Immunoglobulin free light chains and solitary plasmacytoma of bone

Abstract

An abnormal serum immunoglobulin free light chain (FLC) ratio at diagnosis may identify risk of progression to myeloma in patients with solitary bone plasmacytoma (SBP). In the cohort of 116 patients, 43 have progressed to myeloma, with a median time to progression of 1.8 years. The FLC ratio was determined in all 116 patients on serum collected at time of diagnosis and was abnormal in 54 patients (47%). An abnormal FLC ratio was associated with a higher risk of progression to myeloma (P = .039). The risk of progression at 5 years was 44% in patients with an abnormal serum FLC ratio at diagnosis compared with 26% in those with a normal FLC ratio. One to 2 years following diagnosis, a persistent serum M protein level of 5 g/L (0.5 g/dL) or higher was an additional risk factor for progression. A risk stratification model was constructed using the 2 variables of FLC ratio and M protein level: patients with a normal FLC ratio at baseline and M protein level less than 5 g/L (0.5 g/dL) at 1 to 2 years following diagnosis (low risk, n = 31); with either risk factor abnormal (intermediate risk, n = 26); and with both an abnormal FLC ratio and M protein level of 5 g/L (0.5 g/dL) or higher (high risk, n = 18). The corresponding progression rates at 5 years were significantly different in the low, intermediate, and high groups: 13%, 26%, and 62%, respectively (P < .001).

Figure 1.

Kaplan-Meier plots for time to progression and overall survival in our cohort of patients with solitary bone plasmacytoma. For the analysis, 116 patients with available free light chain (FLC) ratios were evaluated. An abnormal FLC ratio at diagnosis had an adverse effect on both time to progression to multiple myeloma (A) and on overall survival (B).

Figure 2.

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.