Clinical Features and Prognostic Factors in Solitary Plasmacytoma: A Single Center Retrospective Study

Background: Solitary plasmacytoma (SP) is a rare form of plasma cell dyscrasia that represents 2.8-5% of all plasma cell disorders. It is characterized by a localized accumulation of neoplastic monoclonal plasma cells, without evidence of systemic disease, such as multiple myeloma (MM). It can be classified into two groups according to the localization: SP of the bone (SBP) and extramedullary plasmacytoma (EMP). The common presentation of the SBP is in the axial skeleton, whereas the EMP is usually seen in the head and neck. The clinical course and prognosis are quite different between these two entities. Here we present the clinical features, treatment strategies and relative prognostic factors of a series of SP patients followed at a single center over four decades. Methods: A retrospective study was carried out in 53 patients with SP referred at our Institution between 1976 and 2012. Patients’ characteristics are shown in Table 1. The median follow-up was 8.9 years (range 0.5-38.1). Thirty-five patients had SBP and 18 patients EMP. The median age was 56.8 years (range 27-80). The male to female ratio was 1.18:1 and 2.6:1 in SBP and EMP, respectively. The vertebral column (48% of cases) and the upper respiratory tract (50% of cases) were the most common sites of involvement SBP and EMP, respectively. Serum M-protein was detected in 62.8% and 27.7% of the SBP and EMP cases (P=0.0156). The tumor sizes were larger in the SBP group than in the EMP group (P=0.0039). Treatment consisted of local radiotherapy (n=26), combined radiotherapy + chemotherapy (n=15), surgery alone (n=4) and chemotherapy alone (n=8). The radiation dose was available for 35 patients and the median radiation dose administered was 41 Gy (range 21-88 Gy). Results: All patients were evaluable for treatment response and outcome. We found no differences between treatment approaches in terms of response rate, progression-free survival (PFS) and overall survival (OS). The local control rate was 94.3% and there was no statistically significant difference between the two groups. Progression to MM was recorded in 20/35 (57.1%) patients with SBP and in 1/18 (5.5%) patients with EMP. The median time to progression was 2.5 years (range 0.6-11). The 5-year PFS rate for all patients was 66.3% (95% CI: 53.9-81.4). Patients with SBP showed a worse PFS rate than EMP patients (53.0% vs 88.5%), with a statistically significant difference (P=0.0003). The total 5-year OS rate was 78.4% (95% CI: 67.3-91.3). Patients with SBP had a significantly worse OS rate than patient with EMP (71.9% vs 88.2%, respectively) (P=0.0290). Univariate analysis revealed that SBP and a larger tumor size (≥5 cm) were adverse prognostic factors affecting PFS (P=0.0027 and P=0.0498, respectively). SBP had an adverse effect also on OS (P=0.0405). In multivariate analysis, bone localization was the only independent worse prognostic factor for both PFS (P=0.0041) and OS (P=0.0216). Conclusions: Patients with SBP have a significantly worse prognosis than patients with EMP, underlining that they are indeed two different entities within the spectrum of plasma cells dyscrasias. The use of modern testing is mandatory to better characterize the patients’ risks of progression and to exclude an occult MM, especially in SBP. There is also a need for further prospective studies with large series of patients evaluating SBP and EMP separately, to elucidate the role of prognostic factors and treatment options for these conditions.