A Rare Case of Plasmablastic Myeloma With Dual Kappa and Lambda mRNA Expression Presenting as a Solitary Hard Palate Tumor.

Plasmablastic myeloma is an aggressive variant of plasma cell myeloma that can mimic plasmablastic lymphoma when it presents as a solitary mass. Dual kappa and lambda light-chain expression is exceptionally rare and may be missed if evaluation relies on protein-level detection alone. We report the case of a woman in her 80s who presented with left hard palate swelling and underwent resection of a solitary hard palate and maxillary tumor. Histology showed sheets of large plasmablastic cells positive for CD38, CD138, multiple myeloma oncogene 1, and immunoglobulin G, and negative for B-cell markers and Epstein-Barr virus-encoded RNA. Within weeks, she developed malaise, nausea, and pancytopenia. Laboratory studies revealed elevated lactate dehydrogenase and serum immunoglobulin G of 4,409 mg/dL with immunoparesis. Serum free light chains showed mildly elevated kappa and markedly elevated lambda, with a kappa/lambda ratio of 0.02. Positron emission tomography-computed tomography demonstrated diffuse skeletal and splenic fluorodeoxyglucose uptake without additional extramedullary lesions, confirming the hard palate tumor as the only extramedullary site. Bone marrow examination showed marked hypercellularity with extensive replacement by plasmablastic plasma cells expressing cyclin D1, while Epstein-Barr virus studies and anaplastic lymphoma kinase were negative. Light-chain immunohistochemistry showed absent kappa staining and lambda staining in only a small subset of tumor cells, whereas RNA in situ hybridization demonstrated dual light-chain messenger RNA expression in most tumor cells. Cytogenetic analysis revealed 1q21 amplification, deletion of 17p13, and immunoglobulin heavy chain/MAF rearrangement. Despite high-dose dexamethasone, sequential proteasome inhibitor-, anti-CD38 antibody-, and immunomodulatory drug-based therapies, followed by B-cell maturation antigen × CD3 bispecific antibody treatment, the disease remained refractory and the patient died 45 days after diagnosis. This case highlights that transcript-based light-chain testing can uncover exceptionally rare dual kappa and lambda expressions when protein assays are negative or misleading. In solitary plasmablastic lesions, integrated clinicopathological assessment, including cyclin D1, Epstein-Barr virus studies, and RNA in situ hybridization, is critical for accurate diagnosis and recognition of this highly aggressive subtype.