Most common primary malignant bone tumour is:

Multiple myeloma is the most common primary neoplasm of the skeletal system.  The disease is a malignancy of plasma cells. Plasma cells are a subset of B cells, which are the producers of humoral immunity factors termed antibodies. However, if malignant transformation occurs in a single plasma cell, its clones produce only a single type of immunoglobulin, and electrophoresis demonstrates a monoclonal peak corresponding to this particular immunoglobulin.  Multiple myeloma has been the prototype of monoclonal malignancies, in this case, of plasma cells. Reference: Shafer’s Textbook of ORAL PATHOLOGY Eighth Edition page no 241