A Case of Unexpected Peritoneal Carcinomatosis From Colon Cancer Detected After Robot‐Assisted Radical Cystectomy for Advanced Bladder Cancer

Introduction Peritoneal carcinomatosis in patients with advanced bladder cancer is a rare but lethal event following robot‐assisted radical cystectomy (RARC). We report a case of an unusual clinical course in which peritoneal dissemination of colon cancer was unexpectedly detected seven months after RARC. Case Presentation A 67‐year‐old man underwent two courses of neoadjuvant chemotherapy and subsequent RARC with extracorporeal urinary diversion for muscle‐invasive bladder cancer. The pathological diagnosis of the RARC specimen was urothelial carcinoma in situ (ypTis). After 7 months, the patient developed small bowel obstruction and underwent laparotomy. Intraoperatively, numerous seeding nodules were observed in the pelvic cavities, involving the small intestine. Contrary to our expectation of bladder cancer recurrence, the pathological examination revealed adenocarcinoma, suggesting primary colorectal cancer. Conclusion Peritoneal dissemination occurring shortly after RARC is often considered due to bladder cancer. However, the possibility of metastasis from other cancers must be taken into consideration.