Plasmacytomas are isolated clonal proliferations of plasma cells. They are localised to a specific site and do not elicit the typical clinical symptoms associated with multiple myeloma (MM). The estimated incidence rate of all plasmacytomas is 0.34 per 100,000 person years, meaning it is approximately 16-fold less common than MM .
In 2003, the International Myeloma Working Group (IMWG) published criteria for the classification of solitary plasmacytoma of bone (SPB), extramedullary plasmacytoma and multiple solitary plasmacytoma as distinct disease entities . These were subsequently adopted by the British Committee for Standards in Haematology . The incidence of SPB has been estimated to be 40% higher than that of extramedullary plasmacytoma  and there are distinct differences in the probable outcomes for patients with these diseases.