Membranous Nephropathy: A Paraneoplastic Manifestation of Renal Cell Carcinoma Discovered Six Years After the Initial Diagnosis

Membranous nephropathy is the leading cause of nephrotic syndrome in adults and is classically associated with solid tumors, particularly in elderly patients. Among the secondary causes of membranous nephropathy, solid tumors account for approximately 15% of cases, with the parallel evolution of the two conditions suggesting a paraneoplastic syndrome. The discovery of anti-phospholipase A2 receptor antibodies has revolutionized the diagnosis and management of membranous nephropathy. Anti-PLA2R antibodies are present in 70% to 80% of patients with primary membranous nephropathy and are highly specific. However, recent studies have shown that some paraneoplastic membranous nephropathies can also be PLA2R- positive, challenging the traditional distinction between primary and secondary forms. We report the case of a 60-year-old man who presented with membranous nephropathy in 2012. At that time, the evaluation for secondary causes was negative, and anti-PLA2R antibodies were not yet available. In 2018, a renal tumor was incidentally discovered on routine imaging. The patient underwent a partial nephrectomy, and histological examination confirmed a clear cell renal cell carcinoma. The improvement of proteinuria after tumor resection supports the paraneoplastic nature of the membranous nephropathy. This case illustrates that membranous nephropathy can be a paraneoplastic manifestation of renal cell carcinoma, even with a long delay between the two diagnoses, and highlights the importance of prolonged surveillance for malignancy in patients with membranous nephropathy, regardless of PLA2R status.