Progression of solitary bone plasmacytoma. Risk of progression of solitary bone plasmacytoma to myeloma using a risk stratification model that incorporates the serum free light chain (FLC) ratio measured at baseline (A) or 1 to 2 years following diagnosis (B) and the persistence of serum monoclonal protein 1 to 2 years following diagnosis. The top curve illustrates risk of progression with time in patients with both risk factors, namely an abnormal serum free light chain ratio (< 0.26 or > 1.65) and M protein level of 5 g/L (0.5 g/dL) or more (high risk); the second gives the risk of progression in patients with any one of the 2 risk factors (intermediate risk); the third curve illustrates the risk of progression with neither risk factor present (low risk). The date at which the test for presence of serum M protein 1 to 2 years following diagnosis was determined was used as time 0 for this analysis.