† These authors contributed equally.
Academic Editor: Erika Di Zazzo
Introduction: Solitary plasmacytoma (SP) is a rare plasma cell disorder characterized by localized neoplastic proliferation of monoclonal plasma cell. Due to its rarity, further understanding of the spectrum of its clinicopathologic features is needed. Methods: A retrospective analysis of cases from a single institution was conducted. Clinical characteristics of the patients were collected; histopathological and semi-quantitative immunohistochemical analyses were performed. Results: Thirteen cases were identified from our pathology archives, including 4 cases of solitary plasmacytoma of bone (SPB) (30.8%) and 9 extraosseous plasmacytoma (EP) (69.2%). The mean age of EP is a decade older than SPB. There is no gender disparity. The most common sites involved are the vertebrae and nasopharynx. Histologically, the tumors can be classified into two grades based on degree of differentiation. Immunohistochemically the tumor cells express CD38, CD138, MUM-1, and exhibit light chain restriction. Ki-67 proliferation index is 30%. In situ hybridization for Epstein-Barr virus-encoded small RNAs (EBER) is negative in six cases tested. Semi-quantitative immunohistochemical analysis showed decreased integrated optical density (IOD) of CD38 in neoplastic cells. IgH gene rearrangement was identified in two cases. Conclusion: SP is a rare plasmacytoid neoplasm that occurs more frequently in older patients. Diagnosis requires a systematic clinical approach combined with the pathological characteristics of plasmacytoid morphology, immunophenotype and light chain restriction. There are more cases of EP than SPB in our series, which is in contrast to that reported in literature. Results from this study suggest that CD38 is a potential immunohistochemical marker associated with prognosis of SP. Further studies with more cases and longer term follow-up may provide more definitive information on risk of progression from SP to multiple myeloma (MM).