A. Ewongwo1, M. Presson2, C. Hui3, E. Laseinde4, S. Jackson3, L. Million3, R. T. Hoppe3, and M. S. Binkley3; 1Department of Radiation Oncology, Stanford University, Stanford, CA, 2Stanford Healthcare, Palo Alto, CA, 3Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, 4Radiation Oncology, Stanford University, Stanford, CA
Purpose/Objective(s): Radiotherapy (RT) is primary treatment for solitary plasmacytoma (SP) and provides palliation for multiple myeloma (MM). As advancements in systemic agents for MM continue to improve survival, the importance of providing durable local control (LC) increases. The optimal RT dose and fractionation to provide durable pain relief and local control is unknown. Thus, the aim of this study was to evaluate outcomes including incidence of local failure for patients with MM and SP treated with RT. Materials/
Methods: We performed a retrospective analysis of 183 adult patients with MM or SP evaluated at our institution from 2000 to 2022, and identified patients treated with palliative or definitive-intent RT. Patients with missing or incomplete clinical or RT records and less than 1 year follow up were excluded. Clinical response was defined as improvement in presenting symptoms such as pain, cord compression, cauda equina syndrome, and pathologic fracture. Clinical response was assessed at 1- and 3-month intervals post RT and categorized as complete response (CR), partial response (PR), or no response (NR). Progression free survival (PFS) and overall survival (OS) were analyzed using Kaplan-Meier methods and were calculated from the time of completion of initial RT. The incidence of local recurrence was measured by Fine and Gray. Cohorts were divided based on the RT biologically equivalent dose (BED) using an a/ß of 10 and outcomes were compared using log-rank tests. Results: A total of 92 patients who received definitive (n=10) or palliative (n= 82) RT were included in the analysis. Median follow up time was 24 months. Median age at time of initial RT was 67 years (range 30-93). Location of treated lesions included: extramedullary sites (5%), axial (64%) or appendicular skeleton (16%), and multi-site disease (14%). Radiation dose varied from 8 Gy in 1 fraction to 45 Gy in 25 fractions, with median dose of 24 Gy. The most frequent RT regimens were 4 Gy x 5 fractions (35%) and 3 Gy x 10 fractions (37%). After initial RT, pain or symptom response was recorded for 81 patients. Of these patients, 50% reported a CR, 41% had PR, and 9% had NR. After initial RT, the cumulative incidence of LR at 3 and 5 years were 51.8% and 56.3% respectively. For the entire cohort, the 3-year and 5-year OS was 72.0% and 68.7%, respectively, and the 3-year and 5-year PFS was 31.4% and 26.2%, respectively. PFS was similar for BED >30 Gy compared to BED <30 Gy (p-value 0.164). In the BED >30 Gy versus BED <30 Gy cohorts, the 3-year LR rate was 48.8% (95% CI 32.4-63.4) and 53.9% (95% CI 35-69.4), and the 3-year PFS was 36.7% (95% CI 24.0-56.0) and 25.5% (95% CI 13.2-49.2), respectively. Conclusion: This study demonstrates that RT provides both excellent local control and symptomatic relief in MM and SP. Definitive doses were associated with low local recurrence rates. Patients with MM treated with lower doses had high local recurrence rates suggesting further study of factors predictive of local recurrence may aid in radiotherapy dosing decisions.
