Clinical features, outcome, and prognostic factors for survival and evolution to multiple myeloma of solitary plasmacytomas: A report of the Greek myeloma study group in 97 patients

Solitary plasmacytoma (SP) is a rare plasma cell dyscrasia characterized by the presence of bone or extramedullary plasma cell tumors. The treatment of choice is local radiotherapy (R/T) ± surgical excision. The role of adjuvant chemotherapy (C/T) or novel agents (NA) is uncertain. Data related to prognostic factors are inconclusive. Herein, we describe the clinical features, survival and prognosis of 97 consecutive patients, 65 with bone SP (SBP), and 32 with extramedullary SP (SEP), diagnosed and treated in 12 Greek Myeloma Centers. Objective response rate (≥PR) and complete response (CR) was 91.8% and 61.9%, respectively, and did not differ between the 2 groups. Overall, 38 patients relapsed or progressed to multiple myeloma (MM). After a median follow-up of 60 months, 5 and 10-year overall survival (OS) probability was 92% and 89% in SEP and 86% and 69% in SBP, respectively (P = 0.2). The 5- and 10-year MM-free survival (MMFS) probability was 90% and 70% for patients with SEP vs. 59% and 50% for patients with SBP, respectively (P = 0.054). Overall, the 5- and 10-year OS probability, plasmacytoma relapse-free survival (PRFS), progression-free survival and MMFS was 84% and 78%, 72% and 58%, 58% and 43%, and 70% and 59%, respectively. In the multivariate analysis, prolonged PRFS and young age were positive predictors of OS. Achievement of CR was the only positive predictor of PRFS. Immunoparesis was the only negative predictor of progression to MM. The addition of C/T or NA-based treatment increased toxicity without offering any survival advantage over R/T.