The definition of EMM can vary widely between groups but generally, it can be classified into two broad groups; bone-related/ osseous EMM and extraosseous EMM.
Introduction: Multiple myeloma (MM) is the second most common blood cancer worldwide and is characterised by clonal proliferation of malignant plasma cells within the bone marrow. These MM plasma cells secrete a monoclonal antibody, often known as M-protein. This can lead to bone destruction, anaemia, myelosuppression and have implications on renal function and other systems.1 Although survival rates are constantly improving with the introduction of new therapies, MM is still incurable as patients will eventually relapse or stop responding to available therapies. Since MM cells usually reside in the bone marrow, most therapies work to target cell types within this microenvironment. However, in rare cases MM can present in a particularly aggressive form whereby the cancer cells have become independent of the bone marrow and may infiltrate other organ systems (Figure 1). This is known as extramedullary MM (EMM) and occurs in only around 7% of patients at diagnosis (primary EMM), and up to 20% of patients at relapsed stage (secondary EMM).2 Even in the era of novel therapies, the incidence of EMM is increasing. Many hypothesise that EMM can arise from particularly aggressive MM clones originating in the bone marrow. Since most current therapies have been designed to target the bone marrow and its niche, it is possible that these therapies are driving malignant plasma cells to evolve and escape the bone marrow and present as EMM. With a particularly poor prognosis, it is important that we are aware of EMM, and improve our understanding of its presentation and pathogenesis. Here I give an overview of the disease, our current understanding of pathogenesis, and treatment approaches.
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