A plasmacytoma is a solitary mass of myeloma cells that occurs in one location. It usually develops in the bone marrow, although it can develop in other areas of the body. These tumors are normally treated with radiation therapy. The prognosis is usually excellent; however, some people with solitary plasmacytoma will develop multiple myeloma, especially if the plasmacytoma is in the bone. Frequent examinations are important to watch for a progression to multiple myeloma.