Solitary plasmacytoma (SP) is the clinical condition characterized by the localized proliferation of clonal plasma cells. The definition of SP has been evolving as a result of improvement in imaging technology as well as the availability of more sensitive techniques that can detect small populations of clonal plasma cells in the bone marrow. Patients with SP do not have other lytic or sclerotic bone lesions or soft tissue masses, hypercalcemia, renal insufficiency, or anemia and no involvement of the bone marrow by clonal plasma cells (Table 16.1) [1–3]. Some series have included patients with two bone lesions and less than 5 or 10 % clonal plasma cells in the bone marrow [4–10]. The presence of a monoclonal protein in the serum or urine or the presence of elevated immunoglobulin free light chain (FLC) does not exclude the diagnosis. On the contrary, such biomarkers may provide important prognostic information and may guide management. SP is further divided into two entities: solitary plasmacytoma of bone (SPB) and extramedullary plasmacytoma (EMP) where the plasma cell clone generally arises from lymphoid tissues away from the bone marrow microenvironment that normally hosts these cells. SP is quite uncommon and constitutes less than 5 % of all plasma cell neoplasms . Out of 45,366 patients with a plasma cell proliferative disorder seen at Mayo Clinic, Rochester, MN, between 1960 and 2011, 883 patients were diagnosed with SP (2 %). SPB is more common than EMP by a ratio of at least 2:1 [2, 12], although the comprehensive literature review by Alexiou et al. suggests a ratio closer to 5:1 . SP is more common in males (~70 %) and the median age at diagnosis varies from 55 to 60 years, depending on the study [2, 5, 7, 8, 14, 15]. Almost a third of patients are below 50 years of age at the time of diagnosis. Thus, patients diagnosed with SP are significantly younger than those diagnosed with multiple myeloma.
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