Characteristics and Survival Outcomes of Solitary Plasmacytomas: A 30-Year Experience of the Greek Myeloma Study Group on 175 Patients.

In this multi-institutional retrospective study, we analyzed the characteristics and outcomes of 118 patients with solitary bone plasmacytoma (SBP) and 57 with extramedullary plasmacytoma (SEP) diagnosed over 30 years. We also evaluated the impact of systemic therapy (ST), which is not routinely recommended, compared to standard radiation therapy (RT). The median age was 62 years (range: 17-85). Treatment included RT (n = 94), RT with ST (n = 47), ST alone (n = 22), and surgical excision alone (n = 12). Overall and complete response (CR) rates were 93% and 53%, respectively; 70 patients relapsed, 56 progressing to multiple myeloma (MM). The median follow-up was 10 years (95% CI: 7.1-13). Median estimated overall survival (OS) was 18.5 years with a 5- and 10-year OS rate of 85% and 70%, respectively, similar across groups (p > 0.05). Median progression-free survival (PFS) was 75 months (95% CI: 53-97), with a 5- and 10-year PFS rates of 57% and 44%. The 5- and 10-year MM-free survival (MMFS) was 66% and 53%, respectively. We identified age ≤ 60 years, achieving CR, and an abnormal serum FLC ratio at diagnosis as the strongest prognosticators for OS (HR: 0.25), PFS (HR: 0.54), and MMFS (HR: 5 + 0.4), respectively (p < 0.05); ST alone or combined with RT did not significantly improve survival outcomes or MMFS (p > 0.05). In conclusion, ST increased toxicity without offering outcome benefits, reaffirming RT as the cornerstone of SP management. Despite therapeutic advancements in MM, the persistent challenge of progression to MM underscores the importance of identifying high-risk SP patients, enabling early intervention with more aggressive treatments.