malignant proliferation (tumour) of cells within the bone marrow that usually occurs during middle age or later and increases in occurrence with age. Myelomas are equally common in males and females and affect any of the marrow-containing bones such as the skull, the flat bones (e.g., ribs, sternum, pelvis, shoulder blades), and vertebrae.

The disease manifests as a proliferation of abnormal plasma cells or plasmablasts that populate the bone marrow throughout the body. These cells produce large quantities of myeloma protein, a monoclonal antibody that can replace the normal antibodies in the blood, reducing the ability of the body to ward off infection. Myeloma proteins can also collect in the tubules of the kidney and cause renal failure. In addition, bone destruction that releases calcium into the circulation may result in calcium deposition in the kidneys and other abnormal sites.

Symptoms and signs of multiple myeloma include pain, anemia, weakness, susceptibility to infection, a tendency to hemorrhage, shortness of breath, and kidney insufficiency. Pathological bone fractures may occur, and neurological symptoms may follow the collapse of affected vertebrae. The disease is progressive and is considered incurable. Current treatments are directed toward changing multiple myeloma into a manageable chronic disease and increasing the overall survival rate. Thalidomide is often used initially to treat multiple myeloma and can prevent progression for a variable length of time. When appropriate, bone-marrow transplantation after high-dose chemotherapy can lead to long-term survival. However, the success rate is variable, with complete remissions lasting from only a few months to many years. In the rare instances that a malignant proliferation of plasma cells is confined to one location, the tumour is called a plasmacytoma and can be treated with irradiation or surgery.