Battle Beyond the Bone Marrow! Case Report of Extramedullary Plasmacytoma with Multiple Myeloma

Introduction: Plasma cell neoplasms includes monoclonal gammopathy of undetermined significance, multiple myeloma (MM), and solitary plasmacytomas. Extramedullary plasmacytomas (EMPs), found in soft tissues, are rare, with mediastinal involvement even rarer. Only 5% of EMPs coexist with MM. We present a rare case of MM and concurrent mediastinal extramedullary plasmacytoma in a patient who presented with AKI. Case Description: A 62 year old male patient with medical history of hypertension, hyperlipidemia with baseline creatinine of 1.1 mg/dl presented to the hospital, asymptomatic with abnormal labs of elevated creatinine to 6.9 mg/dl and anemia of 8.9 g/dl. Kidney injury was initially considered prerenal due to recent NSAID use. Incidental finding of anterior mediastinal mass on imaging was noted. Workup included elevated UPCR (7.56 gram), positive SPEP with monoclonal lambda light chain, elevated free Lambda light chains (13136.56 mg/L) and K/L Ratio of <0.01. Kidney biopsy confirmed monoclonal immunoglobulin deposition disease, monoclonal lambda light chains with light chain cast nephropathy and chronic tubulointerstitial disease without amyloid positivity. Bone marrow biopsy was notable for MM coexpressing lambda light chain. Mediastinal mass biopsy confirmed plasmacytoma. Due to worsening renal function, hemodialysis was initiated. Chemotherapy (bortezomib, cyclophosphamide, dexamethasone and daratumumab) led to significant reduction in lambda light chain allowing discontinuation of dialysis. Discussion: Solitary osseous plasmacytomas are known to progress to MM, the likelihood of soft tissue EMPs (particularly involving anterior mediastinal) evolving into systemic disease is rare. In our case it is unclear which condition developed first, this case underscores the importance of considering a broad differential diagnoses in patients with acute kidney injury and associated mass. Renal complications like light chain cast nephropathy significantly impact morbidity and mortality. Treatment involves stem cell transplantation for eligible patients, radiation or chemotherapy. Early recognition and intervention, by nephrologists and oncologists, is crucial for improving survival.