Fluorescent Ureteral Catheter‐Assisted Retroperitoneoscopic Biopsy for Periureteral IgG4‐Related Disease Mimicking Upper Tract Urothelial Carcinoma

Introduction Periureteral IgG4‐related disease (IgG4‐RD) is rare and can mimic upper tract urothelial carcinoma (UTUC), sometimes leading to unnecessary nephroureterectomy. Diagnosis is challenging when endoscopic biopsy is inconclusive and percutaneous biopsy is unsafe. Case Presentation A 58‐year‐old man presented with right‐sided hydronephrosis. Imaging suggested right renal pelvic or ureteral carcinoma; however, ureteroscopic biopsy was nondiagnostic despite weak IgG4 positivity. Serum IgG4 was elevated to 684 mg/dL. Because percutaneous biopsy was unsafe owing to proximity to major vessels, retroperitoneoscopic biopsy was performed after placement of a fluorescent ureteral catheter. Near‐infrared imaging identified the ureteral course, enabling safe biopsy. Frozen sections showed no malignancy, and histopathology revealed abundant IgG4‐positive plasma cells, establishing IgG4‐RD. Steroid therapy was initiated. Conclusion Intraoperative frozen‐section analysis ruled out malignancy and was the decisive factor in avoiding nephroureterectomy; the fluorescent ureteral catheter served as intraoperative navigation, enabling safe sampling of the periureteral lesion without ureteral injury.