['Prognostic factors for solitary plasmacytoma of bone (SPB), whether measured before or after radiotherapy (RT), have not been established. The authors analyzed multiple factors for myeloma-free survival (MFS) and cause-specific survival (CSS) in SPB patients treated with RT alone.', 'Between 1965 and 2000, 60 patients with carefully staged SPB were treated with RT alone at the M. D. Anderson Cancer Center. Patient ages ranged from 29-77 years (median, 54 years), and 75% of patients had a myeloma (M) protein in the blood and/or urine. No patients showed other lesions on skeletal survey or, in recent years, magnetic resonance imaging (MRI) of the spine; marrow aspirate was normal in all patients. Radiotherapy to the solitary lesion was given to a total dose of 30-70 Gy (median, 46 Gy). The authors analyzed the impact of multiple factors on MFS and CSS, including resolution v. persistence of M protein after RT, secretory v. nonsecretory disease at diagnosis, presence v. absence of an associated soft tissue mass on computed tomography or MRI scan, magnitude of serum M protein elevation at diagnosis, age, spinal v. nonspinal location, Karnofsky performance status, total RT dose, and tumor size.', 'Median follow-up was 7.8 years (range, 1.0-25.5 years). On multivariate analysis, persistence of M protein more than one year after RT was the only independent adverse prognostic factor for MFS (P = 0.005) and CSS (P = 0.04). Most patients with M protein that persisted for more than one year after RT were diagnosed with multiple myeloma within 2.2 years of treatment.'] Is persistence of myeloma protein for more than one year after radiotherapy an adverse prognostic factor in solitary plasmacytoma of bone?