A Plasmacytoma is a plasma cell neoplasm which forms tumors when plasma cells become malignant and grow out of control. Plasmacytomas crowd out normal cells in the bone marrow as well as invade the hard outer part of the bone and then spread into the cavities of the large bones in the body. Plasmacytoma in the bones may cause pain or broken bones. Plasmacytoma of the bone often becomes multiple myeloma. When only one tumor is formed, it is called a solitary plasmacytoma. When multiple small tumors are formed, the disease is multiple myeloma. Plasmacytomas can also invade soft tissue in the body. Plastacytoma in soft tissue can press on nearby areas and cause pain such as the throat, tonsils or sinuses.