Solitary bone plasmacytoma (SBP) and Solitary bone plasmacytoma with minimal marrow involvement (SBPmm) are treated similarly, with definitive radiation therapy (RT) followed by observation. However, the distinction in the natural disease course after RT between the two is not well described. We performed a multi-center study assessing post RT outcomes in 141 SBP and 102 SBPmm patients diagnosed between January 2010 and December 2024. 92% of SBPmm patients progressed to multiple myeloma (MM) vs 76% in patients with SBP (p=0.01); 21% of progressions from SBP were with another solitary bone lesion vs 4% in SBPmm (p 0.01). Patients with a long time to progression (TTP) post first RT had durable responses with RT to the second solitary plasmacytoma in the absence of other myeloma-defining events (MDE). Multi-focal lytic bone lesions were the most common MDE, in 89% and 81% of SBPmm and SBP patients, respectively. SBPmm patients had a median TTP of 15 months (95% CI 14-21) compared to 43 months (95% CI, 34-55) in SBP patients (p0.0001), with 67% SBPmm patients progressing within 2 years of diagnosis. Del(17p) and 1q+ (gain or amplification) predicted for shorter TTP to MM in SBPmm, with 2-year progression rates of 100% and 82%, respectively. We report significant differences in the risk and modes of progression between SBP and SBPmm. SBPmm patients are at particularly high risk for early progression to MM, with majority developing bone disease as the MDE. These findings support exploring prospective studies that incorporate frontline systemic therapy for SBPmm patients.
Lim et al. (Wed,) studied this question.