Mondo Description A plasma cell neoplasm arising at an extraosseous site. There is no involvement of the bone marrow. It most frequently involves the oropharynx, nasopharynx, sinuses, and larynx. Other sites of involvement include the gastrointestinal tract, central nervous system, breast, skin, lymph nodes, and bladder. A minority of patients have a monoclonal gammopathy. Treatment includes radiation therapy. Progression to plasma cell myeloma occurs in a minority of patients.