Gastric plasmacytoma with loss of Cd138 expression mimicking extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue: a case report.

INTRODUCTION: Multiple myeloma (MM) is a malignant plasma cell neoplasm, while extramedullary plasmacytoma (EMP) represents a rare manifestation and is frequently associated with an adverse prognosis. Gastric involvement by EMP is exceedingly uncommon and may present significant diagnostic challenges, particularly in cases with atypical immunophenotypic features that resemble B-cell lymphomas, most notably extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma). CASE PRESENTATION: We report a 64-year-old man who had a large gastric tumor and was diagnosed with MM with gastric EMP, exhibiting loss of CD138 expression. CLINICAL DISCUSSION: Our case presented with epigastric pain and upper gastrointestinal bleeding. Imaging studies revealed a left pararenal mass and a large gastric tumor. Initial gastric biopsy suggested non-Hodgkin lymphoma. However, further investigations demonstrated monoclonal gammaglobulinemia, and bone marrow aspiration and biopsy confirmed malignant plasma cell infiltration. Subsequent re-evaluation of the gastric biopsy with an extended immunohistochemical panel supported the diagnosis of plasmacytoma. Although tumor cells were negative for CD138, they showed positivity for MUM1 and CD79a, along with monoclonal kappa light chain restriction, while CD20 was negative, allowing a definitive diagnosis. The patient was diagnosed with high-risk MM with gastric EMP and achieved a favorable response to daratumumab-based therapy (D-VRd). CONCLUSION: This case highlights the diagnostic pitfalls of gastric EMP, particularly in CD138-negative variants, which may closely mimic MALT lymphoma. Comprehensive clinicopathological correlation and expanded immunophenotypic analysis are essential to establish the correct diagnosis and guide appropriate management.