Evaluation of Whole-Body MRI (WB-MRI) in Smoldering Multiple Myeloma (SMM)

Introduction

Whole-body magnetic resonance imaging (WB-MRI), including multiplanar multisequence technique with diffusion weighted images, is a novel imaging technique being evaluated for patients with multiple myeloma (MM). WB-MRI is ideal for this population due to its high sensitivity for bone marrow signal changes and full anatomic coverage from vertex to mid-thighs. It is well established that patients with unequivocal focal lesions on MRI have worse outcomes. Currently the IMWG recommends that all patients with smoldering multiple myeloma (SMM) undergo WB-MRI (or whole spine MRI if WB-MRI is not available) to rule out two or more focal lesions which would classify the patient as having symptomatic myeloma requiring treatment. There is a clear benefit of using MRI for the detection of early focal myeloma lesions however less is known about findings in the SMM population. Detection of subtle findings such as one small focal lesion or heterogeneous bone marrow in WB-MRI has unknown clinical significance that needs to be further evaluated. This study aimed to evaluate the sensitivity of WB-MRI compared to other highly sensitive functional imaging modalities in patients with SMM both at baseline and after treatment.

Methods

Imaging of patients with WB-MRI performed at the National Institutes of Health Clinical Center Myeloma Program were reviewed and compared to whole spine MRI and ¹⁸F-FDG PET/CT completed at the same timepoint. The majority of patients were being evaluated for enrollment on clinical trials. Patients had undergone a WB-MRI with a 3-Tesla system either as a baseline study, after completion of induction treatment, or during follow up determined by the time DWI became available at our institution. The imaging protocol included sagittalT1 weighted (W) and Short tau inversion recovery (STIR) for spine and coronal, axial T1W and axial T2 TSE pulse sequences. The functional component included diffusion weighted imaging in the axial plane (b=0 and 900sec/mm2). Radiological interpretation was performed by two readers using myeloma response assessment and diagnosis system (My-RADS) {Messiou, 2019 #340}. WB-MRIs were categorized as positive if focal lesions or diffuse/heterogenous pattern of bone marrow infiltration were present. Similarly, ¹⁸F-FDG PET/CTs and whole spine MRIs were classified as positive if focal lesions or diffuse/heterogenous pattern of bone marrow were present.

Results

A total of 34 patients with SMM and 5 patients with relapsed refractory multiple myeloma (RRMM) had sequential WB-MRI and ¹⁸F-FDG PET/CT. Figure 1 summarizes the radiological data of the SMM population. Eleven of these patients had PET/CT, whole spine MRI, and WB-MRI at baseline. Twenty-five patients had PET/CT and WB-MRI completed after at least 8 cycles of treatment. Thirteen patients had consistently negative imaging at baseline, 7 of which also had negative imaging after treatment, while 2 patients were found to have new lesions seen on WB-MRI after treatment. Six patients had resolution of positive imaging seen at baseline after treatment. Among the 17 patients with a positive WB-MRI, 12 (71%; 95% CI 47% - 87%) had a negative correlating PET/CT. Among 5 patients with positive PET/CT at the same time point as a WB-MRI, only 1 (20%; 95% CI 2% - 64%) correlated to a negative WB-MRI. Figure 2 depicts findings from patients with RRMM for comparison. All imaging modalities showed multiple focal findings in all 5 patients.

Conclusions

This study depicts the high sensitivity of WB-MRI in the SMM population. Such a high sensitivity is especially needed in SMM and early myeloma when disease burden is lower and the decision for treatment is being considered. In comparison to the RRMM population where all three imaging modalities easily detect multiple focal lesions, WB-MRI tends to identify myeloma involvement in the SMM patients more than the other imaging techniques. This suggests the importance of utilizing WB-MRI when diagnosing SMM. In the SMM population, the prognostic significance of lesions that are discrepant between MRI and FDG PET/CT is not yet known. Further follow up is needed to evaluate any difference in hard endpoints such as progression free survival between patients with positive findings described by WB-MRI.