Identification of factors that predict progression in MGUS, such as those described above, is important for individual patient management because the risk of transformation is life-long and the evolution of disease between individuals is variable. MGUS may evolve to other asymptomatic pre-malignant states with a significantly higher risk of progression (i.e. smouldering MM, smouldering WM, smouldering LCMM: Section 12.3). Disease evolution has been shown to be characterised by the acquisition of genetic mutations by tumour cell clones and associated changes in the bone marrow microenvironment (Chapter 19) .
13.3.1. MGUS consistently precedes MM
Two independent studies, by Landgren et al.  and Weiss et al. , addressed whether MGUS is consistently present prior to a diagnosis of MM. Landgren et al.  tested pre-diagnostic serum samples from 71 subjects who developed MM. Samples were available up to 10 years prior to the diagnosis of malignant disease. Using serum protein electrophoresis (SPE), serum immunofixation (sIFE) and sFLC analysis, it was shown that >80% of patients had detectable monoclonal protein ≥8 years before diagnosis, increasing to 100% 2 years before diagnosis. In approximately half of the patients who went on to develop MM, the involved/uninvolved sFLC ratio (defined as κ/λ in κ patients and λ/κ in λ patients) showed a year-by-year increase, whilst ratios in the other half remained largely stable. This is an important observation since an evolving MGUS phenotype (in which the monoclonal protein shows a steady increase, Figure 13.8) has been shown to identify patients with a high risk of malignant transformation , with transformation rates of 55% and 10% at 10 years of follow-up for evolving and stable phenotypes respectively .
In a separate group of patients, Weiss et al.  demonstrated that 27 of 30 MM diagnoses were preceded by MGUS. Monoclonal proteins were identified by SPE and/or IFE in 78% of patients and by an abnormal κ/λ sFLC ratio alone in 22% of patients. Of the 3 patients who had no MGUS detected, one patient with IgG myeloma had only one pre-diagnostic serum sample available at 9.5 years before diagnosis. The remaining 2 negative cases were IgD myeloma with sera available only 3 and 5 years before the diagnosis. Similarly to Landgren et al., the authors found that substantial increases in the involved sFLC ratio may precede the diagnosis of MM, with or without a corresponding change in the monoclonal intact immunoglobulin. It was notable that, in all four patients diagnosed with LCMM or NSMM, the disease evolved from light chain MGUS. and comprising 20 patients who went on to develop AL amyloidosis, MGUS was detected prior to diagnosis in all cases. A median of 3 pre-diagnostic samples were obtained up to 10 years prior to diagnosis. The monoclonal protein type was FLC in 55% of cases, and intact immunoglobulin in 45% of cases. The prevalence of MGUS was 100%, 80% and 42% at <4, 4 to 11, and >11 years prior to diagnosis.
Taken together, these findings confirm that a pre-malignant MGUS phase exists prior to disease emergence in most MM and AL amyloidosis patients. In a review of MGUS as a precursor condition, Weiss and Kuehl  included a model illustrating different patterns of MGUS progression comprising: non-evolving, evolving and rapidly evolving (Figure 13.8). Measuring sFLC levels in routine MGUS follow-up may help to identify patients, with an evolving phenotype, who are at a higher risk of progression to malignant disease compared to patients with stable MGUS.
13.3.2. Light chain MGUS as a separate clinical entity and the pre-malignant precursor of LCMM. The authors defined light chain MGUS as an abnormal κ/λ sFLC ratio with an increased concentration of the involved sFLC, no expression of monoclonal intact immunoglobulin, and an absence of end-organ damage that can be attributed to the plasma cell proliferative disorder. An example is shown in Figure 13.9. No abnormality was detected by SPE or urine protein electrophoresis (UPE), but an abnormal κ/λ sFLC ratio indicated the presence of monoclonal FLCs. This finding was confirmed by the detection of λ uBJP by urine IFE (uIFE).
In the study by Dispenzieri et al.  the prevalence and risk of progression of light chain MGUS amongst 18,357 residents of Olmstead County, Minnesota aged 50 years or older was assessed. 610 (3.3%) individuals had an abnormal κ/λ sFLC ratio, of whom 213 had an intact immunoglobulin MGUS. This included 57/213 additional patients whose monoclonal intact immunoglobulin had not previously been detected by screening with SPE, and so the prevalence of conventional MGUS in this population was revised from 3.2% to 3.4% (95% CI 3.2 - 3.7). Of the 397 individuals with an abnormal sFLC ratio but no abnormality detected by SPE, a total of 146 met the definition of light chain MGUS, equivalent to a prevalence of 0.8% (95% CI 0.7 - 0.9). This represented 19% of the total MGUS population; a proportion that matches the relative incidence of LCMM compared to all MM. The light chain type was identified as κ or λ in 108 and 38 individuals, respectively. Overall, involved sFLC concentrations tended to be low; only around 10% of patients had concentrations greater than 200 mg/L. It was noted that 23% of light chain MGUS patients either had or subsequently acquired, renal disease; an observation of relevance to the later proposal for monoclonal gammopathy of renal significance to be considerd as a separate entity (Section 26.4.1). A similar incidence of light chain MGUS (0.7%) was confirmed by a European study of 4702 individuals aged 45 - 75 years .
Although a coexisting abnormal κ/λ sFLC ratio increased the risk of progression to MM and related disorders for patients with conventional MGUS, risk of progression of light chain MGUS did not differ from that for patients with low-risk conventional MGUS (i.e. those with no abnormal κ/λ sFLC ratio) (p=0.1822) . Dispenzieri et al.  speculated that the transformation events resulting in either light chain MGUS or conventional MGUS may be the same, and that the presence of an abnormal sFLC ratio in patients with conventional MGUS indicate that two transformation events had occured and their disease was one step closer to malignant transformation.
13.3.3. Prior knowledge of MGUS improves multiple myeloma survival and Go et al.  assessed the impact of prior knowledge of MGUS diagnosis on MM survival. Sigurdardottir et al.  studied MM survival rates from the time of diagnosis for 14,798 patients enrolled in the Swedish Cancer Registry. A total of 2.7% of patients had a prior diagnosis of MGUS. MM patients with a prior knowledge of MGUS had a better median overall survival than those without prior knowledge of MGUS (median survival 2.1 vs. 2.8 years, p<0.01, Figure 13.10A). Similar findings were reported by Go et al.  Sigurdardottir concluded that “Our observations stress the importance of clinical follow-up in patients with MGUS, regardless of risk stratification.”
Go and colleagues  compared the rates of major complications at cancer diagnosis for patients with a monoclonal gammopathy-associated malignancy (MM, Waldenström’s macroglobulinaemia and lymphoplasmacytic lymphoma) for patients with and without MGUS follow-up. Of the 17,457 study patients, 6% had a prior diagnosis of MGUS. Patients with prior MGUS follow-up experienced significantly fewer complications, including acute kidney injury, hypercalcaemia and fractures (Figure 13.10B).Although both studies provide support for the practice of MGUS follow-up examinations, a recent commentary by Kyle and Rajkumar  stated that prospective studies are now required to confirm the value of MGUS follow-up and the optimal approach to monitor patients.