Rapidly Progressive Retroperitoneal Dissemination of Plasmacytoid Urothelial Carcinoma Presenting With Multiorgan Obstruction After Radical Cystectomy: A Case Report

Introduction Plasmacytoid urothelial carcinoma (PUC) is a rare, aggressive subtype of bladder cancer characterized by diffuse infiltrative growth and a propensity for peritoneal and retroperitoneal dissemination. Case Presentation A patient with muscle‐invasive bladder cancer (cT3N1M0) was diagnosed with conventional high‐grade urothelial carcinoma without subtype histology on transurethral resection. After neoadjuvant gemcitabine–cisplatin chemotherapy, robot‐assisted radical cystectomy with intracorporeal neobladder reconstruction was performed. Final pathological examination revealed PUC (pT4aN2). Within weeks after surgery, the patient developed sequential retroperitoneal fluid accumulation, bilateral hydronephrosis, duodenal obstruction, and biliary obstruction, suggesting rapid diffuse retroperitoneal progression. Enfortumab vedotin plus pembrolizumab was initiated; however, the disease progressed aggressively, and the patient died on postoperative day 80. Conclusion PUC may evade detection on initial TUR and can progress rapidly through diffuse retroperitoneal dissemination even after radical cystectomy. Early postoperative, unexplained multiorgan obstructive findings should raise suspicion of occult progression and prompt timely initiation of systemic therapy.