A case of renal infiltration by Bence Jones protein kappa-producing lymphoplasmacytic lymphoma.

Lymphoplasmacytic lymphoma (LPL) is a type of low-grade B-cell lymphoma, with 90-95% of cases associated with Waldenström macroglobulinemia, characterized by the presence of IgM-type M-protein. We report, for the first time, a case of LPL-producing Bence Jones (BJ) protein kappa. The patient was a 78-year-old woman admitted to our department due to general fatigue and proteinuria that had persisted for 2 months. No M-protein was detected by blood immunofixation, but kappa-type BJ protein was detected in the urine. Light microscopy of a kidney biopsy sample revealed infiltration of lymphocytes and plasma cells into the perirenal adipose tissue and renal interstitium. The infiltrating cells exhibited kappa light chain restriction. Bone marrow examination revealed clusters of immature plasmacytoid lymphocytes that were CD20 positive, CD5 negative, and exhibited light chain restriction. Genetic analysis detected a MYD88 mutation, leading to the diagnosis of LPL in the patient. Six months after starting treatment with tirabrutinib, urinary protein levels improved to 0.2 g/gCr. Renal infiltration was identified due to urinary protein, and currently, no extramedullary lesions outside the kidneys are observed. Tirabrutinib has been extremely effective, but careful follow-up is still required.