RESUMEN
Plasmacytoma is defined as a plasma cell neoplasm forming a solitary osseous or extramedullary tumor without evidence of myeloma or organ damage related to a plasma cell neoplasm. Epstein-Barr virus (EBV) is associated with various B-cell neoplasms, particularly in patients with immune dysregulation; however, plasmacytoma is typically negative for EBV. Here, a case of EBV-positive sternal plasmacytoma in an immunocompetent female is presented. A 76-year-old female with no immunodeficiency presented with a tumor on the anterior thoracic wall. Imaging analysis revealed a 6.3 cm-sized tumor at the manubrium, and a needle biopsy was performed. The tumor in the bone was composed of a diffuse proliferation of plasmacytes with eccentric nuclei and a perinuclear halo. By immunohistochemistry and in situ hybridization, tumor cells were CD20-, CD3-, CD138+, κ+, λ-, EBER+, and the Ki67-labeling index was approximately 20%. Subsequent studies identified IgG κ monoclonal protein in serum but no evidence of plasma cell neoplasm-related organ damage, such as hypercalcemia, anemia, or renal dysfunction. No plasma cell neoplasm was detected in the bone marrow in the morphological and flowcytometric studies. Accordingly, the diagnosis was EBV-positive plasmacytoma. The patient was treated with local radiation therapy and achieved complete remission. EBV-positive plasmacytoma is rare in immunocompetent patients and should be carefully distinguished from plasmablastic lymphoma, another EBV-positive neoplasm with a plasma cell phenotype and an aggressive clinical course. This case also raises an important question: "when to perform EBER in situ hybridization in diagnosing plasma cell neoplasm?", which prompts further large case-series studies.