A Case of Solitary Fibrous Tumor Combined with Phospholipase A2 Receptor (PLA2R)-Related Membranous Nephropathy

Introduction: Cases diagnosed as solitary fibrous tumor (SFT) combined with PLA2R-related membranous nephropathy (MN) are relatively rare. Case Description: This report describes a case of a 35-year-old male patient who presented with a renal mass as the initial symptom, accompanied by nephrotic syndrome and elevated serum PLA2R antibody levels. Post-surgical pathology confirmed the renal tumor as a solitary fibrous tumor, while pathology of the surrounding renal tissue confirmed PLA2R-related MN. Two months after surgery, the patient's nephrotic syndrome showed no improvement; treatment with rituximab was administered, resulting in a partial remission. Discussion: In this patient, MN was first identified due to the detection of nephrotic syndrome during the evaluation of a renal mass. Renal tissue showed PLA2R and IgG4 positivity by immunofluorescence, and the serum anti-PLA2R antibody was elevated. The SFT tumor-associated antigen CD34 was positively expressed in glomerular and peritubular capillary endothelial cells. STAT6 showed weak cytoplasmic positivity in renal tubular epithelial cells, while it was negative in glomeruli. The renal tumor tissue was negative for PLA2R and IgG4, and malignancy-associated MN (NELL1, THSD7A) was also negative, suggesting the PLA2R-associated MN was not paraneoplastic in origin. Therefore, the final diagnosis favored coincidental PLA2R-associated MN coexisting with SFT, rather than tumor-induced secondary MN. Given the potential oncologic risks of traditional immunosuppressants, B-cell–targeted therapy with rituximab (RTX) may be a safer and effective alternative in such patients. In this case, RTX led to partial remission of MN.