Solitary plasmacytoma of bone (SPB) is a localized, biopsy-proven proliferation of monoclonal plasma cells with no other bone lesions (by MRI and CT scan). Additionally, no other abnormalities attributable to a clonal plasma cell disorder are identified (ie, no hypercalcemia, renal insufficiency, anemia, or multiple bone lesions). If clonal plasma cells are identified in the bone marrow but are less than 10% of the cellularity, a diagnosis of "solitary plasmacytoma with minimal bone marrow involvement" may be made. However, if more than 10% clonal plasma cells are found on bone marrow biopsy, a diagnosis of plasma cell myeloma is appropriate.

SPB makes up 1%-2% of plasma cell neoplasms and occurs more commonly in men, with a median age at diagnosis of 55 years. It is very rare under the age of 30.

Patients present with bone pain or pathologic fracture. If a vertebra is involved, spinal cord compression and associated symptoms may occur. Rarely, patients may present with a palpable mass involving surrounding soft tissue.