Long-term Outcomes After Definitive Radiation Therapy for Solitary Plasmacytoma

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.

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.

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.

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.