21.2.2. Prognostic factors in patients with solitary plasmacytoma of bone

Chapter 21

Many attempts have been made to identify prognostic factors associated with disease-free survival and progression to MM. Tumour location, presence of a monoclonal protein at diagnosis [478], persistence of a serum monoclonal protein [479] and low levels of uninvolved immunoglobulins [480] have each been reported to be associated with progression. However, the relatively small number of patients studied has limited the ability to draw robust conclusions [481]; moreover, the prognostic utility of many factors has not been consistent between series. One of the more reliable predictors of progression is the persistence of a serum monoclonal protein [472][479]. However, as this can only be assessed at 1 or 2 years following therapy, there is a clear need for reliable prognostic markers that can be measured at diagnosis [472].

The prognostic utility of baseline sFLC measurements was evaluated by Dingli et al. [472]. A total of 43/116 patients progressed to MM with a median time to progression of 1.8 years. An abnormal κ/λ sFLC ratio was associated with a 44% risk of progression at 5 years compared with a 26% risk in patients with a normal κ/λ sFLC ratio (p=0.039) (Figure 21.2). Patients with an abnormal κ/λ sFLC ratio also had a shorter overall survival (Figure 21.3). At 1 - 2 years after therapy, a persistent serum monoclonal immunoglobulin concentration of ≥5 g/L was an additional risk factor for progression to MM. A risk stratification model was therefore constructed based on these two risk factors (an abnormal κ/λ sFLC ratio at baseline and a monoclonal protein concentration of >5 g/L at 1 - 2 years following diagnosis). Low-, intermediate- and high-risk groups of progression to MM corresponded to none, one or two risk factors, and these gave 5-year progression rates of 13%, 26% and 62% respectively (Figure 21.4). The authors commented that sFLC analysis provided an important prognostic indicator in these patients.

Subsequent studies have further highlighted the prognostic potential of sFLC analysis. Koch et al. [475] reported that 17/32 (53%) patients with an abnormal sFLC ratio at diagnosis progressed to MM compared with 2/17 (12%) patients with a normal κ/λ sFLC ratio at diagnosis. The authors concluded that an abnormal baseline κ/λ sFLC ratio was significantly associated with progression to MM (p=0.012). Most recently, Fouquet and colleagues [476] proposed a risk stratification model that incorporates an abnormal involved FLC (iFLC) concentration and whole body, fluorodeoxyglucose positron emission tomography – computed tomography (FDG-PET CT) at diagnosis. This model was constructed using patients with both bone and extramedullary plasmacytomas and is discussed in detail in Section 21.3.

In summary, these reports indicate that baseline sFLC abnormalities are consistently associated with increased risk of progression from SPB to MM, and sFLC analysis is recommended in IMWG guidelines for all patients with solitary plasmacytoma (Section 25.3.1).