Concurrent Mantle Cell Lymphoma and Paraneoplastic Membranous Nephropathy.

We report a rare case of synchronous stage IV mantle cell lymphoma (MCL) and paraneoplastic membranous nephropathy (MN) in a 48-year-old man. The patient presented with a nasopharyngeal mass; biopsy confirmed cyclin D1-positive MCL harboring the CCND1::IGH rearrangement, along with nephrotic syndrome. Kidney biopsy revealed MN with IgG1/IgG3-dominant glomerular deposits but was negative for PLA2R, THSD7A, and NELL-1 antigens in both renal and tumor tissues, with corresponding serum antibodies undetectable. After lymphoma-directed therapy alone (R-CHOP/R-DHAP induction, autologous hematopoietic stem cell transplantation consolidation, and extended rituximab maintenance), both conditions achieved sustained remission. Complete remission was confirmed per the Lugano 2014 criteria, and nephrotic syndrome resolved, with proteinuria decreasing from 7.38 to 0.29 g/24 hours and serum albumin improving from 26.2 to 35.5 g/L with preserved kidney function. Both MCL and MN remained in remission for >2 years. This case highlights MCL-associated MN as a PLA2R/THSD7A/NELL-1 triple-negative entity where antineoplastic therapy alone induces renal remission, demonstrating both the paraneoplastic mechanism and the feasibility of intensive lymphoma treatment, including transplantation without renal compromise.