21.3. Solitary extramedullary plasmacytoma

Chapter 21

Solitary extramedullary plasmacytomas are plasma cell tumours that arise outside the bone marrow and are defined by the criteria listed below. They are predominantly found in the head and neck mucosa, especially within the upper respiratory tract, although they can occur in any organ. Local tumour irradiation is the treatment of choice, with nearly all patients successfully achieving local control.

Criteria for the diagnosis of extramedullary plasmacytoma [471]

▪ Low concentration or absence of monoclonal protein in serum and/or urine

▪ Extramedullary tumour of clonal plasma cells

▪ Normal bone marrow

▪ Normal skeletal survey

▪ No related organ or tissue impairment (no end organ damage including bone lesions)

The outcome of patients with a solitary extramedullary plasmacytoma is varied, with a 5-year survival rate of 78.9% [469]. As with SPB, the most common outcome is progression to MM, which is reported to occur in 30 - 50% of patients, with a median time to progression of 1.5 - 2.5 years. In fewer than 10% of patients, there is recurrence of disease at the site of the original lesion [474]. A monoclonal protein (typically IgA [470]) is detected in the serum or urine of approximately 25% of patients [480]. Although there is limited evidence, sFLC measurements should also prove useful in monitoring patients with abnormal iFLCs [486], particularly those previously classified as having nonsecretory disease [474].

Fouquet et al. [476] assessed the prognostic utility of sFLCs and whole body FDG-PET CT in 43 patients with solitary plasmacytoma. This comprised 33 patients with solitary plasmacytoma of bone and 10 patients with extramedullary plasmacytoma. At diagnosis, 21/43 (49%) patients had an abnormal iFLC concentration and 11/43 (26%) had an abnormal κ/λ sFLC ratio. By univariate analysis, an abnormal iFLC concentration, an abnormal κ/λ sFLC ratio or the presence of ≥2 hypermetabolic lesions on initial PET CT were associated with significantly shorter time to MM progression (Figure 21.5, p=0.002; and data not shown). On multivariate analysis, an abnormal initial iFLC concentration and the presence of ≥2 hypermetabolic lesions on PET-CT were the strongest independent prognostic factors to identify patients at greatest risk of progression to MM. A risk stratification model was constructed in which patients were grouped into 3 categories, based on the presence of 0, 1, or 2 of these risk factors, and median time to MM progression was “Not reached”, 41 or 21 months, respectively (Figure 21.6).

IMWG guidelines recommend baseline measurements of sFLCs in all solitary plasmacytomas to identify patients at increased risk of progression to MM [167] (Section 25.3.1).