Follicular Lymphoma Arising in an Ileal Neobladder Presenting as a Large Pelvic Mass

Introduction Secondary malignancies arising in intestinal segments used for urinary diversion are rare, and reports of malignant lymphoma are extremely limited. We report a rare case of follicular lymphoma arising in an ileal neobladder 20 years after radical cystectomy. Case Presentation An 81‐year‐old man had undergone radical cystectomy with ileal neobladder reconstruction for muscle‐invasive bladder cancer (pT2bN0M0) 20 years earlier and had remained recurrence‐free. Follow‐up imaging incidentally revealed a large pelvic mass contiguous with the neobladder and multiple enlarged lymph nodes. Transurethral resection led to a diagnosis of follicular lymphoma. Treatment with bendamustine plus rituximab achieved marked tumor reduction without severe adverse events. Conclusion Malignant lymphoma arising in urinary diversion organs is extremely rare. This case highlights the potential for late‐onset lymphoid malignancies following urinary diversion and emphasizes the need for long‐term clinical awareness of secondary malignancies in such patients.