Plasma cell leukaemia (PCL) is a rare and aggressive variant of multiple myeloma (MM), accounting for 2 - 4% of cases. It is defined by the presence of >20% plasma cells in the peripheral blood and/or an absolute plasma cell count >2 x 109/L . It can occur without evidence of MM (primary PCL; 60 - 70%), or may develop from leukaemic transformation of a pre-existing myeloma clone (secondary PCL; 30 - 40%) in 1 - 2% of advanced and refractory patients .
Both primary and secondary PCL have distinct clinical and biological features. The median age of primary PCL patients is approximately 10 years younger than both the general myeloma and secondary PCL populations . Primary PCL also has a more aggressive clinical presentation, with a higher tumour burden and an increased incidence of extramedullary and light-chain only disease (26 - 44%) . Both forms of PCL have very poor outcomes, with the worst prognosis associated with secondary PCL . Overall survival for primary PCL patients is still inferior to that of patients with MM, but has significantly improved in recent years .