Long-term Outcomes After Definitive Radiation Therapy for Solitary Plasmacytoma.

Objective: Definitive radiotherapy (RT) with or without surgery is the standard of care for solitary plasmacytoma. Here, we report clinical outcomes for this rare malignant neoplasm.
Patients and Methods: We retrospectively reviewed the medical records of adults with solitary plasmacytoma treated with definitive RT between 1963 and 2015 at a single institution, and assessed disease control, survival, and toxicity per Common Terminology Criteria for Adverse Events (CTCAE), version 4.
Results: A total of 42 patients with solitary plasmacytoma of the bone (SPB, n=27) or extramedullary plasmacytoma (EMP, n=15) were treated with definitive RT with (n=11) or without (n=31) surgical resection. The median age at diagnosis was 59 years (range: 28 to 76 y).Twenty-two patients had tumors ≥5 cm and 20 had tumors <5 cm. Immunoglobulins were elevated in 23 patients and M-protein in 14. The median RT dose was 45 Gy (range: 15 to 54 Gy) over a median 25 fractions (range: 1 to 38 fractions) with 3 patients receiving twice-daily fractionation and 6 received elective nodal irradiation. No patients received adjuvant chemotherapy. The median follow-up was 10.3 years. The 10-year local control rate after RT was 88%. Five patients who developed a local recurrence had SPB ≥5 cm. The 10-year multiple myeloma-free survival rates were: overall, 47%; SPB, 24%; and EMP, 87% (P=0.0012). The 10-year cause-specific survival rate was 75%: 64% for SPB versus 93% for EMP (P=0.0116). The 10-year overall survival rate was 60%. Three patients experienced late grade 2+ toxicity.
Conclusions: Definitive RT with moderate doses results in excellent local control. We observed a higher rate of progression to multiple myeloma and lower survival in patients with SPB compared with EMP.