3367 - Radiation Therapy for Oligo-Relapsed/Progressive Disease Post-Stem Cell Transplant for Multiple Myeloma: Factors Associated with Favorable Outcomes

B. Fregonese¹, A. Dreyfuss², S. Usmani³, B. S. Imber², and J. Yahalom¹; ¹Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ²Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ³Hematology, Medical Oncology, Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s): Relapsed or refractory Multiple Myeloma (MM) following stem cell transplant (SCT) poses significant clinical challenges. Localized relapse or progression presents a unique clinical scenario where radiation therapy (RT) can be effective in achieving local control and durable systemic responses post-SCT. Here we report the outcomes of MM patients initially treated with RT in the setting of limited relapse or progression after SCT. Materials/Methods: We identified 31 MM patients who received initial RT for oligo-relapsed or oligo-progressive MM following SCT between 2000 and 2021.Oligo-relapse is defined as a single-site relapse not active on pre-SCT scan, and oligo-progressive MM indicates active sites of disease both pre- and post-SCT. Responses to SCT and RT were assessed by PET/CT or CT according to the response criteria established by the International Myeloma Working Group (IMWG) and Society of Nuclear Medicine and Molecular Imaging, categorized as: complete response (CR), very good partial response (VGPR), partial response (PR), stable disease (SD) and progressive disease (PD). Kaplan-Meier analysis was used to assess time to MM progression post-RT, and log-rank test was used to evaluate for predictors of time to MM progression.
Results: Median times from SCT to new site of oligo-relapsed MM (n=20, 64.5%) and pre-existing but inactive sites of oligo-relapsed MM (n=4, 12.9%) were 31 months (range, 3.2-117.5) and 9 months (3.4-15.9), respectively. 22.6% (n=7) of patients received RT for oligo-progressive MM. Prior to RT, 29 (94%) patients had PET/CT, with a median SUV of 4.4 (2.6-7.2) for target lesions. Solitary bone (77%) and extramedullary (23%) disease were treated to median dose of 30 Gy (8-49 Gy). 24 patients (77.4%) underwent PET/CT for initial post-RT lesion response. At first post-RT scan, 13 (14.9%) achieved CR, 11 (35.5%) PR, 4 (12.9%) SD and 2 (6.5%) had evidence of PD. Among the 18 patients that did not achieve CR, 16 subsequently achieved CR at a median time of 6 months (1.7-44.5) post-RT. Most patients (87%) eventually received systemic therapy following RT, with a median number of 3 (1-17) lines of therapy post-RT. Pre-RT lesion SUV (p=0.020) and presence of additional PET-active sites outside the RT field (p=0.048) were significantly associated with shorter time to next MM event. Oligo-relapse within 12 months of SCT (p=0.07) trended towards a statistical significance.
Conclusion: RT for oligo-relapsed or oligo-progressive disease post-SCT offered excellent local control and likely provides a benefit for this patient population. Our data suggests that high pre-RT lesion SUV, short time to relapse post-SCT, and more PET-active disease sites may be important indicators of less favorable post-RT outcome. Still, the main challenge for these patients remains biochemical relapse or progression of MM. Additional experience with this group could guide more personalized use of RT and systemic treatment in patients with oligo-relapse or oligo-progressive sites post-SCT.