Clinical Characteristics and Prognostic Analysis of Extramedullary Disease in Multiple Myeloma: A 15-Year Retrospective Cohort Study

Background: Extramedullary disease (EMD) in multiple myeloma (MM) is associated with poor prognosis, yet the clinical distinctions between bone-related EMD (bEMD) and soft tissue-associated EMD (sEMD) remain incompletely characterized. This study aimed to compare the clinical features and outcomes of bEMD versus sEMD and to develop a simple pre-treatment risk stratification tool using baseline clinical parameters. Methods: We retrospectively analyzed 118 patients with MM and EMD treated at a single center from 2011 to 2025, including 72 bEMD and 46 sEMD cases. Baseline characteristics, laboratory parameters, cytogenetic abnormalities, and survival outcomes were compared. Results: Patients with sEMD had significantly higher serum β2-microglobulin (β2-MG) levels (6.4 mg/L vs. 4.5 mg/L, p = 0.007) and a higher proportion of relapsed/refractory disease (41.3% vs. 15.3%, p = 0.002) compared to bEMD. Median progression-free survival (PFS) and overall survival (OS) were markedly shorter in patients with sEMD than in those with bEMD (PFS: 12.0 vs. 29.0 months, p < 0.001; OS: 25.0 vs. 67.0 months, p = 0.009). Multivariate analysis identified thrombocytopenia (PLT < 100 × 109/L), elevated β2-MG, multisite extramedullary involvement, and TP53 deletion as independent adverse prognostic factors. A risk scoring system incorporating β2-MG (0–2 points), thrombocytopenia (1 point), and multisite involvement (1 point) stratified patients into low-risk (0–2 points) and high-risk (3–4 points) groups with significantly different PFS (27.0 vs. 10.0 months, p < 0.001) and OS (54.0 vs. 22.0 months, p = 0.008). Conclusions: sEMD represents a more aggressive subtype of MM with inferior outcomes. The proposed risk score, based on routinely available clinical parameters, effectively identifies high-risk patients at initial diagnosis and may guide individualized treatment strategies.