease. Thus, more sensitive peripheral markers are warranted to detect malignant cell in peripheral blood. Finally, our definition of CR and sCR include 24 h urine protein with immunofixation, and not Mass-Fix. This could have led us to classify patients as achieving CR when this was not the case.

In summary, bone marrow examination did not add to complete response (CR) assessment in patients who already achieved Mass Fix negativity. Patients achieving negative Mass-Fix status even seem to outperform traditional CR and sCR while achieving a deeper response. Moreover, we demonstrated that bone marrow examination should only be performed if marrow MRD examination is warranted, preferably in patients with negative Mass-Fix.