Clinical Characteristics and Prognostic Analysis of Multiple Myeloma with Extramedullary Disease: A SEER-Based Study

BACKGROUND: Extramedullary disease (EMD), an infrequent manifestation of multiple myeloma (MM), can present at diagnosis or develop during the disease course. EMD can be clinically divided into bone-related EMD (EMD-B) and soft tissue-related EMD (EMD-S). The purpose of our study is to investigate the clinical characteristics, survival outcomes, and prognostic factors of MM patients with EMD. METHODS: A total of 155 MM patients with EMD were ultimately enrolled in our study by retrieving the Surveillance, Epidemiology, and End Results (SEER) database. The Kaplan–Meier survival curves and log-rank test for overall survival (OS) and myeloma-specific survival (MSS) were conducted to compare each potential variable. Variables with a p value <0.1 in the univariate Cox regression were incorporated into the multivariate Cox model to determine the independent prognostic factors, with a hazard ratio (HR) >1 representing adverse prognostic factors. RESULTS: The median age at diagnosis was 63 years old. EMD-B occurred in 99 patients (63.90%), while EMD-S occurred in 56 cases (36.10%). Patients with EMD-S had a significant survival disadvantage in MSS (HR = 1.844, 95% CI 1.117–3.042, p = 0.017) and OS (HR = 1.853, 95% CI 1.166–2.942, p = 0.009) compared to those with EMD-B. Patients with EMD interval ≤24 months were at higher risk of death than those with EMD at diagnosis in MSS (HR = 1.885, 95% CI 1.175–3.346, p = 0.042) and in OS (HR = 1.33, 95% CI 1.119–2.529, p = 0.036). Patients with EMD interval >24 months were at a lower risk of death as opposed to those with EMD at diagnosis. CONCLUSION: Age at MM diagnosis, site of EMD, and time interval from diagnosis to EMD occurrence were independent prognostic factors in MM patients with EMD. EMD-B bore a better prognosis than EMD-S.