Multiple myeloma associated with secondary plasma cell leukemia, gastric mucosal extramedullary plasmacytic...

We report a case of IgG λ-type multiple myeloma (MM) in a patient who received multiple lines of therapy and achieved repeated remissions. In 2025, the patient experienced a third relapse and had been exposed to proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and anti-CD38 monoclonal antibodies (mAbs), meeting the criteria for triple-refractory disease. At relapse, the patient presented with fatigue and progressive anemia, and peripheral blood smear examination revealed approximately 14% atypical plasma cells, fulfilling the diagnostic criteria for secondary plasma cell leukemia (sPCL). Treatment with the KXD regimen (carfilzomib, cyclophosphamide, dexamethasone) resulted in transient disease stabilization for approximately two months, followed by further progression. During the disease course, the patient developed upper abdominal discomfort. Gastroscopy revealed a focal lesion at the gastric angle, and endoscopic submucosal dissection (ESD) was subsequently performed. Histopathological examination demonstrated a moderately differentiated intramucosal papillary adenocarcinoma of the gastric angle, with tumor infiltration confined to the mucosal layer and negative resection margins. In addition, infiltration of CD138-positive plasma cell-like tumor cells with λ light chain restriction was observed in the gastric lamina propria, consistent with extramedullary disease (EMD). The final diagnosis was relapsed MM complicated by sPCL, gastric mucosal extramedullary plasmacytic infiltration, and synchronous moderately differentiated intramucosal papillary adenocarcinoma.