Extramedullary multiple myeloma.

Extramedullary multiple myeloma (EMM) is an aggressive subentity of multiple myeloma, characterized by the ability of a subclone to thrive and grow independent of the bone marrow microenvironment, resulting in a high-risk state associated with increased proliferation, evasion of apoptosis and treatment resistance. Despite improvement in survival for most patients with multiple myeloma over recent decades, outcomes are generally poor when EMM develops. Understanding the molecular underpinnings leading to homing of plasma cells in ecosystems outside the bone marrow will be crucial for therapeutically manipulating the microenvironment and targeting key signaling pathways. Herein, we discuss the evolutionary biology of EMM, underscore the importance of a uniform definition, discuss prognostic significance, and provide current and emerging treatment strategies for managing this rare subentity of multiple myeloma.