Challenges in the treatment of soft-tissue plasmacytoma: a retrospective analysis of 120 patients with extramedullary multiple myeloma.

Purpose: Despite the development of novel drugs and the widespread use of hematopoietic cell transplantation, the prognosis of patients (pts) with multiple myeloma and extramedullary involvement (soft-tissue plasmacytoma, STP) is rather unfavorable.

Methods: A retrospective analysis of 120 pts with STP treated between 2007 and 2022 was performed. The effects of demographic and clinical characteristics on treatment response, progression-free survival (PFS), and overall survival (OS) were evaluated.

Results: The rate of serological response to first-line STP treatment (at least partial remission) was 67%, and the rate of imaging response was 59%. With a median follow-up of 84.2 months, the median PFS was 10.5 months (primary STP: 20.2 months; secondary STP: 5.8 months), and the median OS was 24.5 months (primary STP: 34.5 months; secondary STP: 12.4 months). Based on the multivariate regression analysis, secondary STP (HR 2.75; HR 2.63) and organ involvement (HR 1.45; HR 1.68) were found to be negative prognostic factors of both PFS and OS. In a prognostic model, pts with at least one of these factors had a significantly worse PFS (HR 3.31) and OS (HR 3.45) than those with none risk factor.

Conclusion: In pts with STP, risk-adapted treatment strategies including immunotherapies and cell therapies are urgently required.