IgG4-related retroperitoneal fibrosis mimicking perinephric extension of renal cell carcinoma on CT: a case report.

Immunoglobulin G4-related disease (IgG4-RD) is an immune-mediated fibroinflammatory condition that can closely mimic malignancy on imaging. IgG4-related retroperitoneal fibrosis (RPF) most commonly involves the para-aortic or peri-iliac regions, while focal perinephric involvement is uncommon. Accurate differentiation from true perinephric tumour extension is critical, as misinterpretation may substantially alter renal cell carcinoma (RCC) staging and management. A 41-year-old man undergoing contrast-enhanced CT for transient rectal bleeding was incidentally found to have a heterogeneously enhancing left renal mass suspicious for RCC, with adjacent unilateral irregular perinephric soft tissue extending beyond Gerota's fascia. On CT, these findings were interpreted as locally advanced cT4 RCC. Radical nephrectomy was performed. Histopathology demonstrated clear cell RCC, grade 2, confined to the kidney (pT1b), with intact renal capsule and no perinephric or lymphovascular invasion. The adjacent perinephric tissue instead showed storiform fibrosis, dense IgG4-positive plasma cell infiltrates, and obliterative phlebitis, consistent with IgG4-related RPF. Serum IgG4 was mildly elevated. Postoperative FDG PET/CT demonstrated a focal hypermetabolic perinephric lesion without systemic involvement, and both imaging and serologic findings improved following corticosteroid therapy. This case highlights a rare but important radiologic pitfall in which focal perinephric IgG4-related RPF was radiologically indistinguishable from perinephric RCC extension, resulting in initial overstaging. IgG4-RD should be considered in the differential diagnosis when perinephric soft tissue appears inseparable from a renal mass, and multimodal evaluation integrating imaging, pathology, and serology is essential to avoid overstaging and inappropriate management.