Haploidentical Hematopoietic Stem Cell Transplantation for Relapsed/Refractory Extramedullary Multiple Myeloma: A Conditioning Regimen Combining Selinexor and Helical Tomotherapy-A Case Report and Literature Review.

Relapsed/refractory multiple myeloma (RRMM) and extramedullary multiple myeloma (EMM) present significant challenges for patients with multiple myeloma (MM) after their disease progresses.Despite notable advancements in treatments like autologous hematopoietic stem cell transplantation (ASCT) and chimeric antigen receptor (CAR)-T-cell therapy, most patients with RRMM and EMM face a short survival period. Currently, there are no effective treatments available. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is one of the treatment options for MM. Reduced-intensity conditioning (RIC) regimens have largely replaced myeloablative conditioning (MAC) regimens. RIC is now preferred because it significantly lowers transplant-related mortality, which has dropped to 10-20%. However, RIC regimens are linked to higher relapse rates compared to MAC. To enhance the efficacy of allo-HSCT, it is essential to identify a safer and more effective conditioning regimen. We report a case of EMM involving the breast, supraclavicular region, mediastinum, and pleural effusion, among other sites. The patient did not respond to several treatments, including a proteasome inhibitor (PI) like bortezomib, immunomodulatory drugs (IMiDs) such as lenalidomide, and a monoclonal antibody targeting CD38, like daratumumab. Consequently, we recommended haploidentical hematopoietic stem cell transplantation as a salvage treatment. After undergoing allo-HSCT with a conditioning regimen that mainly included selinexor and helical tomotherapy, the patient achieved a complete remission(CR) and enjoyed long-term disease-free survival for 11 months. Along with existing literature, this case provides encouraging insights for future research on RRMM and EMM, and we anticipate more reports on allo-HSCT cases in the future.