Durable Complete Response to Pembrolizumab for Postoperative Nodal Recurrence of Lynch Syndrome–Associated Upper Tract Urothelial Carcinoma: A Case Report

Introduction Germline mutations in mismatch repair genes cause Lynch syndrome and are associated with upper tract urothelial carcinoma (UTUC). Tumors with deficient mismatch repair (dMMR) or microsatellite instability–high (MSI‐H) may respond favorably to immune checkpoint inhibitors. Case Presentation A 54‐year‐old woman with Lynch syndrome and a germline MSH2 mutation developed right renal pelvic carcinoma. The tumor was diagnosed as a pT3 high‐grade urothelial carcinoma, and the patient underwent laparoscopic nephroureterectomy. She was subsequently administered adjuvant gemcitabine and carboplatin due to a contraindication for cisplatin. Due to nodal recurrence, pembrolizumab was initiated at 200 mg every 3 weeks, achieving a complete radiological and metabolic response after five cycles. The course was complicated by immune‐related adrenal insufficiency, which was managed with corticosteroid replacement therapy. Conclusion Pembrolizumab may achieve a durable complete response in Lynch syndrome–associated UTUC. MSI screening should be considered in UTUC to guide decision‐making.