Long-term outcome of patients with solitary plasmacytoma treated with radiotherapy: A population-based, single-center study with median follow-up of 13.7 years.

In this single-center, population-based, and retrospective study, we analyzed the outcome of 49 patients with solitary bone plasmacytoma (SBP) and 28 patients with solitary extramedullary plasmacytoma (SEP), all treated with radiotherapy. Laminectomy was performed in 18/30 SBP patients with vertebral involvement and tumour resection in 10 SEP patients. Overall survival and cause of death for each patient were compared to 5 sex-, age-, and residency-matched individuals from the normal population. Response (complete and partial) was achieved in 94% of SBP and 96% of SEP patients. Relapse rates were higher in SBP (65%) compared to patients with SEP (18%) (P < .01). Only one in-field relapse was identified for the whole series. Ten- and 15-year overall survival, progression free survival (PFS) and multiple myeloma free survival (MMFS) for patients with SBP were 60%/41%, 25%/17%, and 33%/33%. Corresponding values for patients with SEP were 67%/54%, 57%/44%, and 91%/91%. SBP patients had significantly shorter PFS and MMFS compared to SEP patients (P < .01 for both). Only two of the SEP patients developed multiple myeloma and no patient in the whole series progressed to multiple myeloma later than 10 years after diagnosis. Unlike for SEP, the major cause of death among SBP patients was multiple myeloma (49%). Compared to matched normal population, no increased risk of death from secondary malignancies or cardiovascular disease was observed. Positive predictors in SBP patients were for overall survival age <60 years, combined laminectomy and radiotherapy and radiotherapy dose >40 gray, for PFS tumour size <6 cm and combined laminectomy and radiotherapy and for MMFS tumour size <6 cm. Radiotherapy confers excellent local control in both SEP and SBP patients; however, the challenge is to prevent development of multiple myeloma in patients with SBP.