Multiple myeloma is a cancer that starts in plasma cells. Plasma cells are a type of white blood cell.
Plasma cells make antibodies that help the body fight infection. They are found mainly in the bone marrow but are also in some other tissues and organs. Bone marrow is the soft, spongy tissue inside most bones where different types of blood cells are made.
Plasma cells in the bone marrow sometimes change and no longer grow or behave normally. These changes can cause a precancerous condition called monoclonal gammopathy of unknown significance (MGUS). Precancerous means that the cells are not yet cancer but there is a higher chance these abnormal changes will become cancer. In some cases, MGUS can develop into multiple myeloma.
Myeloma develops when abnormal plasma cells in the bone marrow begin to divide uncontrollably and make more abnormal plasma cells. The abnormal plasma cells are called myeloma cells. Over time, the myeloma cells build up in the bone marrow. This makes it hard for other blood cells in the bone marrow to develop and work normally. The buildup of myeloma cells in the bone marrow can upset the balance of certain minerals in the body. The myeloma cells also make a substance that leads to bone damage and high levels of calcium in the blood. Myeloma cells also make abnormal proteins that can affect other organs such as the kidneys.
Multiple myeloma is described as smouldering or active. Smouldering multiple myeloma is also called indolent or asymptomatic myeloma because it doesn’t cause any symptoms. People with active multiple myeloma have symptoms.
Myeloma cells can form tumours in bones. If there is only one tumour in a bone, it is called a solitary plasmacytoma. When many plasmacytomas are found in the bones, the condition is called multiple myeloma. Plasmacytomas can also form outside of the bones. These tumours are called extramedullary plasmacytomas.
Rare types of multiple myeloma can also develop. These include immunoglobulin D (IgD) myeloma and immunoglobulin E (IgE) myeloma.