Long-Term Outcomes In Patients With Solitary Bone Plasmacytoma Treated With Definitive Radiation Therapy

Solitary plasmacytoma is a rare entity of plasma cell neoplasms. The clinico-pathologic and therapy-related factors that affect local control and survival are not clearly defined. We reviewed long-term outcomes with definitive radiotherapy (RT) and examined prognostic factors for local control (LC) and progression to multiple myeloma (PMM) for solitary bone plasmacytoma (SBP). Cases of SBP treated with definitive RT were reviewed. Exclusion criteria were multiple myeloma at diagnosis, extramedullary plasmacytoma or absence of definitive RT. Kaplan-Meier methods were used for survival analysis and median follow-up was estimated by the Schemper method. Failures were classified as non-myeloma (local, same bone, other bone, or multiple plasmacytoma) or PMM. The following variables were examined using univariate and multivariate analyses: age (≥60 versus <60 years), sex, anatomic site (appendicular versus axial skeleton), tumor size at presentation (<20cc versus ≥20cc), radiation dose (EQD2 <46Gy versus ≥46Gy), presence of serum myeloma protein at diagnosis, persistence of serum myeloma protein after RT and radiologic response after RT (complete response versus partial response or stable disease). Between January 1988 and January 2016, clinical data from 31 patients were collected. The majority were males (71%) and aged <60 years (61%). Median age at diagnosis was 53 ± 11.17 years (range, 37-76). MRI staging was obtained in 77%. The lesion was located in the axial skeleton in 52% and in the appendicular skeleton in 48%. Serum myeloma protein and urine Bence-Jones protein were identified in 58% and 19%, respectively. The median dose of RT was 45Gy ± 3.5 given in 1.8-2.5-Gy fractions (median EQD2 44.3Gy ± 3.7) and adjuvant chemotherapy was given in 26% of the cases. Median follow-up was 55 months ± 56.5 (range 3-185). LC rates at 5 and 10 years were both 82%. Six patients (19%) had non-myeloma failure at median time 5.4 months ± 5.3 (range 5 – 15) and 14 (45%) had multiple myeloma failure at median 20 months ± 37.2 (5 – 125). At 5 and 10 years, overall survival rates were 88% and 81%, respectively, and PMM rates, 35% and 58%. On univariate analysis, probable prognostic factors for LC were site (p=0.215) and radiologic response (p=0.078), and for PMM, age (p=0.235) and radiologic response (p=0.132). On multivariate analysis, no variable was found to be a prognostic factor for LC or PMM. Definitive radiotherapy for SBP yields durable local control. Multicenter collaborative prospective databases may facilitate data collection towards identifying prognostic factors for LC and PMM.