Predictors of Response to Radiation Therapy and of Progression to Multiple Myeloma in Patients with Solitary Bone and Solitary Extramedullary Plasmacytomas

Abstract

<h3>Purpose/Objective(s)</h3> Radiation therapy (RT) plays an important role in treating Solitary Extramedullary (EMP) and Solitary Bone (SBP) Plasmacytomas. The objective of this study is to review the effectiveness of RT in locally controlling the disease and looking at factors that predict progression to Multiple Myeloma (MM) in the PET scan-era. <h3>Materials/Methods</h3> We performed a retrospective analysis of consecutive plasmacytoma patients (pts) treated at our high-volume center. We identified 99 pts, of which 64 were diagnosed with SBP and 35 with EMP, without evidence of MM at diagnosis (end-organ damage, >10% bone marrow involvement, >1 plasmacytoma). We identified 99 total pts, largest series to date in the literature, treated with RT between January 1, 2000, through December 31, 2020. We utilized radiographic staging including PET scans and clinical assessments to determine patient outcomes. Outcomes include initial radiographic responses, local control (LC), overall survival (OS), and progression to MM. Kaplan-Meier was used to assess LC and OS. <h3>Results</h3> Patient and treatment characteristics are summarized in table 1. 24 (69%) EMPs and 54 (84.4%) SBPs received a radiation dose ≥45Gy. 74% (n=26, 5 pts were lost to follow up [FU]) of EMPs and 78% (n=50, 3 pts were lost to FU) of SBPs achieved completed response (CR) with a median time to CR being 7.7 months (0.1-38.9) for EMP and 15.46 months (0.6-89.2) for SBP. All pts achieving a CR were treated with doses ≥40Gy. Only 1 (3%) EMP and 2 (3%) SBPs had an in-field failure after CR. The 2-year OS was 68% for EMPs and 98% for SBPs [p=0.004]. Out of 99 pts, 14 died, and cause of death was not related to the plasma cell neoplasm in 5 EMP pts. The 2-year rate of progression to MM was 15% in the EMP group, and 33% in the SBP group [p<0.001]. On univariate analysis, baseline SUV and size, location [EMP vs. SBP] and achieving a CR after RT were analyzed. Pts with SBP were at higher risk for progression to MM [p<0.05]. <h3>Conclusion</h3> Our data indicates that definitive RT for EMP and SBP can provide excellent LC. Progression to MM remains the main problem and is more common among pts with SBP, which is concordant with previously published literature.