S3429 Extramedullary Plasmacytoma Masquerading as a Pancreatic and Duodenal Mass: A Rare Case

Introduction: Plasmacytomas are rare plasma cell neoplasms with monoclonal proliferation outside the bone marrow. They are classified as solitary bone plasmacytomas (SBP) or extramedullary plasmacytomas (EMP), the latter comprising <5% of plasma cell malignancies. EMPs may arise de novo or alongside systemic multiple myeloma, often signaling aggressive disease. While most affect the head, neck, or lungs, gastrointestinal (GI) involvement is rare and can mimic adenocarcinoma, lymphoma, or neuroendocrine tumors. We present a patient with multiple myeloma and progression to EMPs involving the pancreas and duodenum. Case Description/Methods: A 63-year-old man with immunoglobulin A (IgA) lambda light chain MM presented with several weeks of worsening lethargy. He was diagnosed 3 years prior, with bone marrow biopsy showing 23% clonal plasma cells expressing CD138 and lambda light chain restriction. Cytogenetics revealed immunoglobulin heavy chain (IGH) fusion, fibroblast growth factor receptor 2 (FGFR2) loss (chromosome 4), and tumor protein 53 (TP53) deletion (chromosome 17). He initially received bortezomib-based therapy, followed by 4 cycles of daratumumab, lenalidomide, bortezomib, and dexamethasone (DRVD), then underwent autologous stem cell transplant with daratumumab maintenance. Six months later, PET showed new lytic lesions, prompting initiation of daratumumab, pomalidomide, and dexamethasone (DPd). On admission, labs included: WBC 5.3, hemoglobin 12.6, platelets 366, ALT 124, ALP 139, bilirubin 1.3. Computed tomography revealed bilateral lung nodules and a 4.7 cm pancreatic head mass abutting the superior mesenteric vessels. Brain magnetic resonance imaging showed calvarial enhancement. Endoscopic ultrasound demonstrated a 3.7 cm irregular pancreatic mass; esophagogastroduodenoscopy revealed duodenal nodules. Biopsies from both confirmed EMP with CD138-positive, lambda light chain–restricted plasma cells. He was started on bortezomib, dexamethasone, cisplatin, doxorubicin, cyclophosphamide, and etoposide (VD-PACE). Discussion: GI EMPs are rare and often misdiagnosed due to nonspecific symptoms and imaging overlap with adenocarcinomas. Mechanisms include hematogenous spread or direct extension; loss of CD56 may facilitate dissemination. TP53 deletion correlates with poor prognosis. Diagnosis relies on biopsy and immunohistochemistry. Treatment is typically systemic, though localized EMPs may benefit from radiation. This case highlights the importance of considering GI EMPs in MM patients presenting with new GI symptoms.