Solitary plasmacytoma (SP) is a rare presentation of plasma cell neoplasms. In contrast to multiple myeloma, long-term disease-free survival and cure is possible following local radiotherapy (RT), particularly for soft tissue presentations. In this study, we attempt to identify factors that predict for local failure, progression to multiple myeloma, and disease-free survival (DFS) in patients mainly managed with local RT.

We identified 46 patients referred to the Princess Margaret Hospital between 1982 and 1993. The median age was 63 years (range 35-95), with a male:female ratio of 1.9:1. All patients had biopsy-proven SP (osseous: 32, soft tissue: 14). M-protein was abnormal in 19 patients (41%). All patients were treated with local RT (median dose 35 Gy), with 5 patients (11%) also receiving chemotherapy. Maximum tumor size pre-RT ranged from 0 to 18 cm (median 2.5).

The 8-year overall survival, DFS, and myeloma-free rates were 65%, 44%, and 50%, respectively. The local control rate was 83%. Factors predictive of progression to myeloma (and poorer DFS) included bone presentation and older age. However, these two factors did not influence local control, which was affected by tumor size. All tumors < 5 cm in bulk (34 patients) were controlled by RT. Anatomic location did not predict outcome; however, 3 of the 5 tumors arising in paranasal sinuses did not achieve local control. Lower RT dose (< or =35 Gy) was not associated with a higher risk of local failure.

Solitary plasmacytomas are effectively treated with moderate-dose RT, although osseous tumors have a high rate of recurrence as systemic myeloma. Large tumor bulk locally (> or =5 cm) predicts for local failure. Combined chemotherapy and RT should be investigated in these high-risk patients to increase the local control rate and the cure rate.