Plasmacytoma. Light micrograph showing a somewhat homogenous population of mature plasma cells from a case of solitary plasmacytoma involving a rib. Solitary plasmacytoma of bone is a localized clonal proliferation of immunoglobulin producing plasma cells. By definition, there is no bone marrow involvement at other sites. The clinical features of multiple myeloma are absent. There is no evidence of widespread bone lesions by skeletal survey.