Metastatic Bone Survey in Monoclonal Gammopathy of Undertermined Significance: Useful or Not?

Introduction: Monoclonal gammopathy of undetermined significance (MGUS) is defined as the presence of a serum monoclonal protein <3g/dl, with fewer than 10% plasma cells in bone marrow and absence of lytic bone lesions, anemia, hypercalcemia and renal insufficiency. Incidence increases with age, especially over 70 and its progression to malignant disease occurs at 1% per year. However, so far there are no studies which can reliably distinguish patients who would progress from those who would remain stable. Based on available literature, it is concluded that MGUS has low risk of progression when M-protein is less than 1.5 g/dl, with no reduction in polyclonal immunoglobulins and bone marrow plasma cells less than 5%.

The recommended testing with suspected MGUS is hemoglobin concentration, protein studies, serum calcium, and creatinine. Metastatic bone survey (MBS) and bone marrow aspiration are felt unnecessary if M-protein is less than 1.5 g/dl. However literature to support the use of MBS at diagnosis based on the level of M-protein is limited. Also our observation has been that due to lack of clear guidelines, most physicians obtain a baseline MBS and some follow patients with yearly or every other year MBS irrespective of the level of M-protein. Hence, we decided to review patients diagnosed with MGUS at our institution to determine the importance of MBS and if possible identify risk factors like age, race, M-protein level, hemoglobin concentration, serum calcium or creatinine level, which would identify a subgroup of patients needing a MBS. In doing so we were hoping to separate out those patients in whom we could recommend against unnecessary use of the skeletal survey below a certain defined M protein level.

Study: We reviewed charts on 1906 patients at Henry Ford hospital diagnosed with MGUS between 1990 and 2007. All patients with at least one M-protein and one MBS done were included in the analysis. We excluded patients with a level of M-protein >3.0 g/dl, who never had a skeletal survey in our system, had a light chain myeloma, plasmacytoma, chronic lymphocytic lymphoma(CLL), amyloidosis or protein evaluation done for diagnosis other than MGUS. We had 620 such patients. We collected data regarding their age, sex, ethnicity, date of diagnosis, type and level of the M-protein, hemoglobin level, serum calcium and creatinine at baseline, result of the MBS, date of progression to multiple myeloma (MM) if any and the date of last follow up if they did not progress to MM. Positive MBS is defined as x ray findings consistent with myelomatous changes with bone marrow aspiration confirming diagnosis of MM.

Results: Of 620 patients, 36 had a positive MBS and applying non parametric Mann Whitney test and a chi-squared test, positive results seemed to correlate with higher level of M-protein, IgG subtype, lower hemoglobin and higher creatinine. Male sex and older age were other risk factors.

Using the LOES curve to graph the risk of a positive skeletal event with the level of M-protein, risk was noted to increase significantly with M-protein in the range of 1.8– 3.0 (odds ratio 8.84 compared with 1.31 if level was less than 1.8), which was highly statistically significant as shown in figure 1.

Further for 97/620 who progressed to multiple myeloma, the risk of progression was significantly higher for males, younger age at diagnosis of MGUS, lower hemoglobin, higher level of M-protein, IgG subtype and a positive skeletal event.

Discussion: Our study is a retrospective chart review with its own limitations. However to our knowledge this is the first study to define the level of M-protein in patients with MGUS above which obtaining a MBS may be of value. Our study identifies 1.8 as a cut off value of M-protein below which doing routine MBS without symptoms of bone pains or other laboratory features suggesting progression to multiple myeloma might be unnecessary. Other risk factors for a positive event and progression to MM like lower hemoglobin, higher creatinine, older age, male sex and IgG subtype in our study are in keeping with what has been described in the literature.

Conclusion: Based on our study, obtaining baseline MBS in all patients with suspected MGUS was not beneficial. Hence, we would not recommend obtaining MBS in patients with M-protein <1.8 g/dl in absence of other risk factors for progression to multiple myeloma.

Figure 1:

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LOES curve showing increased likelihood of positive MBS for increasing MPEV level.

Figure 1:

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LOES curve showing increased likelihood of positive MBS for increasing MPEV level.

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