Whole-body MRI Versus [18F]FDG PET/CT in Diagnosing and Monitoring Plasmacytomas: A Comparative Study.

Background/objectives: Current guidelines recommend [18F]FDG PET/CT as the preferred imaging modality for suspected extramedullary bone plasmacytomas, while whole-body magnetic resonance imaging (WB-MRI) is indicated for solitary bone plasmacytomas. Despite these recommendations, the available evidence comparing the diagnostic efficacy of both techniques remains limited. The aim of this study was to compare the diagnostic efficacy of WB-MRI and [18F]FDG PET/CT in the initial evaluation of plasmacytomas.

Methods: We performed a multicenter, observational, and retrospective analysis of patients diagnosed with plasmacytoma who underwent WB-MRI and/or [18F]FDG PET/CT as part of their diagnostic workup. Lesions identified were categorized by anatomic location, and concordance between WB-MRI and [18F]FDG PET/CT findings was assessed. The McNemar test and Pearson χ2 test were used to compare detection rates between WB-MRI and [18F]FDG PET/CT.

Results: The study included 73 patients (33 men) recruited between 2012 and 2023, age range 30-94 years (mean 63.4 ± 12.2 y). Of these, 56 patients underwent both diagnostic tests. Diagnoses revealed solitary plasmacytoma in 16 patients, concurrent multiple myeloma (MM) and plasmacytoma in 18 patients, and plasmacytoma in 22 patients with a prior MM history. Out of the 56 plasmacytomas, 40 were osseous and 16 were extramedullary. WB-MRI detected 98.2% of plasmacytomas compared with 83.9% for [18F]FDG PET/CT, with a statistically significant difference of OR 9 (95% CI: 1.2-394.5), P=0.021. Concordance was very high for osseous plasmacytomas but moderate for extramedullary plasmacytomas.

Conclusions: These findings suggest WB-MRI is an alternative to [18F]FDG PET/CT for detecting plasmacytomas. A comprehensive clinical and radiologic assessment is essential for the optimal evaluation of patients with plasmacytoma.